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A study of the solubility of synthetic hydroxyapatite. Sleeman, Kenneth Jack 1963

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A STUDY OF THE SOLUBILITY OF SYNTHETIC HYDROXYAPATITE by KENNETH J . SLEEMAN B.S.A., U n i v e r s i t y of B r i t i s h Columbia, 1955 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN AGRICULTURE i n the Department of SOIL SCIENCE We accept t h i s t h e s i s as conforming t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA August, 1963 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I f u r t h e r agr.ee that per-mission for extensive copying of t h i s t h e s i s for s c h o l a r l y purposes may be granted by the Head of my Department or b"y h i s representatives. I t i s understood that copying, or p u b l i -c a t i o n of t h i s t h e s i s for f i n a n c i a l gain s h a l l not be allowed without my written' permission. Department of The U n i v e r s i t y of B r i t i s h Columbia,. Vancouver 8, Canada. Date ABSTRACT A study was made of the s o l u b i l i t y of s y n t h e t i c h y d r o x y a p a t i t e ( C a 1 Q ( P 0 4 ) g ( O H ^ ) • The b a s i c c a l c i u m phosphate was s y n t h e s i z e d i n CO^ f r e e systems, over a pH range of 5.0 to 8.0 u s i n g r e a c t i o n temperatures of 40°, 60°, and 90°C, and r e a c t i o n p e r i o d s of 24 and 96 hours. The study was d i v i d e d i n t o two phases. In one phase the pH of the su s p e n s i o n s , and the c a l c i u m and phosphorus c o n c e n t r a t i o n s i n s o l u t i o n were measured a f t e r p r e c i p i t a -t i o n and a f t e r r e d i s p e r s i n g the p r e p a r a t i o n s i n water. The a p p l i c a t i o n of s o l u b i l i t y c r i t e r i a to these s t u d i e s showed t h a t the d i f f e r e n c e s i n s o l u b i l i t y o b t a i n e d by s y n t h e s i s w i t h v a r y i n g r e a c t i o n p e r i o d and temperature were not brought about by c o n d i t i o n s of super s a t u r a t i o n . S o l u b i l i t y was found to decrease w i t h i n c r e a s e d s y n t h e s i s r e a c t i o n p e r i o d and i n c r e a s e d temperature of s y n t h e s i s . With a g i v e n temperature and s y n t h e s i s r e a c t i o n p e r i o d the h y d r o x y a p a t i t e p r e p a r a t i o n s had a u n i f o r m s o l u b i l i t y . S o l u b i l i t y changed o n l y when the c o n d i t i o n s of s y n t h e s i s were changed. X-ray d i f f r a c t i o n a n a l y s i s of the s o l i d phase i n d i c a t e d t h a t the s o l u b i l i t y of h y d r o x y a p a t i t e d e c r e a s e d w i t h i n c r e a s i n g c r y s t a l l i n i t y . E xamination w i t h the e l e c t r o n microscope, however, showed t h a t i i i n c r e a s e d c r y s t a l l i n i t y was not due to i n c r e a s i n g c r y s t a l s i z e . Owing to the s m a l l n e s s of c r y s t a l s , i t was not p o s s i b l e t o examine the h y d r o x y a p a t i t e w i t h a p e t r o g r a p h i c microscope, and hence i t was not p o s s i b l e to d e t e c t the presence of amorphous phases. I t i s p o s s i b l e , then, t h a t s m a l l amounts of amorphous substances were p r e s e n t i n the s o l i d phase, and, as a r e s u l t the apparent degree of c r y s -t a l l i n i t y was a l t e r e d . In the second phase, s t u d i e s of the Ca:P mole r a t i o s of the p r e p a r a t i o n s showed t h a t Mole r a t i o s were not c o n s t a n t f o r any mode of s y n t h e s i s . F u r t h e r s t u d i e s i n d i c a t e d t h a t a d s o r p t i o n was of no, or at b e s t , minor importance i n d e t e r m i n i n g the Ca:P r a t i o s of h y d r o x y a p a t i t e . Isomorphous s u b s t i t u t i o n w i t h 0H~ions r e p l a c i n g P0^= i o n s , and H^O i o n s r e p l a c i n g Ca i o n s might have o c c u r r e d , but i t was not p o s s i b l e to d e t e c t s o l u b i l i t y d i f f e r e n c e s which s h o u l d r e s u l t from t h i s s u b s t i t u t i o n . Hence i t was not p o s s i b l e t o c o r r e l a t e s o l u b i l i t y w i t h c o m p o s i t i o n of h y d r o x y a p a t i t e . Even though the h y d r o x y a p a t i t e appeared to be of v a r i a b l e c o m p o s i t i o n , i t was found t h a t t h i s b a s i c c a l c i u m phosphate had a d e f i n i t e pKsp v a l u e i n the pH range of 5.0 to 8.0. ACKNOWLEDGEMENT In the summer of 1956, a s o i l survey of the U.B.C. Research Farm No. 2 , at Oyster R i v e r on Vancouver I s l a n d , was made as a p o r t i o n of the t h e s i s program. The maps f o r t h i s survey and the accompanying r e p o r t s are on f i l e i n the l i b r a r y of the S o i l S c i e n c e Department at the U n i v e r s i t y of B r i t i s h Columbia. The author would l i k e t o extend h i s g r a t i t u d e t o Mr. L. F a r s t a d of the Canada Department of A g r i c u l t u r e and t o Dr. C. A. Rowles of the Department of S o i l S c i e n c e , f o r t h e i r a i d i n t h i s study. F o r t h i s h y d r o x y a p a t i t e study, the author wishes t o express g r a t i t u d e t o a l l those whose h e l p has made i t p o s s i b l e ; t o Dr. J . S. C l a r k , f o r m e r l y o f the Department of S o i l S c i e n c e , who suggested the t o p i c of study, and, to Dr. C. A. Rowles and Dr. Hugh Gardner of the Department of S o i l S c i ence whose c o n s t r u c t i v e c r i t i -cisms have been i n v a l u a b l e i n p r e s e n t i n g the data. A s p e c i a l vote of thanks must go to the Department of M e t a l l u r g y f o r use of the X-ray equipment n e c e s s a r y i n t h i s study. F i n a l l y , t o my w i f e , I r e n e , whose p a t i e n c e and d e v o t i o n has a l l o w e d t h i s p r o j e c t t o proceed through some d i f f i c u l t times, a gr e a t d e a l of c r e d i t i s due. i TABLE OF CONTENTS PAGE ACKNOWLEDGEMENT i ABSTRACT i i INTRODUCTION i v SECTION I REVIEW OF LITERATURE 1 I I METHODS FOR THE SOLUBILITY STUDIES 9 (1) The P r e p a r a t i o n of H y d r o x y a p a t i t e . . . . 9 (2) D i s s o l u t i o n Experiments 11 (3) A n a l y t i c a l Procedures 12 a) X-ray A n a l y s i s 12 b) The Measurement of pH 13 c) The D e t e r m i n a t i o n of Ca l c i u m by Ver senate 14 d) The D e t e r m i n a t i o n of Phosphorus by the Aqueous Molybdenum Blue Method . 16 (4) C a l c u l a t i o n s 18 a) C a l c u l a t i o n s o f A c t i v i t y C o e f f i c i e n t s 18 b) C a l c u l a t i o n of H 2 P 0 4 a c t i v i t y . . . . 19 c) C a l c u l a t i o n of PO^ A c t i v i t y 22 d) D e t e r m i n a t i o n of R e g r e s s i o n L i n e s . . 24 (5) The D e t e r m i n a t i o n o f Mole R a t i o s 24 I I I RESULTS AND DISCUSSION • 27 (1) R e s u l t s of the S o l u b i l i t y S t u d i e s . . . . 27 (2) R e s u l t s of the Mole R a t i o S t u d i e s . . . . 33 SUMMARY AND CONCLUSIONS 36 IV LITERATURE CITED 38 SECTION V APPENDICES 1 S o l u b i l i t y of H y d r o x y a p a t i t e Measured a f t e r P r e c i p i t a t i o n at 40°C f o r 24 hours. 1A S o l u b i l i t y o f H y d r o x y a p a t i t e formed a t 40°C f o r 24 Hours Measured a f t e r D i s s o l u t i o n . 2 S o l u b i l i t y of H y d r o x y a p a t i t e Measured a f t e r P r e c i p i t a t i o n at 60°C f o r 24 Hours. 2A S o l u b i l i t y o f H y d r o x y a p a t i t e formed at 60°C f o r 24 Hours Measured a f t e r D i s s o l u t i o n . 3& S o l u b i l i t y of H y d r o x y a p a t i t e Measured a f t e r 3 A P r e c i p i t a t i o n at 60° f o r 96 Hours and Measured a f t e r D i s s o l u t i o n . 4 S o l u b i l i t y of H y d r o x y a p a t i t e Measured a f t e r P r e c i p i t a t i o n at 90°C f o r 96 Hours. 4A S o l u b i l i t y of H y d r o x y a p a t i t e formed at 90°C f o r 96 Hours Measured A f t e r D i s s o l u t i o n . 5 The Ca:P Mole R a t i o and Apparent S o l u b i l i t y Product of H y d r o x y a p a t i t e . 6 The Ca:P Mole R a t i o of H y d r o x y a p a t i t e b e f o r e and a f t e r Treatment w i t h D e s o r b i n g S o l u t i o n s . 7 S o l u b i l i t y Diagrams, Apparatus, and X-ray D i f f r a c t i o n P a t t e r n s . INTRODUCTION S t u d i e s of the p r o p e r t i e s of h y d r o x y a p a t i t e have been made mainly by b i o l o g i c a l and s o i l c h e m i s t s . These groups have been a s s i s t e d by m i n e r o l o g i s t s who have e s t a b l i s h e d the s t r u c t u r e o f the a p a t i t e l a t t i c e . The i n t e r e s t of the b i o l o g i c a l c hemists, i n h y d r o x y a p a t i t e , stemmed from i n v e s t i g a t i o n s o f the m i n e r a l m a t e r i a l i n the t e e t h and bone of mammals. X-ray d i f f r a c -t i o n s t u d i e s of the m i n e r a l phase of bone i n d i c a t e d t h a t o n l y an a p a t i t e - l i k e m a t e r i a l was pr e s e n t of which f l u o r a p a t i t e , ( C a 1 0 ( P 0 4 ) g F 2 ) , was the p r o t o t y p e . S i n c e bone m i n e r a l s had o n l y t r a c e s of f l u o r i n e p r e s e n t (49), i t was g e n e r a l l y a c c e p t e d t h a t h y d r o x y a p a t i t e , (Ca-j^PO^g(OH *)<•>) > best approximated the substance c o m p r i s i n g the m i n e r a l phase of t e e t h and bone 13, 20, 47, 48, 51, 52, 60, 61, 62, 66. S o i l chemists have s t u d i e d the b a s i c c a l c i u m phosphates because, i n n e u t r a l and a l k a l i n e s o i l s i n s o l u b l e c a l c i u m phosphates are formed. I t was w i d e l y conceded t h a t the f i x a t i o n of s o l u b l e phosphorus i n these s o i l s i n v o l v e d a stepwise p r o c e s s by which s u c c e e d i n g l y l e s s s o l u b l e c a l c i u m phosphates were formed. The end and s t a b l e product of t h i s p r o c e s s was c o n s i d e r e d t o be h y d r o x y a p a t i t e (11, 63). i v In both cases the studies have centered around the determination of the s o l u b i l i t y composition, and surface properties of hydroxyapatite. The purpose of t h i s study was to seek information on the s o l u b i l i t y of hydroxyapatite, synthesized over a wide range of conditions, by applying the s o l u b i l i t y c r i t e r i a of Clark and Peech (11). An attempt was also made to c o r r e l a t e the s o l u b i l i t y of the p r e c i p i t a t e s with t h e i r chemical composition. v REVIEW OF LITERATURE The s o l u b i l i t y of h y d r o x y a p a t i t e i s of g r e a t importance i n b i o l o g i c a l and s o i l systems. B a s i c a l l y , t h e o r i e s p e r t a i n i n g t o the p r e c i p i t a t i o n of h y d r o x y a p a t i t e , whether i n v i v o or i n v i t r o , i m p l i e d t h a t some s o l u b i l i t y product must be exceeded i n order t h a t p r e c i p i t a t i o n may occu r . Many s t u d i e s have been made of the s o l u b i l i t y of t h i s m i n e r a l , but, u n f o r t u n a t e l y , i