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The spectrophotometric identification of the permitted synthetic food colours Davies, Francis Raymond Edward 1949

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L-1= "5 THE SPECTROPHOTOMETRY IDENTIFICATION OF THE PERMITTED SYNTHETIC FOOD COLOURS. Francis Raymond Edward Davies A Thesis Submitted i n Partial Fulfilment of the Requirements for the Degree of MASTER OF ARTS i n the Department of CHEMISTRY THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER, 1949 \ Page \. ABSTRACT T h i r t e e n s p e c i f i e d water s o l u b l e dyes a r e p e r -m i t t e d f o r use i n e d i b l e products consumed i n Canada and the U n i t e d S t a t e s . The o f f i c i a l c h e m i c a l - p h y s i c a l method of a n a l y s i s i s not very s u c c e s s f u l when s m a l l amounts of one dye are present w i t h l a r g e r amounts of o t h e r s , or even i n the a n a l y s i s of a s i n g l e dye i f present i n s m a l l q u a n t i t i e s . The spectrophotometer has been shown to be very u s e f u l i n i d e n t i -f y i n g the t h i r t e e n p e r m i t t e d dyes i n d i v i d u a l l y , and by i t s means many p r e v i o u s l y u n r e s o l v a b l e b i n a r y mixtures have been r e a d i l y a n a l y z e d . F u r t h e r i n v e s t i g a t i o n o f the p o s s i b i l i t i e s of t h i s method of a n a l y s i s i s proceeding. Page 1. ACKNOWLEDGEMENT Although t h i s T h e s i s i s based upon independent r e s e a r c h , c a r r i e d out e n t i r e l y i n the L a b o r a t o r i e s o f the Department of N a t i o n a l H e a l t h and Welfare, Vancouver, B.C., I am p r o f o u n d l y indebted t o Dr. J . ALLEN HARRIS, P r o f e s s o r o f Chemistry a t the U n i v e r s i t y o f B r i t i s h Columbia, f o r h i s a d v i c e and encouragement i n the completion of t h i s work. I have p l e a s u r e i n acknowledging h i s s p o n s o r s h i p . Page The S p e c t r o p h o t o m e t r y I d e n t i f i c a t i o n o f the P e r m i t t e d Syntheti© Food C o l o u r s . CONTENTS S e c t i o n Page Number A b s t r a o t 5 I n t r o d u c t i o n 4 Methods of Procedure 6 A n a l y s i s of Data 8 References 15 E r r a t a 15 P l a t e 1 The Blue Dyes 16 P l a t e 2 .... The Green Dyes 16 P l a t e J .... The Yellow and Orange Dyes 17 P l a t e 4 .... The Red Dyes 17 P l a t e s 5 and 6.. The L i g h t Green and 10% B i n a r i e s 18 P l a t e 7 .... Guinea Green and Fast Green 19 P l a t e 8 .... Sunset Yellow and 10% B i n a r i e s 1.9 P l a t e 9 .... T a r t r a z i n e and 10% B i n a r i e s 20 P l a t e 10 ... T a r t r a z i n e , B r i l l i a n t B;iue, and I n d i g o t i n e 20 P l a t e 11 ... Amaranth and 10% B i n a r i e s 21 P l a t e 12 ... Grape Colour Mixtures 21 L i s t of Pe r m i t t e d Dyes 2 2 Page 4. INTRODUCTION The Food and Drugs A c t of the Dominion of Canada and the food laws of the U n i t e d S t a t e s o f America permit the use of only t h i r t e e n s p e c i f i e d w a t e r - s o l u b l e a n i l i n e dyes. These dyes are l i s t e d on the addendum page f o l l o w i n g P l a t e 12. For the purpose of a d m i n i s t e r i n g the A c t , i t i s t h e r e f o r e neoessary t o be able to demonstrate the presence of a r t i f i c i a l food c o l o u r s wherever they have been added t o a foo d , beverage, or c o n f e c t i o n e r y , and, t h e i r presence having been e s t a b l i s h e d , to be able to i d e n t i f y the dyes wi t h a c c u r a c y and with reason-able speed and s i m p l i c i t y . Since these dyes are added s o l e l y f o r the purpose of improving the appearance of the foo d , thus i n c r e a s i n g i t s s a l e s a t t r a c t i o n , they c o n s t i t u t e a very minute p a r t o f the e n t i r e food. Indeed, i n the case of h i g h c l a s s c o n f e c t i o n e r i e s , the dyes are o f t e n p r e s e n t i n mere "bl u s h e s " or p a l e t i n t s , and o f t e n a re there o n l y as v e r y t h i n s u r f a c e washes. I t i s not at a l l uncommon f o r a c o l o u r to be p l a i n l y v i s i b l e to the eye and y e t to be presen t i n such m i c r o s c o p i c t r a c e s as to d e f y detection, i n the food by o r d i n a