n a gr e a t many cases these have not always been thermodynamically sound. In the main, the s o l u b i l i t y s t u d i e s have been of a simple n a t u r e , u s i n g e i t h e r s y n t h e s i z e d or n a t u r a l l y o c c u r r i n g h y d r o x y a p a t i t e . I t has been shown (34, 37, 42), t h a t the s o l u b i l i t y of h y d r o x y a p a t i t e was dependent on pH. The c a l c i u m and phosphate c o n c e n t r a t i o n s i n s o l u t i o n s markedly i n c r e a s e d w i t h i n c r e a s e d a c i d i t y . A l s o these s t u d i e s showed t h a t , as the c a l c i u m phosphate d i s s o l v e d the pH i n c r e a s e d g i v i n g an i n v e r s e r e l a t i o n s h i p between c a l c i u m and hydrogen. T h i s e f f e c t has been d e s c r i b e d by the em-p i r i c a l r u l e of Hodge (34) which has been s i m p l i f i e d by L e v i n s k a s (42) to the r e l a t i o n s h i p [ C a + + ] / [ H + ] c=i K. The i n v e r s e r e l a t i o n s h i p between c a l c i u m and hydrogen ion; c o n c e n t r a t i o n s has been a t t r i b u t e d (58) to a mole f o r mole - 2 -+ ++ exchange of H^O f o r Ca at the c r y s t a l s u r f a c e r e s u l t i n g i n an i n c r e a s e d pH. To study the e f f e c t s o f common i o n s on the s o l u b i l i t y of b a s i c c a l c i u m phosphate, many i n v e s t i g a t o r s (41, 42, 45, 57) have added h y d r o x y a p a t i t e to s o l u t i o n s c o n t a i n i n g c a l c i u m or phosphate i o n s . Although q u a n t i t a -t i v e agreement among the v a r i o u s workers was l a c k i n g , i t was found t h a t the a d d i t i o n o f c a l c i u m i o n s decreased the c o n c e n t r a t i o n o f phosphorus i n s o l u t i o n , and c o n v e r s e l y a d d i t i o n of phosphate i o n s d e c r e a s e d the c a l c i u m concen-t r a t i o n . I t was a l s o found (42,57) t h a t the e q u i l i b r i u m pH was decreased w i t h c a l c i u m as the common i o n , and i n -c r e a s e d w i t h phosphorus as the common i o n . T h i s has been a t t r i b u t e d (58). t o the replacement of s u r f a c e hydronium i o n s by c a l c i u m , t h e r e b y c a u s i n g a pH decrease i n the s o l u t i o n . With excess phosphorus p r e s e n t , i t was p o s t u -l a t e d t h a t the a v a i l a b l e c a l c i u m was drawn i n t o the c e n t r e p a r t o f the c r y s t a l s t r u c t u r e , l e a v i n g c a l c i u m v a c a n c i e s i n the s u r f a c e hexagonal "column" p o s i t i o n s . These v a c a n c i e s c o u l d be f i l l e d by drawing hydronium i o n s from s o l u t i o n , and i n c r e a s i n g the pH. S t u d i e s to determine the e f f e c t of v a r y i n g s o l i d to s o l u t i o n r a t i o s have shown, w i t h two e x c e p t i o n s (41, 44), t h a t t h e r e was a d e c i d e d tendency f o r more c a l c i u m and phosphorus t o d i s s o l v e w i t h more s o l i d phase p r e s e n t (9, 17, 28, 42). As more s o l i d was added t o the s o l u t i o n , the c a l c i u m c o n c e n t r a t i o n tended t o become many - 3 -times g r e a t e r than t h a t of phosphorus. T h i s e f f e c t has been a t t r i b u t e d (58) to an i n c r e a s e of the r e a c t i v e s u r -f a c e s of the s o l i d . S i n c e s t u d i e s of s o l u b i l i t y r e l a t i o n s must be made at e q u i l i b r i u m , many i n v e s t i g a t o r s (6, 8, 10, 12, 25, 36, 37, 42, 46, 57,71) have t r i e d t o determine the time r e q u i r e d to r e a c h e q u i l i b r i u m . A g a i n t h e r e appeared t o be no agreement between workers. With p r e c i p i t a t e d hydroxy-a p a t i t e , i t has been demonstrated t h a t e q u i l i b r i u m was o b t a i n e d i n a few hours (57) y e t o t h e r work has shown (6) t h a t 19 months or even l o n g e r was n e c e s s a r y . I t has been suggested (58) t h a t on m ixing of c a l c i u m and phosphate i o n s , amorphous, or at b e s t , s m a l l i m p e r f e c t c r y s t a l s were formed, which on a g i n g r e s u l t e d i n changing s u r f a c e s and hence changing s o l u b i l i t y . By s y n t h e s i z i n g h y d r o x y a p a t i t e at 90°C, i t has been shown (12) t h a t s t a b l e c o n d i t i o n s were r e a c h e d i n f o u r hours. T h i s suggests t h a t h i g h temperatures of s y n t h e s i s were r e q u i r e d to make s t a b l e c r y s t a l s and r e a c h e q u i l i b r i u m q u i c k l y . There i s l i t t l e doubt (12, 37) t h a t e q u i l i b r i u m w i l l be r e a c h e d q u i c k l y w i t h the d i s s o l u t i o n of s t a b l e w e l l formed c r y s t a l s . Many attempts have been made to r e a c h the same s o l u b i l i t y v a l u e from s u p e r s a t u r a t e d and u n d e r s a t u r a t e d c o n d i t i o n s . To date f o u r workers have c l a i m e d to be suc-c e s s f u l . Kuyper (40) used l a r g e amounts of s o l i d phase t o minimize changes i n s u r f a c e c o m p o s i t i o n . L e v i n s k a s and - 4 -Newman (42) added c r i t i c a l amounts of s o l i d phase t o s o l u t i o n s of c a l c i u m and phosphate i o n s which were o n l y s l i g h t l y i n excess of e q u i l i b r i u m v a l u e s . C l a r k (1-2) was s u c c e s s f u l i n o b t a i n i n g the same s o l u b i l i t y v a l u e f o r p r e -c i p i t a t e d and d i s s o l v e d h y d r o x y a p a t i t e . E r i c s s o n (22), by adding l a r g e amounts of s o l i d t o more a c i d s o l u t i o n s (and a d j u s t i n g pHO, and s m a l l e r amounts t o l e s s a c i d s o l u t i o n s , then by a d j u s t i n g i o n i c s t r e n g t h so t h a t both s o l u t i o n s were e q u a l , was a b l e t o approach the same s o l u b i l i t y v a l u e from b o t h d i r e c t i o n s . These r e s u l t s , however, showed con-s i d e r a b l e d i f f e r e n c e s i n s o l u b i l i t y v a l u e s . Perhaps, as has been suggested by Greenwald (28) the s o l u b i l i t y be-h a v i o r was dependent t o a gre a t e x t e n t on the c r y s t a l l i n e s u r f a c e s . These then were the methods used f o r s t u d y i n g the s o l u b i l i t y of h y d r o x y a p a t i t e . Although the methods employed were r a r e l y comparable, the v a r i a b l e r e s u l t s found, a l o n g w i t h the s u g g e s t i o n t h a t the l a t t i c e was a b l e to r e f l e c t i t s c o m p o s i t i o n i n i t s f l u i d s u r r o u n d i n g s by i o n i c exchange (40, 41), l e d Newman and co-workers (58) to conclude t h a t h y d r o x y a p a t i t e had no d e f i n i t e Ksp v a l u e . Indeed, as Hodge (34) p o i n t e d out i n 1951 no one had ever r e p o r t e d a d e f i n i t e Ksp v a l u e of h y d r o x y a p a t i t e . In view o f the nature of many of the s o l u b i l i t y s t u d i e s made, t h i s was not s u r p r i s i n g . F or example, the c o n c l u s i o n t h a t the s o l u b i l i t y of h y d r o x y a p a t i t e v a r i e d w i t h pH was - 5 -based on the simple o b s e r v a t i o n t h a t c a l c i u m and phosphorus c o n c e n t r a t i o n i n s o l u t i o n changed w i t h pH. R a r e l y were ade-quate attempts made to take i n t o account i o n a c t i v i t i e s , and t o use sound s o l u b i l i t y c r i t e r i a f o r the i n t e r p r e t a t i o n of e x p e r i m e n t a l o b s e r v a t i o n s . Recent s t u d i e s have shown t h a t h y d r o x y a p a t i t e has f i x e d thermodynamic p r o p e r t i e s . The c h e m i c a l e n g i n e e r i n g d i v i s i o n of the T._V„..A._ (68) has determined the thermodynamic p r o p e r t i e s of h y d r o x y a p a t i t e from which i t i s p o s s i b l e to c a l c u l a t e the s o l u b i l i t y p r o d u c t . S e v e r a l workers (2, 11, 23) have a i d e d i n d e v e l o p i n g s o l u b i l i t y c r i t e r i a f o r h y d r o x y a p a t i t e . C l a r k (12), by use of these c r i t e r i a has shown t h a t h y d r o x y a p a t i t e p o s s e s s e d a c o n s t a n t s o l u b i l i t y product from pH 5.0 to 9.0 i f adequate p r e c a u t i o n s were taken to ensure the attainment of e q u i l i b r i u m . I t i s f e l t t h a t one of the main reasons f o r the f a i l u r e t o o b t a i n c o n s t a n t s o l u b i l i t y v a l u e s i n e a r l i e r s t u d i e s was the absence of e q u i l i b r i u m c o n d i t i o n s . Even though the Ksp v a l u e s r e p o r t e d by C l a r k and T.V.A. were d i f f e r e n t , they were s i m i l a r enough to i n d i c a t e t h a t e x p e r i m e n t a l e r r o r , and d i f f e r e n t a n a l y t i c a l p rocedures may have caused the observed d i f f e r e n c e s . Almost without e x c e p t i o n , i t has been shown (1, 3, 7, 12, 13, 20, 21, 22, 24, 26, 31, 32, 33, 50, 54, 55, 59) t h a t the b a s i c c a l c i u m phosphates found i n n a t u r e and those formed i n the l a b o r a t o r y possess an a p a t i t e - l i k e - 6 -l a t t i c e . U s u a l l y these b a s i c c a l c i u m phosphates do not have a ch e m i c a l c o m p o s i t i o n c o r r e s p o n d i n g t o t h a t of a pure a p a t i t e , but g e n e r a l l y c o n t a i n an excess (or d e f i c i e n c y ) of e i t h e r c a l c i u m or phosphorus. I d e a l l y , f l u o r a p a t i t e and h y d r o x y a p a t i t e ( C a 1 ( ) ( P 0 4 ) 6 ( F ) 2 ) and ( C a 1 0 ( P 0 ) 6 ( 0 H ) 2 ) have a mole r a t i o of Ca:P of 1.66. I t has been found t h a t many s y n t h e t i c and n a t u r a l a p a t i t e s possess mole r a t i o s of c a l -cium t o phosphorus r a n g i n g from 1.33 to 2.0, even though they e x h i b i t an a p a t i t e s t r u c t u r e a c c o r d i n g t o X-ray d i f f r a c t i o n a n a l y s i s (1, 3, 29, 38). The phosphates w i t h mole r a t i o s l e s s than 1.33 have been shown t o c o n t a i n CaHP0 4. 2H 20 (14, 25, 27, 64), and those g r e a t e r than 2.0 have been found t o c o n t a i n Ca(OH),-, (1) or some oth e r c a l c i u m compound. Between the range 1.33. t o 2.0, isomorphous sub-s t i t u t i o n and a d s o r p t i o n have g e n e r a l l y been proposed to account f o r the observed v a r i a t i o n of the Ca:P r a t i o s . C a l c i u m and phosphorus may be adsorbed (3, 4, 5, 15, 16, 22, 27, 32, 33, 38, 43, 44, 59) at the s u r f a c e of the a p a t i t e c r y s t a l s . On t h i s b a s i s r a t i o s below 1.66 have been a t t r i b u t e d t o the presence of adsorbed phosphate i o n s (3, 4, 5, 32, 35, 59), and the presence of adsorbed c a l c i u m i o n s has been used t o e x p l a i n the mole r a t i o s o f Ca:P above 1.66 (15, 16, 38). The manner i n which a d s o r p t i o n takes p l a c e has not been s a t i s f a c t o r i l y e x p l a i n e d , P o s s i b l y , a form o f c h e m i s o r p t i o n o c c u r s , where a new c r y s t a l l a t t i c e b e g i n s t o form at the s u r f a c e o f another c r y s t a l . I f i n -complete c r y s t a l s were formed, mole r a t i o s might be a l t e r e d - 7 -c o n s i d e r a b l y . Even though the c r y s t a l s are i n d e e d s m a l l (56), measurements of the s u r f a c e a r e a of a p a t i t e c r y s t a l s i n d i c a t e d t h a t the s u r f a c e a r e a was not s u f f i c i e n t t o account f o r the observed mole r a t i o s (57, 70) on the b a s i s of a d s o r p t i o n . The concept of s u b s t i t u t i o n of other i o n s f o r c a l c i u m and phosphorus has a l s o been put f o r t h to account f o r mole r a t i o s d i v e r g i n g from the i d e a l (1, 30, 31, 33, 50, 51, 52, 53, 67, 69). I t has been proposed, f o r example t h a t H +, H^0 +, OH -, Rv^ O, and other i o n s or molecules may s u b s t i -t u t e f o r c a l c i u m and phosphorus i n the a p a t i t e l a t t i c e w ithout p r o d u c i n g any marked a l t e r a t i o n s i n i t s s p a t i a l arrangements. The n e g a t i v e groups would s u b s t i t u t e f o r phosphate i o n s and the p o s i t i v e groups f o r c a l c i u m i o n s . No c o n c l u s i v e evidence has been found, however, f o r the e x i s t -ence of a s u b s t i t u t e d h y d r o x y a p a t i t e s e r i e s where H + and 0H~ have r e p l a c e d r e s p e c t i v e l y c a l c i u m and phosphorus. A d s o r p t i o n of Ca and P i o n s at the s u r f a c e of the a p a t i t e l a t t i c e s h o u l d not i n f l u e n c e i t s s o l u b i l i t y , s i n c e these i o n s are h e l d i n the same way as the l a t t i c e i o n s themselves. T h i s i s , of c o u r s e , dependent on the c o n d i t i o n t h a t complete e q u i l i b r i u m e x i s t between a l l p a r t s of the c r y s t a l and the s u r r o u n d i n g s o l u t i o n . Isomorphous s u b s t i t u t i o n of t h i s k i n d i m p l i e s the f o r m a t i o n of a s o l i d s o l u t i o n . Newman and co-workers (58) have c i t e d the apparent absence of any d e f i n i t e s o l u b i l i t y product f o r h y d r o x y a p a t i t e , as evidence f o r i t s e x i s t e n c e as a s o l i d - 8 -s o l u t i o n . I n a system, i n which o n l y c a l c i u m , phosphate, h y d r o x y l , and hydrogen i o n s were p r e s e n t , d i v e r g e n c i e s from i d e a l r a t i o s c o u l d o n l y be accounted f o r on the b a s i s o f s u b s t i t u t i o n i n v o l v i n g these f o u r c o n s t i t u e n t s . A lthough C l a r k (12) demonstrated t h a t h y d r o x y a p a t i t e p o s s e s s e d a co n s t a n t s o l u b i l i t y product over a wide pH range, no a n a l y s e s were made of the s o l i d phase t o determine i f the a p a t i t e p r e p a r a t i o n s s t u d i e d were of i d e a l c o m p o s i t i o n . A major p a r t of t h i s study was made to determine the composi-t i o n o f a p a t i t e p r e p a r e d under a wide range of c o n d i t i o n s and t o r e l a t e t h i s t o s o l u b i l i t y . I I METHODS FOR THE SOLUBILITY STUDIES (1) The " P r e p a r a t i o n of H y d r o x y a p a t i t e I t was found p r e v i o u s l y (12), t h a t h y d r o x y a p a t i t e p r e p a r e d i n the presence of CO2 d i d not have r e p r o d u c i b l e s o l u b i l i t i e s , even when pr e p a r e d at r e l a t i v e l y h i g h tempera-t u r e s f o r l o n g p e r i o d s . Thus, CO^ f r e e was passed through the r e a c t i o n f l a s k s d u r i n g s y n t h e s i s , which, b e s i d e s e x c l u d i n g CO2, s e r v e d to mix the s o l u t i o n s . The d e s i r e d range of f i n a l pH v a l u e s was o b t a i n e d by adding v a r y i n g amounts of H^PO^ to a c o n s t a n t volume of C a ^ H ^ . In t h i s way, the f i n a l pH c o u l d be c l o s e l y predetermined. Although r a p i d m ixing of H^PO^ and Ca(0H)2 gave e x c e l l e n t r e s u l t s i f the f i n a l pH was below pH 6.3, i t was n e c e s s a r y to combine the r e a c t a n t s s l o w l y to o b t a i n u n i f o r m s o l u b i l i t i e s above t h i s pH. Reagents: a) H^PO^ s t o c k s o l u t i o n 1.0 M. b) H 3 P 0 4 0.1 M. c ) S a t u r a t e d Ca(0H)2« An excess of reagent grade Ca(0H)2 was added to b o i l e d d i s t i l l e d water. CO2 f r e e N2 was passed through the s u s p e n s i o n f o r 24 hours, t o mix and exclude CO2. - 9 -- 10 -T h i s mixture was then s t o r e d , f r e e of C0 2, u n t i l the supernatant was c l e a r . Procedure A:pH 5.0 t o 6.0 To a r e a c t i o n f l a s k , c o n t a i n i n g 200 ml. of s a t u r a t e d C a ( 0 H ) 2 , 22 to 35 ml. of 0.1M H 3P0 4, and enough d i s t i l l e d water were added t o make the t o t a l volume 450 ml. The f l a s k s were p l a c e d i n a con s t a n t temperature b a t h of d e s i r e d temperature and C 0 2 f r e e N 2 was passed through them. B:pH 6.0 to 8.0 A r e a c t i o n f l a s k c o n t a i n i n g 50 ml. of d i s t i l l e d water was p l a c e d i n the o i l bath, and N Q was bubbled through the system. Then by u s i n g c a p i l l a r y tubes, 200 ml. of s a t u r a t e d C a ( 0 H ) 2 , and from 17 to 25 ml. of 0.1M H 3 P 0 4 d i l u t e d to 200 ml. were added t o the f l a s k s i m u l t a n e o u s l y at a r a t e from 0.5 to 1.0 ml. per minute. The apparatus used f o r slow mixing i s shown i n Appendix 7, F i g u r e 6. From i p r e l i m i n a r y experiments, i t was found t h a t more r e p r o d u c i b l e r e s u l t s were o b t a i n e d by adding the C a ( 0 H ) 2 at a r a t e s l i g h t l y f a s t e r t han t h a t of H 3P0 4. . The ..reaction temperatures used were 40, 60 and 90°.C. The r e a c t i o n p e r i o d s were e i t h e r 24 or 96 hours. A l l samples s y n t h e s i z e d at 40°C and some s y n t h e s i z e d at 60°C were r e a c t e d f o r 24 hours. A l l samples s y n t h e s i z e d at 90°C and some syn-t h e s i z e d at 60°C had 96 hour r e a c t i o n p e r i o d s . The f l a s k s , - 11 -h e l d i n an o i l b a t h d u r i n g the r e a c t i o n p e r i o d were su b s e q u e n t l y removed, and p l a c e d i n a water b a t h at 25 - .02°C f o r 96 hours to a l l o w them to r e a c h e q u i l i b r i u m at t h i s temperature. A f t e r e q u i l i b r a t i o n at 25°C, the pH of the sample was measured u s i n g a p o r t i o n of the suspen-s i o n . The remainder of the s u s p e n s i o n was f i l t e r e d through a f i n e s i n t e r e d g l a s s f u n n e l . A drop of 1.0 N HCI was added to the f i l t r a t e t o prevent p r e c i p i t a t i o n , and i t was a n a l y s e d f o r c a l c i u m and phosphorus as d e s c r i b e d below. The s o l i d phase r e t a i n e d on the f i l t e r was washed t w i c e w i t h e t h a n o l and d r i e d at 110°C. For each r u n f o u r p r e p a r a t i o n s of a p a t i t e were made. Of t h e s e , one sample was used t o de-termine mole r a t i o s , two were used f o r d i s s o l u t i o n e x p e r i -ments and one was used f o r X-ray d i f f r a c t i o n a n a l y s i s . (2) D i s s o l u t i o n Experiments C l a r k (12) found t h a t the s o l u b i l i t i e s o f hydroxy-a p a t i t e p r e p a r e d at 90°C f o r 120 and 4 hours were s i m i l a r . He concluded, t h e r e f o r e , t h a t h e a t i n g at 90°C f o r 96 hours would be s u f f i c i e n t to o b t a i n s t a b l e c o n d i t i o n s i n a p a t i t e systems. At lower temperatures of s y n t h e s i s , 40 and 60°C i t was found t h a t the s o l u b i l i t i e s of the c a l c i u m phosphates were g r e a t e r than when the s y n t h e s i s was performed at 90°C. In t h i s study p r e c i p i t a t e s s y n t h e s i z e d at 40 and 60°C appeared t o be more s o l u b l e than those s y n t h e s i z e d at 90°C. S i n c e s y n t h e s i s at 40 and 60°C might have ind u c e d a s t a t e of simple s u p e r s a t u r a t i o n i n these c a l c i u m phosphate - 12 -systems, the s o l i d phase was r e d i s p e r s e d i n water and al l o w e d t o r e a c h e q u i l i b r i u m . S o l u b i l i t i e s were determined a f t e r d i s p e r s i o n and these s o l u b i l i t y v a l u e s were compared t o the s o l u b i l i t y v a l u e s f o r the p r e c i p i t a t e d systems. Procedure: For the d i s s o l u t i o n s t u d i e s the p r e c i p i t a t e d c a l c i u m phosphate was p l a c e d i n a r e a c t i o n f l a s k w i t h 450 ml. of water. The temperature was main-t a i n e d at 25°C, and CO2 f r e e was passed through the system. A f t e r a g i t a t i o n f o r 96 hours the pH, and the c a l c i u m and phosphorus c o n c e n t r a t i o n s were determined i n the manner d e s c r i b e d l a t e r . (3) A n a l y t i c a l Procedures a) X-ray A n a l y s i s To be c e r t a i n t h a t the s y n t h e s i z e d p r e c i p i t a t e s were h y d r o x y a p a t i t e , and to t e s t t h e i r c r y s t a l l i n i t y , X-ray d i f f r a c t i o n a n a l y s e s were made. The X-ray u n i t used was a N o r e l c o X-ray d i f f r a c t o m e t e r equipped w i t h G e i g e r counter and a Brown r e c o r d e r . The r a d i a t i o n was Cu K CL . A tube p o t e n t i a l and c u r r e n t of 40 K i l o v o l t s and 20 m i l l i a m p e r e s were used. The f i n e l y ground powder was scanned at a r a t e of 1.0° of 2 9 per minute. The samples were scanned o n l y from 24 to 56 degrees, s i n c e s c a n n i n g over the f u l l range showed t h a t the s i g n i f i c a n t l i n e s o c c u r r e d between these a n g l e s . - 13 -Procedure: The d r i e d s o l i d phase was ground by means of an agate p e s t l e and mortar. T h i s f i n e l y ground sample was then d e p o s i t e d over an a r e a of one square i n c h on g l a s s s l i d e s and p l a c e d i n the p a t h of the X-ray beam. b) The Measurement of pH A Cambridge bench model pH meter c a l i b r a t e d i n 0.02 pH u n i t was used f o r the measurement of pH. The Cambridge Ag. Ag-Cl type g l a s s e l e c t r o d e (No. 42518) was used. In order t o prevent leakage of KC1, however, i t was n e c e s s a r y t o use the Leeds and Northrup ( S t d . 1199-31) cal o m e l e l e c t r o d e i n p l a c e of the calomel e l e c t r o d e s u p p l i e d w i t h the i n s t r u m e n t . A s p e c i a l beaker w i t h two s i d e arms was c o n s t r u c t e d f o r pH measurements (see Appendix 7, F i g u r e 7 ) . With t h i s beaker C 0 2 - f r e e N^ c o u l d be passed through or over the s o l u t i o n s t o prevent c o n t a m i n a t i o n w i t h C0 2-B u f f e r s o l u t i o n s were pr e p a r e d from N a t i o n a l Bureau of Standards b u f f e r s a l t s under CO^- f r e e c o n d i t i o n s . These s o l u t i o n s and the h y d r o x y a p a t i t e systems were p l a c e d i n a water b a t h at 25 - o.02°C to o b t a i n a u n i f o r m temperature f o r pH measurements. Due t o the s e n s i t i v i t y of the pH meter, i t was n e c e s s a r y t o s t a n d a r d i z e a g a i n s t the b u f f e r s o l u t i o n f r e q u e n t l y w h i l e measuring pH. In the s o l u b i l i t y s t u d i e s , i t was found t h a t pH d e t e r m i n a t i o n was the most c r i t i c a l measurement. For example, the H + i o n a c t i v i t y appeared as the s i x t h power of a c u b i c e x p r e s s i o n i n the c a l c u l a t i o n of PO.