r y methods. As an example of the great t i n c t o r i a l powers of some of the dyes, i t was found that a s o l u t i o n o f Fa s t Green, one hundred p a r t s i n ten m i l l i o n of water, was -much more i n t e n s e than many o f the t i n t s used i n the c o n f e c t i o n e r y t r a d e , and two p a r t s of F a s t Green i n t e n m i l l i o n of water oould e a s i l y be d e t e c t e d when c o n t a i n e d i n a 100 ml. pyrex beaker and viewed by t r a n s m i t t e d l i g h t . Page 3, Since any dye present i n a food sample w i l l almost never be a v a i l a b l e i n o r d i n a r y "weighable" quantities., i t i s m a n i f e s t -l y i m p o s s i b l e to nave p o s i t i v e means of c h e m i c a l l y i d e n t i f y i n g the thousands of dyes which i n theory c o u l d be p r e s e n t . There-f o r e the i d e n t i f i c a t i o n of food dyes has assumed the n e g a t i v e d i r e c t i o n of attempting to d e t e c t the p e r m i t t e d dyes by e s t a b -l i s h i n g c e r t a i n procedures f o r i s o l a t i n g each o f these dyes from the other p e r m i t t e d dyes, and r e j e c t i n g any dye which does not conform. Thus one attempts to prove the presence or absence of the p e r m i t t e d dyes, but makes no attempt to i d e n t i f y any other dyes, merely c l a s s i f y i n g them i n a group as "non-permitted dyes". There i s , of course, an e s t a b l i s h e d method of procedure to i s o l a t e the p e r m i t t e d dyes, a s d e t a i l e d i n Chapter ZXI of the "Methods of A n a l y s i s of the A s s o c i a t i o n of O f f i c i a l A g r i c u l -t u r a l Chemists", S i x t h E d i t i o n , 194-5. ( 1 ) . T h i s scheme depends p r i m a r i l y on the p a r t i t i o n o f the dye between two r e l a t i v e l y i mmiscible s o l v e n t s . C o n d i t i o n s are v a r i e d by f o u r stages, so as to d i v i d e the p e r m i t t e d dyes i n t o f o u r primary groups, each of which i n turn i s d i v i d e d by f u r t h e r m a n i p u l a t i o n i n t o i t s component dyes. I f the dyes are q u i t e pure and present i n reasonable q u a n t i t i e s and i n not too unequal amounts, t h i s scheme of a n a l y s i s i s f a r i l y s a t i s f a c t o r y . U n f o r t u n a t e l y these i d e a l c o n d i t i o n s seldom e x i s t i n p r a c t i c e . Moreover, Sunset Yellow FCF does not remain e n t i r e l y i n the aqueous phase u n t i l the l a s t group, but, i f present i n ponderable q u a n t i t y , w i l l Page 6. appear i n a l l groups and w i l l not c l e a r out of Groups I to I I I . w i t h the washings. A l s o i t i s found t h a t t a r t r a z i n e shows very-l i t t l e tendency to pass i n t o the amyl a l c o h o l from the f i v e per cent, h y d r o c h l o r i c a c i d , as the scheme c a l l s f o r . However, the most troublesome f a c t o r by f a r i s the great i n e q u a l i t y of the amounts of each dye present i n many m i x t u r e s . The amount of I n d i g o t i n e r e q u i r e d to change the c o l o u r o f T a r t r a z i n e to a " l i m e " green shade i s r e l a t i v e l y a mere t r a c e . A l t h o u g h c l e a r l y v i s i b l e as a shade of green, the c o l o u r , when t e s t e d by o r d i n a r y methods, c o n s i s t e n t l y y i e l d s o n l y T a r t r a z i n e . METHODS OF PROCEDURE. An attempt was made to use the Spectrophotometer as a means of i d e n t i f y i n g each food dye. The instrument a v a i l a b l e f o r t h i s r e s e a r c h was a Beckman Model DU P h o t o e l e c t r i c 'Quartz S p e c t r o -photometer, complete w i t h u l t r a v i o l e t and i n f r a r e d , a c c e s s o r i e s . Two p a i r each of both Corex and S i l i c a c e l l s were a v a i l a b l e , but s i n c e i t was convenient to s w i t c h from the v i s i b l e range to the u l t r a v i o l e t and v i c e v e r s a with the same s o l u t i o n s use was made e n t i r e l y of the s i l i c a c e l l s . By t e s t i t waS d i s -covered that a l l four o e l l s were matched w e l l enough to use i n t h i s i n v e s t i g a t i o n without a p p l y i n g any c o r r e c t i o n . About the o n l y r e f e r e n c e of d i r e c t value t o the s p e c i f i c