= i o n a c t i v i t y , so t h a t - 14 -e r r o r s i n pH measurement were m a g n i f i e d s e r i o u s l y i n the subsequent c a l c u l a t i o n s . B u f f e r s o l u t i o n s : Potassium Hydrogenphthalate 0.05M. The s a l t was d r i e d f o r one hour at 105°C, and 10.130 g. were weighed, and d i s s o l v e d i n 1.0 1. of d i s t i l l e d water. At 20°C and 25°C the pH v a l u e s o f t h i s s o l u t i o n were 4.00 and 4.01 r e s p e c t i v e l y . K H 2P0 4 o.25M. and Na 2HP0 4 0.025M. These s a l t s were d r i e d at 130°C f o r 2 hours. Then, 3.402 g. of KH 2P0 4 and 3.549 g. of Na 2HP0 4 were d i s s o l v e d i n 1.0 1. of d i s t i l l e d water. At 20°C and 25°C the pH v a l u e s of t h i s s o l u t i o n were 6.88 and 6.86 r e s p e c t i v e l y . Procedure: The l i n e a r i t y of the pH meter was t e s t e d by s t a n d a r d i z i n g i t a g a i n s t the two b u f f e r s o l u t i o n s , through which C 0 2 f r e e N 2 was b e i n g bubbled. Care was taken to make c e r t a i n t h a t the temperature compensator of the pH meter was set to the temperature of the s o l u t i o n s . When the meter was s t a n d a r d i z e d , some sample was siphoned from a r e -a c t i o n f l a s k , N 2 was passed through i t , and the pH measured. Measurements were r e p e a t e d u n t i l the pH r e a d i n g s were c o n s t a n t w i t h i n - 0.02 pH u n i t . c) The D e t e r m i n a t i o n of C a l c i u m by Versenate The method f o r the d e t e r m i n a t i o n of c a l c i u m w i t h v e r s e n a t e g i v e n i n S a l i n e and A l k a l i S o i l s (18) was f o l l o w e d . S i n c e more d i l u t e v e r s e n a t e s o l u t i o n s were used, - 15 -the amount of magnesium added was adjusted to give sharper end-points with the eriochrome black T. indicator. The i n -dicator gave good end-point colors i f dissolved in ethanol without hydroxylamine hydrochloride, even after the elapse of one month. It was found, that more uniform end-points were obtained i f the indicator was f i l t e r e d to eliminate suspended particles. The optimum amount of buffer was —2 found to be 3.0 mis. for calcium concentrations from 1x10 —6 to 1x 10 molar. No interference from phosphorus was found up to 1x 10"^ M. Phosphorus. Reagents: a) Ethylenediaminetetraacetate (versenate) solution 0.01 M: four (4.0) g. of versenate were dissolved in 1.0 1. of water. b) Versenate solution 0.005 M. the 0.01 M. versenate solution was diluted 1:2 and 10 to 18 drops of 0.1 M. MgCl 2 were added to produce a satisfactory end-point with the indicator. c) NH4C1 - NH40H buffer solution: 67.5 g. of NH4C1 were dissolved in 570 ml. of concentrated NHjOH and made up to one 1. d) Eriochrome Black T. indicator: approximately 0.5 g. of the indicator was dissolved in 100 ml. of ethanol and f i l t e r e d . - 16 -e) Standard C a l c i u m s o l u t i o n 0.1M: CaC0 3 was d r i e d at 110°C f o r 2 hours; 10.009 g. of the CaC0 3 was d i s s o l v e d i n j u s t enough HCI to e f f e c t s o l u t i o n . A f t e r the r e a c -t i o n was complete, the s o l u t i o n was d i l u t e d t o 1.0 1. A few drops of CHC1 3 were added to the c a l c i u m standards t o prevent b a c t e r i a l growth. f ) Standard C a l c i u m s o l u t i o n 0.005M: the 0.1M CaC0 3 s o l u t i o n was d i l u t e d 1:20. Procedure: To s t a n d a r d i z e the v e r s e n a t e s o l u t i o n 5.0 ml. of the d i l u t e CaC0 3 s t a n d a r d s o l u t i o n were d i l u t e d t o 25 ml., 3.0 ml. of b u f f e r s o l u t i o n , and 4 to 5 drops of i n d i c a t o r were added. T h i s s o l u t i o n was t i t r a t e d w i t h 0.005 M v e r s e n a t e s o l u t i o n u n t i l a b l u e - g r e e n end-point was o b t a i n e d . The c a l c i u m content of samples was determined i n the same way, however, best r e s u l t s were o b t a i n e d when the sample a l i q u o t e c o n t a i n e d l e s s t han 0.01 e q u i v a l e n t s of c a l c i u m . d) The D e t e r m i n a t i o n of Phosphorus by Aqueous Molybdenum Blue Method For the d e t e r m i n a t i o n of phosphorus a m o d i f i c a t i o n of the Dickman-Bray aqueous molybdenum blu e method (19) was used. T h i s procedure, a l t h o u g h r a p i d and simple was chosen because of i t s s e n s i t i v i t y t o low phosphorus c o n c e n t r a t i o n s . - 17 -The per cent t r a n s m i t t a n c e was measured on a Bausch and Lomb photo e l e c t r i c c o l o r i m e t e r at 660 m i l l m i c r o n s u s i n g a one c e n t i m e t e r c e l l . Reagents: a) Ammonium Molybdate s o l u t i o n 8.4%: 42.0 g. of ( N H 4 ) g Mo 0 2 4 4H 20 were d i s s o l v e d i n 500 rml. of water. b) HCI 9.8 N: 816 ml. of c o n c e n t r a t e d HCI were d i l u t e d t o 1:1. c) Molybdate HCI M i x t u r e : 50 ml. of 8.4% ammonium molybdate were mixed w i t h 100 ml. of 9.8 HCI. T h i s s o l u t i o n was pre p a r e d monthly. d) C o n c e n t r a t e d S n C l 2 S o l u t i o n s : 10 g. of S n C l 2 - 2H 20 was d i s s o l v e d i n c o n c e n t r a t e d HCI and s t o r e d i n a b l a c k b o t t l e . e) D i l u t e S n C l 2 S o l u t i o n : 1 ml. of c o n c e n t r a t e d S n C l 2 s o l u t i o n was added t o 19 ml. of IN HCI. T h i s s o l u t i o n was pr e p a r e d d a i l y . f ) Phosphorus Standard S o l u t i o n 0.01M: 0.6806 g. of reagent grade KH 2P0 4 d r i e d at 110°C was d i s s o l v e d i n 500 mis. of H o0. - 18 -Procedure: A s e r i e s of d i l u t e s t a n d a r d phosphorus —6 —6 s o l u t i o n s , v a r y i n g i n c o n c e n t r a t i o n from 1x10 to 3 0 x 1 0 M were p r e p a r e d from the 0.01 M s t a n d a r d s o l u t i o n . To 25 ml. a l i q u o t s of the d i l u t e d s t a n d a r d s o l u t i o n s i n 50 ml. E r l e n -meyer f l a s k s , 3.0 ml. of molybdate - HCI s o l u t i o n were added. J u s t b e f o r e measuring the t r a n s m i t t a n c y of the sample, 2.0 drops of the d i l u t e S n C l 2 were added. The s o l u t i o n was mixed, t r a n s f e r r e d t o the sample c e l l , and the c o l o r i n t e n -s i t y measured i n the c o l o r i m e t e r . S i n c e the b l u e c o l o r , developed on the a d d i t i o n of S n C l 2 fades on s t a n d i n g , i t was e s s e n t i a l t h a t a l l samples be measured at some predetermined time — say 20 seconds — a f t e r adding the SnClg. I t was o f t e n n e c e s s a r y t o make d i l u t i o n s so t h a t sample a l i q u o t s —6 c o n t a i n e d l e s s than 5 0 x 1 0 M.P. (4) C a l c u l a t i o n s a) C a l c u l a t i o n of A c t i v i t y C o e f f i c i e n t s I t was n e c e s s a r y t o c a l c u l a t e i o n a c t i v i t y c o e f f i c i e n t s so t h a t c o r r e c t e d c o n c e n t r a t i o n s , or a c t i v i t i e s c o u l d be used i n the c a l c u l a t i o n s of the v a l u e s pH-1/2 pCa and p H 2 P 0 4 + 1/2 pCa (2, 11, 23, 65). Since a n a l y t i c a l de-t e r m i n a t i o n s o f c a l c i u m and phosphorus measure the t o t a l c o n c e n t r a t i o n s of these i o n s i n s o l u t i o n , i t was n e c e s s a r y t o c a l c u l a t e i o n a c t i v i t y by making use of a p p r o p r i a t e a c t i v i t y c o e f f i c i e n t s and d i s s o c i a t i o n c o n s t a n t s . I n d i v i -d u a l i o n a c t i v i t y c o e f f i c i e n t s were c a l c u l a t e d from the Debye-Huckel e q u a t i o n (39). - 19 -A Z 2 V/T + Ba. / — where: ^ = the a c t i v i t y c o e f f i c i e n t of the i o n . Z = the v a l e n c e of the i o n . LI = the i o n i c s t r e n g t h o f the s o l u t i o n . A and B = c o n s t a n t s f o r a g i v e n s o l v e n t and temperature. In water at 25°C, A has the v a l u e 0.5085 and B, 0.3281 x 1 0 8 . a^ = the "average e f f e c t i v e diameter" of the h y d r a t e d i o n s i n aqueous s o l u t i o n . For C a + + a. = 6.0 x 1 0 ~ 8 x ' f o r H oP0 ~ and HP0. = a. = 4.0 x 1 0 ~ 8 Angstrom eL 4 4 x u n i t . To determine the i o n i c s t r e n g t h the e q u a t i o n o f Lewis and R a n d a l l was used. I t was assumed, t h a t c a l c i u m r e p r e s e n t e d the predominant c a t i o n i n s o l u t i o n and t h a t an e q u i v a l e n t amount of monovalent a n i o n was p r e s e n t . Thus the i o n i c s t r e n g t h would be [X = 0.5 E C ^ 2 , where: C. = the i o n i c c o n c e n t r a t i o n i n moles x per l i t r e of s o l u t i o n and Z. = the v a l e n c e of the i o n . x b) C a l c u l a t i o n of Hv>P04 A c t i v i t y The d e t e r m i n a t i o n of t o t a l phosphorous measured the t o t a l c o n c e n t r a t i o n of a l l the i o n s p e c i e s p r e s e n t i n - 20 -the o r i g i n a l sample. For example, i n any aqueous s o l u t i o n of phosphorous depending on pH, the s p e c i e s P0 4~, HPO^ , H 2 P ^ 4 ~ ' a n ( i H3 P (^4 w o u l d a 1 1 b e p r e s e n t t o a g r e a t e r or l e s s e r degree, and each would be i n c l u d e d i n the d e t e r m i n a t i o n of t o t a l phosphorous. The determined phosphate c o n c e n t r a t i o n , however, must be s u b - d i v i d e d i n t o i o n s p e c i e s b e f o r e i t has p h y s i c o - c h e m i c a l s i g n i f i c a n c e . T h i s was done i n the f o l l o w i n g manner: [ t o t a l P] = [ H 3 P 0 4 ] + [ H 2P0 4"] + [ H P 0 4 _ ] + [ P 0 4 = ] , ( l ) (H PO.) (H PO -) (HPO = ) (PO = ) or [ t o t a l P] = 3 4 + * * + ——±— + I (2) where the b r a c k e t s r e p r e s e n t c o n c e n t r a t i o n , the parentheses i o n a c t i v i t i e s and j- the a c t i v i t y c o e f f i c i e n t o f the r e s p e c -t i v e i o n s p e c i e s . I f K^, K^, and are the f i r s t , second, and t h i r d thermodynamic d i s s o c i a t i o n c o n s t a n t s of p h o s p h o r i c a c i d , then, ( H + ) ( H p P 0 ") ( H + ) ( H 2 P 0 ") K l = ( H 3 p g 4 ) ' ^ d = K f - 1 - ' ( 3> (H +)(HP0 = ) = K (H PO ") K p = , and (HPO.) = ^ 2 4 , (4) 2 ( H 2 P 0 4 _ ) 4 (H +) ( H + ) ( P 0 / ) . K„(HP0, = ) K K (H PO ") K- = | v - , and (PO ") = 3 4 ' = 2 3 + 2 ( 5 ) 3 (HP0 4=> 4 (H +) ( H + ) 2 S u b s t i t u t i n g e q u a t i o n s ( 3 ) , ( 4 ) , and (5) i n t o e q u a t i o n (2) g i v e s , r + + , D 1 ( H + ) ( H 2 P 0 4 " ) ( H 2 P 0 4 - ) K 2 ( H 2 P 0 4 - ) K 2 K 3 ( H 2 P 0 4 - ) [ t o t a l P] = + x + — r - r — + K i - 21 -S o l v i n g f o r ( H 2 P 0 4 ) i t i s found that. (H pPO ") = + [ t o t a l P ]  (H_) 1 + K 2 + K 2 K 5 In e q u a t i o n (6) t o t a l P may be determined a n a l y t i c a l l y , ( H +) determined by pH measurement, and the a c t i v i t y c o e f f i c i e n t s c a l c u l a t e d from the Debye-Huckel e q u a t i o n . The d i s s o c i a t i o n c o n s t a n t s f o r p h o s p h o r i c a c i d were taken from F a r r (1950) and have the v a l u e s ^ = 7.51 x 1 0 ~ 3 K 2 = 6.88 x 1 0 ~ 8 and K 3 = 4.73 x 1 0 " 1 3 S i n c e the presence of the v a r i o u s i o n s p e c i e s was dependent on pH, s e v e r a l s i m p l i f i c a t i o n s of e q u a t i o n (6) were p o s s i b l e . For example the f o l l o w i n g s i m p l i f i c a t i o n s may be used i n the denominator of the e q u a t i o n , a) below pH 4.0 use (E±)/K1 + b) from pH 4.0 to 5.2 use c) from pH 5.2 to 10 use + K 2/ ( H + y » = , d) above pH 10 use + K^/(R+^J* = . - 22 -c) C a l c u l a t i o n o f P 0 4 A c t i v i t y The P 0 4 i o n a c t i v i t y may be determined i n much the same manner. I t was shown t h a t Ctotal P] . (H 3P0 4) + W + + <<> , (2) (H +)(PO ") _ (H+)(PO ") but 1— = K ; (HPO ) = ^ — , (7) (HP0 4 ) ^ 4 K 3 ( H + ) ( H P 0 4 = ) _ ( H + ) ( H P 0 / ) ( H + ) 2 ( P 0 / ) ^ — = K 2; (H 2P0 ) = ^ — = — , (8) (H 2 P 0 4 " ) K 2 K 2 K 3 ( H + ) ( H P O " ) ( H + ) ( H J P O ~ ) ( H * ) 2 ( H P O ") ^ 4 ir . /TT nr\ \ 4 4 = K l 5 ( H 3 P 0 4 )( H 3 P 0 4 ) K x K l K 2 < H + ) 5 ( P Q 4 E ) . (9) K 1 K 2 K 3 S u b s t i t u t i n g e q u a t i o n ( 7 ) , ( 8 ) , and (9) i n t o e q u a t i o n ( 2 ) , the r e s u l t i s / = f ( H + ) 3 ( H + ) 2 , „ + s 1 T [ t o t a l P] = (PO ~ K + + !JL4 + — _ >or, [ t o t a l l P ] \ K 1 K 2 K 3 / ~ K 2 K 3 K 3 / J ) ( P 0 4 > , i KiK2K 3jv\r + 3 — = = At h i g h pH (H ) m a y b e n e g l e c t e d , so t h a t ( P 0 A ) [ t o t a l P] ) K 2 K 3 / / (If - 23 -At low pH K^K^K^ is negligible and K l K 2 K 3 v / ^ (PO. ) = [ t o t a l P] 4 ' ( H + ) 3 / + ( H + ) 2 K I / " + ( H +) KiW The r e l a t i o n s h i p between (H 2 P 0 4 ~ ) and (P0 4 ~) may be c a l c u l a t e d from the d i s s o c i a t i o n c o n s t a n t s f o r pho s p h o r i c a c i d , (H + ) ( H P 0 4 = ) ( H 2 P 0 4 " ) * ( H + ) ( P O " ) r 4 — = K (HPO ") 3 K (H PO ) and (HPO. -) ± — — i — - — 4 (H +) ( H + ) 2 ( P 0 " ) T h e r e f o r e , - — = K0K_. ( H 2 P 0 4 ) Z 3 T a k i n g n e g a t i v e l o g a r i t h m s of the l a s t e q u a t i o n the r e s u l t i s p P 0 4 pH 2 P 0 4 = p K 2 K 3 - 2pH and p P 0 4 = p K 2 K 3 - 2pH + p H 2 P 0 4 -- 24 -d) D e t e r m i n a t i o n of R e g r e s s i o n L i n e s . R e g r e s s i o n l i n e s were p l o t t e d from the determined s o l u b i l i t y d ata by use of the method of l e a s t squares to c a l c u l a t e "m" and "b", n lxy - Ix l y ^ 1 „ , u , . where m = — = = - 7 5 — j — — = the s l o p e of the l i n e H x ^ £y ~ H x y and b = =—f 7 = — = the y i n t e r c e p t . The symbols x and y r e p r e s e n t the r e s p e c t i v e v a l u e s of pH - l/2pCa and p H 2 P 0 4 + l/2pCa f o r each s o l u b i l i t y p o i n t , The r e g r e s s i o n l i n e s were l o c a t e d by s u b s t i t u t i n g the d e t e r -mined v a l u e s f o r m and b i n t o e q u a t i o n y = mx + b, and by s o l v i n g f o r y where x was any convenient v a l u e of pH-l/2pCa. (5) The D e t e r m i n a t i o n of Mole R a t i o s T h e o r e t i c a l l y , pure h y d r o x y a p a t i t e would have a c a l c i u m t o phosphorus r a t i o of 1.66. S e v e r a l workers (1, 3, 14, 25, 27, 29, 38, 64) have r e p o r t e d , however, t h a t b a s i c c a l c i u m phosphates h a v i n g a p a t i t e c r y s t a l s t r u c t u r e s d i d not have c o n s t a n t Ca:P r a t i o s . In o r d e r t o determine i f s y n t h e s i s over a wide range of c o n d i t i o n s would a f f e c t the Ca:P r a t i o s of h y d r o x y a p a t i t e , and t o determine i f d i f f e r e n c e s i n c o m p o s i t i o n c o u l d be c o r r e l a t e d w i t h s o l u b i l i t y , the mole r a t i o and s o l u b i l i t y of s e v e r a l p r e p a r a t i o n s were compared. - 25 -S i n c e i n c o n s i s t e n t mole r a t i o s c o u l d be caused by e i t h e r adsorbed c a l c i u m or phosphorus, d i f f e r e n t s o l u t i o n s were used i n attempts to. desorb these i o n s from the s o l i d -3 -4 phase. The s o l u t i o n s used were 1 x 10 M C a C ^ l 1 x 10 M H 3P0 4, and 5 x 1 0 ~ 3 M KC1. Where C a C l 2 was added, i t was p o s s i b l e t h a t the compounds w i t h mole r a t i o s below 1.66 c o u l d be a l t e r e d by the a d s o r p t i o n of c a l c i u m i o n s from s o l u t i o n . C o n v e r s e l y , w i t h H^PO^, the compounds w i t h mole r a t i o s above 1.66 c o u l d be a l t e r e d by the a d s o r p t i o n of phosphate i o n s from s o l u t i o n . Where KC1 was added, the potassium i o n s c o u l d desorb c a l c i u m and the c h l o r i d e i o n s c o u l d desorb phosphorus from the c r y s t a l . Thus, i f s o r p t i o n was a f a c t o r i n d e t e r m i n i n g the Ca:P r a t i o s of h y d r o x y a p a t i t e , the e x p e r i m e n t a l c o n d i t i o n s a l l o w e d f o r a l t e r a t i o n s i n com-p o s i t i o n through the p r o c e s s of e i t h e r a d s o r p t i o n or desorp-t i o n . To o b t a i n h y d r o x y a p a t i t e w i t h Ca:P r a t i o s above 1.66, i t was n e c e s s a r y to s y n t h e s i z e the h y d r o x y a p a t i t e above pH 9.0, i n which case i t was not p o s s i b l e t o o b t a i n r e p r o d u c i b l e s o l u b i l i t y v a l u e s . Procedure: The s o l i d phase c a l c i u m phosphate was weighed, d i s s o l v e d i n c o n c e n t r a t e d HCI, and made up to a volume of 250 mis. The c a l c i u m and phosphorus content of t h i s s o l u t i o n was determined by the methods d e s c r i b e d p r e v i o u s l y . - 26 -To determine the e f f e c t of C a C l 2 , H 3P0 4, and KC1 s o l u t i o n s on the s o l i d phase, the h y d r o x y a p a t i t e sample was d i v i d e d i n t o two p o r t i o n s . With one p o r t i o n the mole r a t i o was determined as above. To the other p o r t i o n , i n a r e a c t i o n f l a s k , 450 ml. of one of the above s o l u t i o n s was added. N i t r o g e n gas was bubbled through t h i s s o l u t i o n f o r 96 hours and the temperature was m a i n t a i n e d at 25°C. A f t e r f i l t e r i n g , the s o l i d phase c o l l e c t e d on a s i n t e r e d g l a s s f u n n e l was washed twice w i t h e t h a n o l and d r i e d a t 110°C. The Ca:P r a t i o s of t h i s c a l c i u m phosphate was then determined i n the u s u a l way. I l l RESULTS AND DISCUSSION (1) R e s u l t s of the S o l u b i l i t y S t u d i e s In appendices 1 to 4 are shown v a l u e s f o r the s o l u b i l i t y of p r e c i p i t a t e d h y d r o x y a p a t i t e measured a f t e r r e a c t i o n at 40 and 60°C f o r 24 hours, and 60 and 90°C f o r 96 hours. S o l u b i l i t y diagrams f o r the f o u r r e a c t i o n con-d i t i o n s are p r e s e n t e d i n appendix 7 ( F i g u r e s 1 t o 4 ) , i n which the v a l u e f o r the o r d i n a t e s pH-l/2pCa and p H 2 P 0 4 + l/2pCa have been p l o t t e d . The r e g r e s s i o n l i n e s determined f o r each s e t of s o l u b i l i t y d ata are a l s o shown i n these s o l u b i l i t y diagrams. In appendices 1A to 4A are shown the s o l u b i l i t y v a l u e s f o r h y d r o x y a p a t i t e samples which were s y n t h e s i z e d at 40 and 60°C f o r 24 hours, and 60 and 90°C f o r 96 hours, and then r e d i s p e r s e d i n water. The v a l u e s f o r pH - l/2pCa, and pRv,P04 + l/2pCa of these d a t a have been p l o t t e d i n the s o l u b i l i t y diagrams i n appendix 7 ( F i g u r e s 1 to 4 ) . Tabl e I summarizes the data shown i n appendices 1 t o 4 and 1A to 4A and i n appendix 7 ( F i g u r e s 1 t o 4 ) . In t h i s t a b l e are i n c l u d e d the s l o p e of each r e g r e s s i o n l i n e , and the average pKsp v a l u e of h y d r o x y a p a t i t e measured a f t e r - 27 -- 28 -p r e c i p i t a t i o n and a f t e r d i s s o l u t i o n f o r each c o n d i t i o n o f s y n t h e s i s . TABLE I Slopes of the R e g r e s s i o n L i n e s f o r H y d r o x y a p a t i t e S o l u b i l i t y Measured a f t e r P r e c i p i t a t i o n at V a r i o u s Temperatures and R e a c t i o n P e r i o d s and A f t e r D i s s o l u t i o n S o l u b i l i t y l i n e Slope of l i n e Average pKsp Measured a f t e r P r e c i p i t a t i o n Average pKsp Measured a f t e r D i s s o l u t i o n T . V . A . ( T h e o r e t i c a l ) 2.33 111.185 40°C 24 hours 2.28 109.43 109.49 60°C 24 hours 2.36 112.90 113.04 60°C 96 hours 2.29 113.53 113.38 90°C 96 hours 2.28 113.60 113.40 From the comparison of the average pKsp v a l u e s measured a f t e r p r e c i p i t a t i o n , and a f t e r d i s s o l u t i o n , i t i s seen t h a t f o r a g i v e n temperature and r e a c t i o n p e r i o d , the s o l u b i l i t y measured a f t e r d i s s o l u t i o n d i d not change a p p r e c i a b l y from the s o l u b i l i t y measured a f t e r p r e c i p i t a t i o n . The s i m i l a r s o l u b i l i t i e s would i n d i c a t e t h a t s y n t h e s i s a t temperatures below 90°C d i d not b r i n g about c o n d i t i o n s of simple super-s a t u r a t i o n i n these b a s i c c a l c i u m phosphate systems, and some other f a c t o r must be i n v o l v e d . The s o l u b i l i t y v a l u e s i n T a b l e I show t h a t as the r e a c t i o n p e r i o d and temperature were i n c r e a s e d , the s o l u b i l i t y decreased. H y d r o x y a p a t i t e formed at 90°C was l e s s s o l u b l e than t h a t formed at 60°C, - 29 -and t h i s i n t u r n was l e s s s o l u b l e than t h a t formed at 40°C. Except at 90°C, h y d r o x y a p a t i t e became l e s s s o l u b l e the l o n g e r the r e a c t i o n p e r i o d and p r e c i p i t a t e s formed at 60°C f o r 96 hours were l e s s s o l u b l e than those r e a c t e d at t h i s temperature f o r a 24 hour p e r i o d . No t e s t s were made to compare the e f f e c t of r e a c t i o n p e r i o d on s o l u b i l i t y at 40°C. The e f f e c t of temperature and r e a c t i o n p e r i o d of s y n t h e s i s i s i l l u s t r a t e d i n appendix 7 ( F i g u r e 5 ) , i n which the e x p e r i -m e n t a l l y determined r e g r e s s i o n l i n e s are compared w i t h the s o l u b i l i t y l i n e taken from the d a t a of T.V.A. (67). The s o l u b i l i t y of compounds s y n t h e s i z e d at 40°C was g r e a t e r , and the s o l u b i l i t y of compounds s y n t h e s i z e d at 60 and 90°C was l e s s than the v a l u e g i v e n by T.V.A. I t i s seen, however, (Ta b l e I ) t h a t the s l o p e s of these l i n e s were remarkably s i m i l a r , and c l o s e t o the t h e o r e t i c a l v a l u e of 2.33. In appendix 7 ( F i g u r e s 8 t o 13) are p r e s e n t a t i o n s of X-ray d i f f r a c t i o n p a t t e r n s of h y d r o x y a p a t i t e p r e p a r e d w i t h v a r y i n g temperature and r e a c t