purpose of t h i s r e s e a r c h i s the p u b l i c a t i o n o f the U n i t e d S t a t e s Department of A g r i c u l t u r e , T e c h n i c a l B u l l e t i n No. 310, June, 1932, by W.O. Holmes, J.T. Scanlan, and A.R. P e t e r s o n , Page 7. e n t i t l e d , "The V i s u a l Spectrophotometry of Dyes." (2). I t should be noted t h a t the a u t h o r s recommend t h a t one av o i d the use of water as a s o l v e n t due to tautomeric a l t e r a t i o n , and i n s t e a d suggest f i f t y p e r c e n t , a l c o h o l . However, i n t h i s r e s e a r c h the aqueous s o l u t i o n was adhered to p r i m a r i l y because t h i s i s the form i n which the p r a c t i c a l s o l u t i o n s are primarily encountered. I t may w e l l be that i n f u t u r e q u a n t i t a t i v e work, an a l c o h o l i c or o t h e r s o l u t i o n w i l l have to be employed. In view of the p a u c i t y o f pre v i o u s work, i t was decided to l i m i t the p r e l i m i n a r y i n v e s t i g a t i o n s to the f o l l o w i n g : (1) a sea r c h of the d e n s i t y curve f o r each pe r m i t t e d dye through the range 220 m i l l i m i c r o n s to 700 m i l l i m i c r o n s . (2) a s e a r c h of the d e n s i t y curves f o r c e r t a i n b i n a r y mixtures through c e r t a i n s p e c i f i c ranges of wavelength. I n a l l but a few of these binary s o l u t i o n s the components were presen t i n the r a t i o of 10 percent, minor component i n 90 percent, major component. That p a r t of the f u l l range (220 m i l l i m i c r o n s t o 700 m i l l i m i c r o n s ) was r u n f o r each s o l u t i o n which would show t o what degree, i f any, the presence of the minor c o n s t i t u e n t m o d i f i e d t h e standard curve of the major c o n s t i t u e n t . An attempt was made to make use o f dye c o n c e n t r a t i o n s such that the maximum a b s o r p t i o n would y i e l d a d e n s i t y g r e a t e r than u n i t y but l e s s than 2; th a t i s , the minimum t r a n s m i s s i o n would l i e between one percen t , and 10 percent. For Guinea Green B and F a s t Green, the c o n c e n t r a t i o n used was one p a r t i n 100,000. Page 8. For the other e l e v e n dyes the c o n c e n t r a t i o n was one p a r t i n 2:5,000, alth o u g h f o r f u t u r e work i t i s suggested t h a t the con-c e n t r a t i o n s o f I n d i g o t i n e and B r i l l i a n t Blue be reduced to some po i n t i n t e r m e d i a t e to the above c o n c e n t r a t i o n s . Thus the data obtained can be used a t a f u t u r e time i n c a l c u l a t i n g c o n c e n t r a -t i o n s of unknown simple and composite s o l u t i o n s . The i n v a r i a b l e s o l v e n t used was d i s t i l l e d water. I t i s a well-known f a c t t h a t at pH other than 7 , the shade and i n t e n s i t y of s e v e r a l of these dyes are q u i t e changed. T h i s i s n o t a b l y tame of Orange I , H r y t h r o s i n e and Naphthol Y e l l o w S. T h i s f e a t u r e of the problem, namely v a r i a t i o n s caused by changes i n pH or changes i n s o l v e n t s was not i n v e s t i g a t e d a t t h i s time. A l l measurements were made at room temperature (20°C. 1 2°.)'. ANALYSIS OF DATA. A t t e n t i o n i s f i r s t drawn to the d e n s i t y curves f o r t h e t h i r t e e n p e r m i t t e d dyes. These are c l a s s i f i e d a c c o r d i n g t o c o l o u r . P l a t e 1 shows the two blue dyes, I n d i g o t i n e and B r i l l i a n t B l u e . Both these dyes have very simple curves w i t h one main a b s o r p t i o n maximum i n the red-orange. The maximum d e n s i t y f o r B r i l l i a n t Blue occurs at 640 m i l l i m i c r o n s , while t h a t f o r I n d i g o t i n e i s a t 611 m i l l i m i c r o n s . The spectrophotometer d i s t i n g u i s h e s between these two dyes w i t h the utmost ease. I t i s of i n t e r e s t to note that a f u r t h e r d i s t i n c t d i f f e r e n c e i n the d e n s i t y curves of these dyes occurs i n the range 300 m i l l i -Page 9 . microns to 440 m i l l i m i c r o n s . P l a t e 2 i l l u s t r a t e s the d e n s i t y curves f o r the three green dyes;. As one might expect from t h e i r s i m i l a r i t y i n c o l o u