i o n p e r i o d s . These X-ray d i f f r a c t i o n p a t t e r n s showed t h a t the c r y s t a l l i n i t y of hydroxy-a p a t i t e i n c r e a s e d w i t h i n c r e a s e d r e a c t i o n p e r i o d and temperature u n t i l the c o n d i t i o n s at 90°C f o r 96 hours were ac h i e v e d . A p p a r e n t l y , i n c r e a s i n g the r e a c t i o n p e r i o d and temperature s e r v e d to age the p r e c i p i t a t e s , r e s u l t i n g i n b e t t e r d e f i n e d c r y s t a l s . The o r d e r of c r y s t a l l i n i t y f o r the s y n t h e s i s of h y d r o x y a p a t i t e w i t h v a r y i n g temperature and r e a c t i o n p e r i o d s was as f o l l o w s : 90°C f o r 96 hours^> 60°C f o r 96 h o u r s 6 0 ° C f o r 24 hours ^> 40°C f o r 96 hours >^ 40°C - 30 -f o r 24 hours. As has been s t a t e d p r e v i o u s l y the order of s o l u b i l i t y i s the r e v e r s e of t h i s . I t i s e v i d e n t t h e r e f o r e , t h a t t h e r e i s a r e l a t i o n s h i p between s o l u b i l i t y and c r y s t a l -U n i t y . T h i s r e l a t i o n s h i p i s t h a t below 90°C, more s o l u b l e , and l e s s c r y s t a l l i n e h y d r o x y a p a t i t e i s formed the s h o r t e r the r e a c t i o n p e r i o d and the lower the temperature of s y n t h e s i s . . I t was found, at the 5% c o n f i d e n c e l e v e l , t h a t the s t a n d a r d e r r o r s of the mean de p a r t u r e s from r e g r e s s i o n f o r the 90°C and the 60°C (24 hour) r e g r e s s i o n l i n e s were -0.048 and -- 0.039 r e s p e c t i v e l y . Thus i t i s seen t h a t t h e r e was not a s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e between the s o l u b i l i t y v a l u e s making up these two r e g r e s s i o n l i n e s . The l a c k of a s i g n i f i c a n t d i f f e r e n c e between the two groups of s o l u b i l i t y v a l u e s might have been due t o the v a r i a b i l i t y of the s o l u b i l i t y v a l u e s o b t a i n e d a f t e r r e a c t i o n at 60*0 f o r 24 hours. P o s s i b l y s t a t i s t i c a l l y s i g n i f i c a n t s o l u b i l i t y d i f f e r e n c e s would be o b t a i n e d o n l y when the degree of c r y s t a l l i n i t y was l e s s than t h a t a c h i e v e d by s y n t h e s i s at 60° f o r 24 hours, or when the c o n d i t i o n s of s y n t h e s i s c o u l d be e x a c t l y r e p r o d u c e d t o e l i m i n a t e v a r i a b i l i t y . In o r d e r t o o b t a i n r e p r o d u c i b l e s o l u b i l i t i e s above pH 6.0, i t was n e c e s s a r y t o mix the C a ( 0 H ) 2 and E^PO^ v e r y s l o w l y . At 90°C w i t h slow mixing of a c i d and base, a more c r y s t a l l i n e p r e p a r a t i o n was formed than w i t h the r a p i d m i x i n g o f r e a g e n t s (see appendix 7, F i g u r e 13). These h i g h -l y c r y s t a l l i n e a p a t i t e s , formed at 90°C h a v i n g had the same s o l u b i l i t y as the l e s s c r y s t a l l i n e compounds formed at 90°C. - 31 -I t appears t h a t s y n t h e s i s at 90°C w i t h r a p i d mixing of Ca (OROg and H^PO^ p r o v i d e d the minimum degree of c r y s t a l l i n i t y r e q u i r e d t o produce h y d r o x y a p a t i t e of c o n s t a n t s o l u b i l i t y , and t h a t beyond t h i s p o i n t , s o l u b i l i t y was not a f f e c t e d by i n c r e a s i n g the c r y s t a l l i n i t y of the compounds. I t i s b e l i e v e d t h a t the r a t e of f o r m a t i o n of h y d r o x y a p a t i t e i s v e r y slow above pH 6.0. R a p i d m i x i n g of a c i d and base f r e q u e n t l y produced a s t a t e of simple super-s a t u r a t i o n i n the c a l c i u m phosphate systems, which was d i f f i c u l t t o overcome. T h e r e f o r e , slow m i x i n g was r e q u i r e d t o e l i m i n a t e the f o r m a t i o n of a metastable s t a t e and t h e r e -by o b t a i n r e p r o d u c i b l e s o l u b i l i t y v a l u e s . E x amination w i t h the e l e c t r o n microscope of h y d r o x y a p a t i t e samples formed at 40, 60, and 90°C showed t h a t c r y s t a l s i z e d i d not change w i t h i n c r e a s i n g tempera-t u r e s of s y n t h e s i s . The d i f f e r e n c e s i n c r y s t a l l i n i t y , d e t e c t e d by X-ray d i f f r a c t i o n a n a l y s i s might have been due to the f o r m a t i o n of a p a t i t e s v a r y i n g i n c r y s t a l l i n e p e r f e c -t i o n . P o s s i b l y , d i f f e r e n c e s i n c r y s t a l l i n i t y might be due to the i n c l u s i o n of amorphous substances which would decrease the apparent c r y s t a l l i n i t y . S ince the c r y s t a l s were too s m a l l f o r p e t r o g r a p h i c examination, amorphous phases c o u l d be p r e s e n t and remain undetected. The presence of such amorphous substances c o u l d be r e s p o n s i b l e f o r the d i f f e r e n c e s i n s o l u b i l i t y of h y d r o x y a p a t i t e p repared w i t h d i f f e r e n t r e a c t i o n p e r i o d s and temperature. I t appears t h a t w i t h a g i v e n method of s y n t h e s i s the amount of amorphous - 32 -m a t e r i a l remained r e l a t i v e l y c o n s t a n t w i t h the r e s u l t t h a t s o l u b i l i t i e s a l s o tended t o remain c o n s t a n t . The f a i l u r e t o o b t a i n r e p r o d u c i b l e c o n d i t i o n s f o r the f o r m a t i o n o f a u n i f o r m l y , or an ade q u a t e l y c r y s t a l l i n e s o l i d phase would appear t o be the reason t h a t most workers have been unable t o determine a d e f i n i t e pKsp v a l u e f o r h y d r o x y a p a t i t e . I t seems u n l i k e l y , however, t h a t the v a r i a b i l i t y i n pKsp v a l u e s found here c o u l d be a t t r i b u t e d t o the f a i l u r e t o o b t a i n e q u i l i b r i u m i n these b a s i c c a l c i u m phosphate systems. To some e x t e n t , the v a r i a b l e s o l u b i l i t y v a l u e s o b t a i n e d may have r e s u l t e d from a n a l y t i c a l e r r o r s . In these u n b u f f e r e d systems, e r r o r s i n pH, and e r r o r s i n c a l c i u m and phosphorus d e t e r m i n a t i o n s were p r o b a b l y s u f f i c i e n t l y g r e a t t o account f o r some of the d i s c r e p a n c i e s i n pKsp v a l u e s . Moreover, such a n a l y t i c a l e r r o r s were m a g n i f i e d g r e a t l y i n the c a l c u l a t i o n o f s o l u b i l i t y product where i o n a c t i v i t i e s were r a i s e d t o h i g h exponents. Even though the c a l c i u m and phosphorus c o n c e n t r a t i o n s o f the s o l u t i o n s v a r i e d from 1 x 10~ 3 to 1 x 10~ 6 M, and pH from 5.0 to 8.0, these ex-periments show t h a t h y d r o x y a p a t i t e does, indeed, have a d e f i n i t e s o l u b i l i t y p r o d u c t . - 33 -(2) R e s u l t s of the Mole R a t i o S t u d i e s The r e s u l t s of the mole r a t i o s t u d i e s are p r e s e n t e d i n appendix 5, and i t i s seen t h a t the mole r a t i o s o f h y d r o x y a p a t i t e were not co n s t a n t f o r any mode of s y n t h e s i s . No c o r r e l a t i o n of the mole r a t i o w i t h pH or s o l u b i l i t y c o u l d be found f o r any of the s y n t h e t i c hydroxy-a p a t i t e s . G e n e r a l l y , w i t h v e r y h i g h pH, the mole r a t i o s were g r e a t e r than the v a l u e 1.66, w h i l e at low and i n t e r -mediate pH v a l u e s the mole r a t i o s were l e s s than t h i s v a l u e . In the l i t e r a t u r e review, i t was shown t h a t a d s o r p t i o n , or isomorphous s u b s t i t u t i o n were used t o e x p l a i n the l a c k of constancy o f mole r a t i o s . I t appeared p o s s i b l e , then, t h a t h i g h mole r a t i o s were due t o the a d s o r p t i o n of c a l c i u m i o n s at h i g h pH, and low mole r a t i o s due t o the a d s o r p t i o n o f phosphate i o n s at low pH. From the r e s u l t s shown i n appendix 6, i t i s seen t h a t i t was not p o s s i b l e to change the mole r a t i o s of h y d r o x y a p a t i t e by treatment w i t h s o l u t i o n s of H^PO^, C a C l ^ or KC1. That i s , i t was not p o s s i b l e t o d e t e c t changes i n mole r a t i o r e s u l t i n g from a d s o r p t i o n , or d e s o r p t i o n o f i o n s at the c r y s t a l s u r f a c e s . T h i s i s not s u r p r i s i n g i n view o f the f a c t t h a t the t o t a l amount of a d s o r p t i o n must be s m a l l (57, 69) s i n c e i t i s l i m i t e d t o c r y s t a l s i z e . I t must be concluded, t h e r e f o r e , t h a t i f s o r p t i o n has any e f f e c t on d e t e r m i n i n g the compo-s i t i o n of h y d r o x y a p a t i t e , the e f f e c t must be s m a l l . - 34 Under the e x p e r i m e n t a l c o n d i t i o n s of s y n t h e s i s , c o n t a m i n a t i n g i o n s were prese n t i n such low c o n c e n t r a t i o n s , i t i s extremely d o u b t f u l t h a t they c o u l d have indu c e d i s o -morphous s u b s t i t u t i o n of the magnitude r e q u i r e d to produce such a range of mole r a t i o s . The o n l y i o n s p r e s e n t i n g r e a t enough c o n c e n t r a t i o n would have been H_0 + and OH - i o n s . I f isomorphous s u b s t i t u t i o n accounted f o r the v a r i a b i l i t y i n c o m p o s i t i o n , 0H~ i o n s must have s u b s t i t u t e d f o r P 0 4 ~ i o n s (51), and H^G + i o n s must have taken the p l a c e of C a + + i o n s i n the c r y s t a l l a t t i c e (58). The h i g h mole r a t i o s o b t a i n e d at h i g h pH f i t the t h e o r y t h a t 0H~ i o n s r e p l a c e d P 0 4 ~ i o n s . S i m i l a r l y the low mole r a t i o s o c c u r r i n g w i t h s y n t h e s i s at ++ + low pH i n d i c a t e d t h a t Ca i o n s were r e p l a c e d by H^O i o n s . Since the e x p e r i m e n t a l work i n d i c a t e s t h a t a d s o r p t i o n cannot account f o r the observed d i f f e r e n c e s i n c o m p o s i t i o n , and s i n c e H^0 + and OH - i o n s were pr e s e n t to s u b s t i t u t e i s o -m o r p h i c a l l y , the d i f f e r e n c e s i n c o m p o s i t i o n would appear to be best accounted f o r on the b a s i s of isomorphous sub-s t i t u t i o n . Compounds v a r y i n g i n c o m p o s i t i o n due t o isomorphous s u b s t i t u t i o n should, however, have d i f f e r e n t s o l u b i l i t i e s . I t was not p o s s i b l e t o c o r r e l a t e s o l u b i l i t y w i t h Ca:P r a t i o s of h y d r o x y a p a t i t e i n any way. S i n c e t h i s l a c k of c o r r e l a t i o n i s opposed to thermodynamic p r i n c i p l e s two p o s s i b l e explana-t i o n s a r i s e . I t i s p o s s i b l e t h a t the s u b s t i t u t i o n of C a + + or P 0 4 ~ i o n s by H^O* or OH - i o n s i s o m o r p h i c a l l y produced changes i n s o l u b i l i t y which were too s m a l l t o be d e t e c t e d . I t i s - 35 -p o s s i b l e t h a t f o r a d e f i n i t e c o n d i t i o n of s y n t h e s i s amorphous phases were pr e s e n t which a l t e r e d the Ca:P r a t i o s , but d i d not markedly a f f e c t the s o l u b i l i t y of the h y d r o x y a p a t i t e . I f the s o l i d phase c o n s i s t e d c h i e f l y o f h i g h l y c r y s t a l l i n e hydroxy-a p a t i t e s m a l l q u a n t i t i e s of amorphous phases c o u l d be p r e s e n t and escape d e t e c t i o n w i t h X-ray d i f f r a c t i o n t e c h n i q u e s . T h e i r p resence, however,would a l t e r the mole r a t i o s c o n s i d e r a b l y . Moreover, the h y d r o x y a p a t i t e c r y s t a l s produced i n these s t u d i e s were too s m a l l (600 x 100 A*) f o r p e t r o g r a p h i c a n a l y s i s and d e t e c t i o n of amorphous phases by t h i s method was not pos-s i b l e . There i s l i t t l e doubt t h a t when c a l c i u m and phosphorus compounds are f i r s t r e a c t e d an amorphous substance i s formed which on s t a n d i n g , g r a d u a l l y changes t o an a p a t i t e compound (58, 64). Some of the l e s s c r y s t a l l i n e or. amorphous phases c o u l d have p e r s i s t e d even a f t e r r e a c t i o n at h i g h temperature f o r l o n g p e r i o d s . In order t o determine i f the presence of amorphous phases i s the f a c t o r a l t e r i n g the mole r a t i o s of hydroxy-a p a t i t e , i t would be n e c e s s a r y t o make l a r g e c r y s t a l s which c o u l d be s t u d i e d p e t r o g r a p h i c a l l y , and which would p r o v i d e a more d i s t i n c t X-ray d i f f r a c t i o n p a t t e r n . I t would a l s o be n e c e s s a r y t o use a method of s y n t h e s i s whereby amorphous phases c o u l d be e l i m i n a t e d s h o u l d they be p r e s e n t . SUMMARY AND CONCLUSIONS The r e s u l t s of the d i s s o l u t i o n s t u d i e s showed t h a t d i f f e r e n c e s i n the s o l u b i l i t y o f h y d r o x y a p a t i t e syn-t h e s i z e d under v a r y i n g c o n d i t i o n s were not due to simple s u p e r - s a t u r a t i o n . From s o l u b i l i t y s t u d i e s and X-ray a n a l y s i s , i t was demonstrated t h a t the s o l u b i l i t y of h y d r o x y a p a t i t e was r e l a t e d t o the degree of c r y s t a l l i n i t y o f the s y n t h e s i z e d b a s i c c a l c i u m phosphate. S o l u b i l i t y d e creased and c r y s t a l -l i n i t y i n c r e a s e d w i t h i n c r e a s e d s y n t h e s i s r e a c t i o n p e r i o d and temperature of s y n t h e s i s u n t i l the c o n d i t i o n s at 90° C f o r 96 hours were reached. In order t o s y n t h e s i z e h y d r o x y a p a t i t e samples of equal s o l u b i l i t y above a pH v a l u e of 6.0, i t was n e c e s s a r y t o mix the EL^PO^ and CaCOROg more s l o w l y than at lower pH v a l u e s . A p p a r e n t l y , i n p r e c i p i t a t i n g h y d r o x y a p a t i t e i n a l e s s a c i d medium a slow m i x i n g of re a g e n t s was r e q u i r e d to prevent the f o r m a t i o n of a metastable s t a t e , and as a r e s u l t , b e t t e r d e f i n e d c r y s t a l s were produced. As was shown by e l e c t r o n microscope examination, however, the observed d i f f e r e n c e s i n c r y s t a l l i n i t y were not due to d i f f e r e n c e s i n c r y s t a l s i z e . A l l of the h y d r o x y a p a t i t e samples t e s t e d had o c r y s t a l dimensions of about 600 x 100 A and were too s m a l l f o r study w i t h the p e t r o g r a p h i c microscope. The d i f f e r e n c e s i n c r y s t a l l i n i t y d e t e c t e d by X-ray d i f f r a c t i o n a n a l y s i s might - 36 -- 37 -have been due to the presence of amorphous substances which would have the e f f e c t of d e c r e a s i n g the apparent degree of c r y s t a l l i n i t y . I t i s c o n c e i v a b l e t h a t w i t h a g i v e n method of s y n t h e s i s the amount of amorphous m a t e r i a l p r e s e n t r e -mained r e l a t i v e l y c o n s t a n t w i t h the r e s u l t t h a t the s o l u b i l i t i e s a l s o tended t o remain c o n s t a n t . From the mole r a t i o s t u d i e s , i t was c o n c l u d e d t h a t a d s o r p t i o n was not important i n d e t e r m i n i n g the Ca:P r a t i o s of h y d r o x y a p a t i t e . I f isomorphous s u b s t i t u t i o n + ++ — o c c u r r e d , H^O i o n s must have r e p l a c e d Ca i o n s , and OH i o n s must have r e p l a c e d PO^ - i o n s from the c r y s t a l l a t t i c e . Even though the r e s u l t i n g d i f f e r e n c e s i n s o l u b i l i t y might have been s m a l l , isomorphous s u b s t i t u t i o n s h o u l d have a f f e c t e d s o l u b i l i t y . While i n c o n s i s t a n t mole r a t i o s were o b t a i n e d , i t was not p o s s i b l e t o c o r r e l a t e s o l u b i l i t y w i t h c o m p o s i t i o n . The i n c o n s i s t a n t mole r a t i o might be e x p l a i n e d on the b a s i s of s m a l l amounts of amorphous substances b e i n g p r e s e n t , which would a l t e r the mole r a t i o s , but not the s o l u b i l i t y of the h y d r o x y a p a t i t e samples. I t would appear from these s t u d i e s , t h a t the s y n t h e t i c h y d r o x y a p a t i t e p r e -p a r a t i o n s d i d not have a f i x e d c o m p o s i t i o n , but n e v e r t h e l e s s the h y d r o x y a p a t i t e p o s s e s s e d a u n i f o r m pKsp v a l u e even when s y n t h e s i z e d over a wide range of pH. LITERATURE CITED 1. A r n o l d , P. W. The Nature o f p r e c i p i t a t e d c a l c i u m phosphates. Trans. Farady S o c , 46,1061 (1950). 2. A s l y n g , H. C. 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Les r e a c t i o n s d ' hydrolyse des phosphates d i c a l c i q u e et t r i -c a l c i q u e . B u l l . Soc. Chim., 53, 1210, (1933). - 43 -65. S c h o f i e l d , R. K. Can a P r e c i s e Meaning Be Given t o " A v a i l a b l e " S o i l Phosphorus? S o i l s and F e r t i l i -z e r s , 18, 373, (1955). 66. Sobel, A. E. et a l . C a l c i f i c a t i o n of t e e t h I I . Comparison o f some p r o p e r t i e s o f human and r a t t e e t h . J o u r . Dent. Res., 28, 61, (1949). 67. T a y l o r , N. W. , and Shear, C. M i c r o s c o p i c and X-ray i n v e s t i g a t i o n s on the c a l c i f i c a t i o n of t i s s u e . J o u r . B i o l . Chem. 81, 479, (1929). 68. Tennessee V a l l e y A u t h o r i t y , Phosphorus, p r o p e r t i e s of the element and some of i t s compounds. Chemical E n g i n e e r i n g Report No. 8, (1950). 69. Thewhs, J . , Bl o c k , G. N., and Murray, M. W. Chemical and X-ray a n a l y s i s o f d e n t a l m i n e r a l and s y n t h e t i c a p a t i t e s . 70. Watson, M., and Robinson, R. A. Not seen, c i t e d by Newman (58). 71. Wendt, C. R., and C l a r k e , A. N. An e l e c t r o m e t r i c study of the n e u t r a l i z a t i o n of. phosphoric a c i d by c a l c i u m h y d r o x i d e , J o u r . Amer. Chem. Soc., 45, 881, (1923). APPENDICES - 44 -APPENDIX 1 SOLUBILITY OF HYDROXYAPATITE MEASURED AFTER PRECIPITATION AT 40°C FOR 24 HOURS Sample Phosphorus Calcium Concen- Concen-t r a t i o n t r a t i o n M. M. 1 2.04x10 3 9.90x10 2 2.70 " 1.27x10 3 2.00 " 9.85x10 4 2.67 " 1.27x10 5 2.60 " 1.27 " 6 2.63 " 1.23 " 7 1.87 " 9.66x10 8 2.00 " 9.85 " 9 1.31 " 7.03 " 10 1.44 11 6.84 " 11 1.63 " 8.14 11 12 1.57 " 7.13 " 13 1.61 " 7.75 " 14 1.58 " 7.75 " 15 1.53 " 7.32 " 16 1.08 " 5.63 " 17 1.13 11 5.63 11 18 1.08 " 5.83 " 19 1.12 " 5.45 " 20 21 8.30x10 * 5.45 11 3.54x10 22 23 1.08x10 J 8.60x10 4.97 " 4.40 " 24 8.00 11 4.26 " 25 8.50 11 4.26 " 26 8.60 " 4.40 11 27 4.45 " 2.29 " 4 4 PH pH-l/2pCa P H 2 P 0 4 pCa +l/2pCa 5.95 4.40 4.29 3.11 5.96 4.46 4.14 3.00 5.96 4.41 4.61 3.10 5.97 4.47 4.14 3.00 5.98 4.48 4.16 3.00 5.98 4.47 4.16 3.01 5.98 4.33 4.33 3.11 5.98 4.43 4.61 3.10 6.00 4.38 4.56 3.24 6.02 4.42 4.48 3.20 6.04 4.45 4.44 3.18 6.06 4.34 4.48 3.24 6.06 4.46 4.45 3.20 6.07 4.47 4.46 3.20 6.08 4.37 4.59 3.23 6.14 4.47 4.70 3.34 6.15 4.48 4.68 3.34 6.17 4.51 4.69 3.33 6.24 4.56 4.70 3.36 6.24 4.56 4.68 3.36 6.27 4.51 4.92 3.53 6.31 4.61 4.75 3.40 6.39 4.60 4.83 3.41 6.39 4.69 4.87 3.40 6.40 4.70 4.84 3.40 6.40 4.71 4.83 3.41 6.60 4.76 5.33 3.69 pH 9PO. pPO. pKsp=lCpCa * 4 4 +6pPOA+2pOH 2.74 10.36 * 109.36 2.64 10.20 107.28 3.06 10.66 111.04 2.66 10.24 107.50 2.66 10.22 107.40 2.65 10.21 107.40 2.78 10.34 109.18 3.06 10.62 110.76 2.94 10.46 111.16 2.88 10.36 109.70 2.85 10.29 109.28 2.86 10.26 109.84 2.85 10.25 109.38 2.86 10.24 109.30 2.98 10.34 110.18 3.03 10.27 110.74 3.01 10.23 110.48 3.03 10.21 110.22 3.03 10.07 109.54 3.01 10.05 109.52 3.16 10.14 111.60 3.05 9.95 109.08 3.14 9.88 108.60 3.15 9.89 108.56 3.17 9.89 108.54 3.14 9.86 108.46 3.48 9.82 110.62 APPENDIX 1A SOLUBILITY OF HYDROXYAPATITE FORMED AT 40 °C FOR 24 HOURS MEASURED AFTER DISSOLUTION Sample Phosphorus Concen-t r a t i o n M. Calcium Concen-t r a t i o n M. PH pH-l/2pCa PH 2P0 4 +l/2pCa pCa 1-D 9 . 6 6 x l 0 - 4 9.94x10" -5 6.88 - 4.86 5.23 4.04 5-D 3.30 " 1.19x10" -4 6.97 4.96 5.76 4.04 6-D 3.46 " 1.21 " 7.04 5.01 5.79 4.06 18-D 1.22 " 1.06 " 7.11 5.11 6.21 4.01 16-D 1.47 " 1.06 " 7.29 5.29 6.22 4.01 21-D 1.26 " 9.18x10" -5 7.34 5.31 6.34 4.07 27-D 1.40 " 9.79 11 7.34 5.32 6.28 4.04 28-D 1.03 '* 6.89 " 7.36 5.27 6.49 4.19 29-D 7 . 6 4 x l 0 - 5 6.26x10" •5 7.52 5.41 6.74 4.23 P H 2 P 0 4 p P 0 4 pKsp=lCpCa +6pP0 4+2P0H 3.21 8.97 108.46 3.75 9.33 110.44 3.76 9.20 109.72 4.20 9.50 110.88 4.21 9.15 108.42 4.30 9.14 108.86 4.26 9.10 108.32 4.40 9.20 110.38 4.63 9.11 109.92 APPENDIX 2 SOLUBILITY OF HYDROXYAPATITE MEASURED AFTER PRECIPITATION AT 60°C FOR 24 HOURS Sample Phosphorus Concen-t r a t i o n M. Calcium Concen-t r a t i o n M. pH pH-l/2pCa p H 2P0 4 +l/2pCa pCa p H 2 P 0 4 p P 0 4 pKsp=lCpCa +6pP0 4+2pOH 1 5.00x10 3 2.30x10" -3 5.30 3.91 3.73 2.78 2.35 11.27 112.82 2 4.33 " 2.20 " 5.32 3.93 3.81 2.80 2.41 11.29 113.10 3 4.47 " 2.19 " 5.33 3.93 3.79 3.80 2.40 11.26 112.90 4 4.40 " 2.14 " 5.34 3.94 3.80 2.81 2.40 11.24 112.86 5 4.64 " 2.09 " 5,. 34 3.93 3.89 2.82 2.48 11.32 112.44 6 4.00 11 2.18 5.35 3.95 3.84 2.80 2.44 11.26 112.86 7 4.80 " 2.10 " 5.35 3.94 3.77 2.82 2.36 11.18 112.58 8 4.68 " 2.09 " 5.35 3.94 3.78 2.82 2.38 11.20 112.70 9 4.20 " 2.14 " 5.36 3.96 3.83 2.81 2.42 11.22 112.70 10 4.68 " 2.13 " 5.36 3.96 3.78 2.81 2.37 11.17 112.40 11 4.25 " 2.04 " 5.40 3.99 3.81 2.83 2.40 11.12 112.22 12 3.33 " 1.59 " 5.42 3.96 . 3.97 2.92 2.51 11.19 113.50 13 3.34 " 1.46 " 5.46 3.98 4.01 2.96 2.53 11.13 112.46 14 4.27 " 1.29 11 5.46 3.96 3.92 2.99 2.42 11.02 113.10 15 3.23 " 1.43 " 5.47 3.99 4.02 2.97 2.53 11.11 113.42 16 3.43 " 1.50 " 5.49 4.03 3.97 2.93 2.51 11.05 112.62 17 3.63 " 1.40 " 5.49 4.00 3.97 2.98 2.48 11.02 113.94 18 1.94 " 1.14 11 5.59 4.07 4.27 3.04 2.75 11.09 113.76 19 3.13 " 1.14 " 5.60 4.08 4.08 3.05 2.56 10.88 112.58 20 3.07 " 1.21 " A 5.64 4.13 4.08 3.02 2.57 10.81 111.78 21 2.63 " 9.50x10" -t± 5.64 4.09 4.17 3.11 2.62 10.86 112.98 22 1.73 11 1.02x10" - o 5.65 4.10 4.35 3.09 2.80 11.02 113.72 23 2.23 " 1.02 " A 5.66 4.11 4.24 3.09 2.69 10.89 112.92 24 2.20 " 9.85x10" -ft 5.68 4.13 4.24 3.10 2.69 10.85 112.74 25 2.71 " 9.47 " A 5.71 4*15 4.16 3.11 2.61 10.71 111.94 26 1.90x10 ° 6.70x10" -4 5.84 4.24 4.35 3.20 2.75 10.59 111.86 APPENDIX 2A SOLUBILITY OF HYDROXYAPATITE FORMED AT 60°G FOR 24 HOURS MEASURED AFTER DISSOLUTION Sample Phosphorus Calcium Concen-t r a t i o n M. Concen-t r a t i o n M 27 -D 1.26x10" •4 9.74x10" 26 -D 7.60x10" •5 7.71 " 23 -D 1.20x10" •4 8.82 " 25 -D 1.06 " 6.90 " 16 -D 9.32x10" •5 6.08 " 24 -D 1.26x10" •4 8.72 " 21--D 8.64x10* •5 8.12 " 18 -D 1.06x10" •4 8.52 " 28--D 5.32x10" •5 9.33x10" 5 pH pH-l/2pCa P H 2 P 0 4 pGa +l/2pCa 6.90 4.84 6.19 4.11 6.98 4.91 6.42 4.15 7.00 4.96 6.19 4.09 7.06 4.97 6.32 4.19 7.10 4.98 6.42 4.24 7.11 5.07 6.22 4.09 7.13 5.07 6.41 4.12 7.14 5.09 6.31 4.10 7.22 5."19 6.63 4.06 p H 2 P 0 4 p P 0 4 pKsp=lCpCa +6pP0 4n2p0H 4.13 9.85 114.40 4.34 9.90 114.94 4.15 9.67 112.92 4.23 9.63 113.56 4.30 9.62 1.13.92 4.18 9.48 111.56 4.35 9.61 112.60 4.26 9.58 112.20 4.60 9.68 112.24 APPENDIX 3 AND 3A SOLUBILITY OF HYDROXYAPATITE MEASURED AFTER PRECIPITATION AT 60°C FOR 96 HOURS AND MEASURED AFTER DISSOLUTION Sample Phosphorus Calcium Concen- Concen-t r a t i o n t r a t i o n M. M. 1 5.40x10 3 2.71x10 2 5.53 " 2.77 " 3 5.40 " 2.77 " 4 3.05 " 1.62 " 5 3.23 " 1.65 " 6 3.18 » 1.62 " 7 3.35 11 1.78 11 8 3.13 " 1.44 " 9 1.96 " 1.34 " 10 2.95 " 1.55 " 11 2.68 " 1.26 11 12 2.67 " 1.45 " 13 3.30 " 1.41 " 14 2.88 " 1.50 11 15 2.68 " 1.26 " 16 2.62 11 1.47 " 17 2.93 " 1.51 " 18 2.98 " 1.55 11 19 2.55 " 1.22 " 20 1.97 '" 1.37 " 21 2.00 " 1.16 " 22 2.05 " 1.11 " 23 2.03 " 1.10 " 24 2.05 " 1.03 " 25 1.69 11 8.06x10 26 1.33 " 6.64 " -3 -4 PH pH-l/2pCa P H 2P0 4 pCa +l/2pCa 5.17 3.80 J5.68 2.73 5.21 3.85 3.61 2.71 5.23 3.87 3.67 2.71 5.33 3.87 4.02 2.88 5.40 3.95 3.99 2.74 5.41 3.95 3.99 2.91 5.41 3.97 3.90 2.95 5.50 4.00 4.05 2.99 5.50 4.01 4.25 2.98 5.52 4.06 4.03 2.93 5.53 4.03 4.12 3.01 5.53 4.04 4.12 2.99 5.53 4.03 4.03 3.00 5.54 4.07 4.06 2.95 5.54 4.04 4.12 3.01 5.54 4.05 4.13 2.98 5.55 4.08 4.04 2.94 5.55 4.08 4.03 2.93 5.55 4.04 4.15 3.02 5.58 4.10 4.24 2.97 5.60 4.08 4.25 3.04 5.61 4.08 4.24 3.06 5.62 4.11 4.24 3.06 5.65 4.11 4.27 3.09 5.67 4.08 4.40 3.19 5.78 4.15 4.55 3.27 pH 9P0. pPO. pKsp=lCpCa ^ 4 4 +6pP0 4+2pOH 2. 32 11.50 113.96 2. 26 11.36 112.84 2. 32 11.38 112.92 2. 50 11.36 114.30 2. 46 11.18 111.68 2. 54 11.24 113.72 2. 57 11.27 114.30 2. 56 11.08 113.38 2. 76 11.28 114.48 2. 57 11.05 112.56 2. 61 11.07 113.46 2. 63 11.09 113.38 2. 53 10.99 112.88 2. 58 11.02 112.54 2. 61 11.05 113.32 2. 64 11.08 113.20 2. 57 10.90 112.24 2. 57 10.99 112.14 2. 63 11.05 113.40 2. 76 11.12 113.26 2. 74 11.06 113.56 2. 71 11.01 113.44 2. 73 11.01 113.42 2. 73 10.95 113.30 2. 81 10.99 114.50 2. 92 10.88 114.42 APPENDIX 3 AND 3A SOLUBILITY OF HYDROXYAPATITE MEASURED AFTER PRECIPITATION AT 60°C FOR 96 HOURS AND MEASURED AFTER DISSOLUTION Sample Phosphorus Calcium Coneen- Concen-t r a t i o n t r a t i o n M. M. pH pH-l/2pCa p H 2 P 0 4 pCa +l/2pCa P H 2 P 0 4 p P 0 4 pKsp=lCpCa +6pP0 4+2pOH 27 1.32x10" -3 7.25x10" •4 5.79 4.18 4.54 3.23 2. 92 10.86 113.88 28 1.17 ti 6.24 " 5.84 4.19 4.62 3.30 2. 98 10.82 114.24 29 1.16 II 9.64 " 5.85 4.29 4.54 3.12 2. 98 10.80 112.30 30 5.50x10" •4 3.52 " 6.12 4.36 5.08 3.53 3. 32 10.60 114.66 31 5.50 ti 3.32 " 6.19 4.41 5.10 3.55 3. 32 10.46 113.88 32 4.41 II 3.52 " 6.20 4.44 5.18 3.53 3. 42 10.54 114.14 33 4.83 II 3.32 11 6.22 4.44 5.16 3.55 3. 38 10.46 113.82 31--D 1.66 II 9.86x10" •5 6.81 4.79 5.97 4.04 3. 95 ,9.85 113.98 12-•D 1.08 ti 1.05x10" •4 6.98 4.87 6.20 4.01 4. 20 9.76 112.80 24--D 9.16x10" •5 1.04x10" •4 7.00 4.99 6.28 4.02 4. 28 9.80 113.00 APPENDIX 4 SOLUBILITY OF HYDROXYAPATITE MEASURED AFTER PRECIPITATION AT 90°C FOR 96 HOURS Sample Phosphorus Calcium Concen- Concen-t r a t i o n t r a t i o n M. M. 1 6.59x10" -3 3.46x10 2 5.43 " 2.72 " 3. 6.34 " 2.97 " 4 6.54 11 3.02 " 5 4.70 " 2.75 " 6 5.20 " 2.63 11 7 6.91 11 3.30 11 8 4.41 " 2.22 " 9 3.94 " 1.94 " 10 3.68 11 1.78 " 11 3.94 " 2.01 " 12 3.88 " 1.96 " 13 3.58 " 1.80 " 14 3.52 11 1.76 " 15 2.42 " 1.33x10 16 17 1.09 " 7.56x10" •4 4.40x10 3.45x10 18 5.21 " 2.68 " 19 5.33 " 2.13 " 20 4.41 11 1.86 " 21 2.08 " c 1.20 " 22 3.60x10 •D 3.84 " 23 1.36x10" •O 2.90x10 -3 -5 pH pH-l/2pCa P H 2 P 0 4 pCa +l/2pCa 5.05 3.73 3.47 2.63 5.11 3.75 3.59 2.72 5.11 3.77 3.59 2.69 5.11 3.77 3.57 2.68 5.16 3.81 3.68 2.71 5.16 3.79 3.70 2.73 5.21 3.73 3.57 2.65 5.26 3.86 3.75 2.86 5.31 3.89 3.88 2.85 5.33 3.90 3.91 2.87 5.36 3.94 3.87 2.84 5.36 3.94 3.89 2.84 5.37 3.94 3.92 2.87 5.40 3.96 3.94 2.88 5.53 4.04 4.15 2.98 5.94 4.23 4.72 3.43 6.21 4.44 4.96 3.54 6.26 4.43 5.14 3.66 6.30 4.44 5.20 3.72 6.43 4.54 5.33 3.73 6.80 4.82 5.83 3.96 7.35 5.61 7.58 3.84 7.87 5.59 7.92 4.56 p H 2 P 0 4 p P 0 4 pKsp=lCpCa +6pP0 4+2pOH 2. 16 11.58 113.68 2. 23 11.58 114.16 2. 24 11.54 113.92 2. 23 11.53 113.76 2. 33 11.53 113.96 2. 33 11.53 114.26 2. 24 11.34 112.12 2. 46 11.48 114.98 2. 45 11.35 113.98 2. 47 11.33 114.02 2. 45 11.25 113.18 2. 47 11.27 113.30 2. 49 11.27 113.57 2. 46 11.18 113.08 2. 66 11.12 113.46 3. 01 10.65 114.32 3. 19 10.29 112.72 3. 30 10.30 113.88 3. 34 10.26 114.16 3. 44 10.10 113.04 3. 85 9.77 112.62 5. 84 10.66 112.06 5. 64 9.42 114.38 \ APPENDIX 4A SOLUBILITY OF HYDROXYAPATITE FORMED AT 90°C FOR 96 HOURS MEASURED AFTER DISSOLUTION Sample Phosphorus Concen-t r a t i o n M. 26-D 8.96x10 28-D 8.96x10 10-D 6.80 " 20-D 9.80 " 25-D 5.80 " 27-D 8.52 " 24-D 4.80 11 29-D 7.84 " 14-D 3.00 " 17-D 4.38 " 30-D 2.93 " 19-D 1.89x10 -5 Calcium pH Concen-t r a t i o n M. 6.51x10" 5 6.97 6.32x10" 5 7.07 9.31 " 7.08 6.90 11 7.12 8.79 " 7.13 5.74 " 7.14 7.24 " 7.15 6.66 " 7.28 6.72 " 7.31 6.90 " 7.33 7.28 " 7.45 6.46x10" 5 7.67 pH-l/2pCa PH 2P0 4 +l/2pCa 4.86 6.37 4.95 6.42 5.05 6.47 5.03 6.38 5.09 6.56 5.00 6.49 5.06 6.71 5.18 6.63 5.21 6.69 5.24 6.84 5.37 7.01 5.46 7.45 pCa p H 2 P 0 4 4.21 4.27 4.24 4.31 4.06 4. 44 4.19 4.29 4.09 4.52 4.27 4.36 4.17 4.62 4.20 4.53 4.20 4.59 4.19 4.75 4.18 5.00 4.22 5.34 pP 0 4 pKsp=lCpCa +6pP0 4+2pOH 9.85 115.26 9.69 114.40 9.80 113.24 9.57 113.08 9.78 113.32 9.60 114.02 9.84 114.44 9.49 112.38 9.49 112.32 9.61 112.90 9.62 112.62 9.52 111.98 APPENDIX 5 THE Ca:P RATIO AND APPARENT SOLUBILITY PRODUCT OF HYDROXYAPATITE Reacted at 90°C f o r 96 hours Re Sample pH of S y n t h e s i s Ca/P pKsp 9a 5.29 1.23 114.52 31 5.86 1.34 112.62 16 5.94 1.50 114.32 32 6.00 1.59 112.60 17 6.40 1.45 112.72 21 6.50 1.34 112.62 28 6.57 1.36 114.40 23 7.87 1.38 114.38 29 8.45 1.29 112.38 30 8.87 1.54 113.50 i c t e d at 60° f o r 24 hours 1 5.30 1.05 112.82 4 5.34 1.18 112.86 6 5.35 1.18 112.86 11 5.40 1.10 112.22 14 5.46 1.35 113.10 16 5.49 1.33 112.62 29 5.54 1.59 114.40 18 5.59 1.04 113.76 20 5.64 1.25 111.78 22 5.65 1.44 113.72 APPENDIX 5 (cont'd) THE Ca:P MOLE RATIO AND APPARENT SOLUBILITY PRODUCT OF HYDROXYAPATITE Reacted at 60°C f o r 96 hours Re Sample pH of S y n t h e s i s Ca/P pKsp 9 5.50 1.26 114.48 20 5.58 1.54 113.26 23 5.62 1.48 113.42 28 5.84 1.64 114.24 30 6.12 1.26 114.66 29a 6.27 1.49 112.35 13a 7.73 1.34 - - -34 9.41 1.74 _ _ _ 35 10.09 2.23 _ _ _ 9 10.33 2.08 - - -i c t e d at 40°C f o r 24 hours 4 5.97 1.98 107.50 8 5.98 1.38 110.76 9. 6.00 1.48 111.16 14 6.07 1.41 109.30 17 6.14 1.24 110.74 22 6.31 1.48 109.08 23 6.39 1.44 108.60 27 6.60 1.39 110.62 30 8.05 1.88 _ _ _ 31 10.63 1.81 APPENDIX 6 THE Ca:P MOLE RATIO OF HYDROXYAPATITE BEFORE AND AFTER TREATMENT WITH DESORBING SOLUTION Sample I n i t i a l Ca:P mole r a t i o Treatment F i n a l Ca:P mole r a t i o A22 1.40 CaC 2 l x l O " 3 M 1.40 A22 1.40 H 3P0 4 l x l O ~ 4 M 1.42 A l l 1.50 KC1 5xlO" 3M 1.52 32-2 1.52 -KCl 5xlO~ 3M 1.52 34-2 1.47 KCl 5xlO" 3M 1.47 18-1 1.68 KC1 5xlO~ 3M 1.66 20-2 1.67 KCl 5xlO~ 3M 1.68 1-a 1.86 KC1 5xlO~ 3M 1.84 2-a 2.10 KCl 5 x l 0 ~ 3 M 2.12 3-a 1.67 KCl 5xlO~ 3M 1.68 APPENDIX 7 F i g u r e 1. E n l a r g e d s o l u b i l i t y diagram w i t h s o l u b i l i t y measurements a f t e r 40°C f o r 24 hours. 2. S o l u b i l i t y measured a f t e r r e a c t i o n at 60°C f o r 24 hours. 3. S o l u b i l i t y measured a f t e r r e a c t i o n at 60°C f o r 96 hours. 4. S o l u b i l i t y measured a f t e r r e a c t i o n at 90°C f o r 96 hours. 5. Comparison of r e g r e s s i o n l i n e s f o r h y d r o x y a p a t i t e s y n t h e s i z e d at v a r i o u s temperatures. 6. Diagram of the apparatus f o r r e a c t i n g a c i d and base s l o w l y . 7. Diagram of the beaker used f o r pH d e t e r m i n a t i o n s . 8. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 40°C f o r 24 hours. 9. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 40°C f o r 96 hours. 10. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 60°C f o r 24 hours. 11. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 60°C f o r 96 hours. 12. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 90°C f o r 96 hours. 13. X-ray d i f f r a c t i o n p a t t e r n of h y d r o x y a p a t i t e s y n t h e s i z e d at 90°C f o r 96 hours a f t e r slow mixing. .Fig. 1. Knlarged s o l u o i i i t y diagram witn s o l u b i l i t y measurements arter 40°C. f o r 24 hours. \ F i f . 2 S o l u b i l i t y measured a f t e r r e a c t i o n at 6 0 ° c f o r 24 nrs., S o l u o i l i t y a f t e r d i s s o l u t i o n 8-01 : ^ 1  3 0 4 0 p H . , / 2 p C a 5 0 Fig.3 S o l u b i l i t y measured a f t e r reaction at 60°C f o r ^ 6 hrs. o Q . C V J 5 0 6 0 701 ••'ir.'1 S o l u b i l i t y measured a f t e r r e a c t i o n i.t 90<>c f o r 96 hours. 8 0 ® ) S o l u b i l i t y a f t e r pr^cipita-,ion O) S o l u b i l i t y ^ f t e r d i s s o l u t i o n 30 4 0 p H - f / 2 p C a 5 0 Fig.5. Comparison of regression l i n e s f o r hydroxyapatite syntnesizod "" at various temperatures. TO sep:j.ratory funnels containing a c i d and base. COo f r e e Ng r.. 6 ;)i;-:grom of the apparatus for r e a c t i n g acid and base slowly. 0 -00? free N P intake * r, 7 Diagram of the beaker used f o r pH determinations. Interplaxar Distance fyft-... ^ 1 50 -25 50 45 . 35 Bragg Angle 36 1— 25 Fig. 8 X--ray diffract ion patten* of hydroxyap atito o/nthooiaod '•, 40°C for 24 hours. lo7 1 50 25 1 Intorplanar Distance CA") 2 . 0 ' 2„5 3^ 0 ~ 1— T 35 30 r 25 50 45 40" Bragg Angle Fig^9 X-ray diffraction pattern of hydroxys t>atite aynthoai2ed at 40°C for 06 hours. h 2 -50 25 -Interplanar Diotanoe 1 15 1^ 25 Bragg Angle Fig. 16 Xnray diffraction pattorn of hydroxyap Atite synthesized at 60°C for 24. hours. 50 -25 Intorplanar Distance 2.0 2^1 1 Bragg Angle Fig.11 X-ray diffraction pattern of hydroayapatiw synthesized at 60°C for houra. 1.7 Interplonar Distance ' 7 ^1 50 -0 T T 50 /.5 40 35 ifc-agg Angle Figo 12 X-ray'diffraction pat torn of Ivlroxyapatiti: for- -')(•>r hours. T " 25 T 25 5b "73 ' 73 j> 3o Bra^ -g Anglo Fig.13 X-ray diffraction .pattern of hydroxyapatite synthesized at 90°C r^QA hours after slow mixing. 

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