r and chemical nature, these three dyes have very s i m i l a r curves, w i t h maxima l o c a t e d very c l o s e t o g e t h e r . These maxima occur at 654 m i l l i m i c r o n s f o r L i g h t Green SF Y e l l o w i s h , at 627 m i l l i m i c r o n s f o r F a s t Green, and at 6 2 1 m i l l i m i c r o n s f o r Guinea Green. Although these are c l o s e f o r r o u t i n e work, t h e y are very s p e c i -f i c , and the three dyes are not d i f f i c u l t to d i s t i n g u i s h i n simple s o l u t i o n . Z t i s noteworthy to repeat that both Guinea Green and F a s t Green are very i n t e n s e dyes, and were used i n d i l u t i o n s of 1/100,000 In s t e a d of the d i l u t i o n of 1/2^,000 employed f o r the other e l e v e n dyes. P l a t e 5 shows the orange and y e l l o w dyes. ( D e s p i t e i t s name, Sunset Y e l l o w F0F i s r e a l l y an orange dye, and i s proper-l y c l a s s e d w i t h Orange I.) These l a t t e r two dyes show s t r o n g a b s o r p t i o n i n the v i s i b l e p a r t o f the spectrum, Sunset Y e l l o w FCF producing maximum a b s o r p t i o n at 480 m i l l i m i c r o n s and Orange I a r a t h e r f l a t curve with a " p l a t e a u " over the range 470 m i l l i -microns -490 m i l l i m i c r o n s . Thus these two dyes are i n d i s t i n g u i s h -able i n t h i s p a r t of t h e i r range. F o r t u n a t e l y , however, there i s s t r o n g a b s o r p t i o n i n the u l t r a v i o l e t r e g i o n , and from 250 m i l l i m i c r o n s t o 5 1 1 m i l l i m i c r o n s the curves r u n "counter" to each other. T h i s , then, i s the range to search i f one wishes to i d e n t i f y e i t h e r of these orange dyes. Page 10, The density curves for the two yellow dyes, Tartrazine and EFaphthol S show a most unusual s imi la r i ty throughout the range 200 millimicrons to 700 millimicrons and would be very d i f f i c u l t to dist inguish. However, Naphthol Yellow S has the property of decolourizing almost completely i n di lute mineral acid (say 2%f0) and therefore i t s density curve should change rapidly with changing pH. It is the intention of the writer to investigate the effects of changes i n hydrogen ion concentration on the absorption curves of these and other dyes. I t remains to discuss the four red dyes, of which the absorption curves are shown i n Plate 4, I t i s seen at once that Erythrosine shows a double maximum at 528 millimicrons and 522 mil l imicrons, with a d is t inc t dip at 525 mil l imicrons. This double absorption maximum i s unique amongst the thirteen dyes tested, and is a splendid identifying factor for Erythrosine. Amaranth shows a single maximum at J?21 millimicrons and can be readily ident i f ied . However the maxima for Ponceau SX and Ponceau 5R are only two millimicrons apart, namely at 504 m i l l i -microns and 502 millimicrons respectively, and reference must be made to the u l t raviole t region, Here i t is seen that whereas Ponceau SX shows a dis t inct minimum at 2 58 millimicrons and a maximum at 505.5 mil l imicrons, Ponceau 5R shows a mini-mum at 506 millimicrons and a steeply r i s ing curve from about 280 millimicrons to the lower l i m i t of the spectrophotometer. Hence i n the region 220 millimicrons -530 millimicrons the two Page 11. Ponceau curves r u n completely counter to each o t h e r . The q u e s t i o n now arose as to how s u c c e s s f u l l y the s p e c t r o -photometer would be a b l e to d e t e c t t h e presence of r e l a t i v e l y s mall amounts of p e r m i t t e d dyes i n the presence of l a r g e r q u a n t i t i e s o f a second, masking, dye. Since the t h i r t e e n dyes permit of (2 x 13 x 12) 312 b i n a r y mixtures of t h i s type i t was only p o s s i b l e to attempt runs on r e l a t i v e l y few of these 312 m i x t u r e s . Since experience has shown that c e r t a i n m ixtures are d i f f i c u l t or impossible t o separate by the s o l v e n t - p a r t i t i o n method these were n a t u r a l l y chosen for . a n a l y s i s , and some others were chosen as l i k e l y to demonstrate any p o s s i b l e shortcomings.: of the spectrophotometric method. The b i n a r y mixtures with l i g h t Green SF Y e l l o w i s h as the major c o n s t i t u e n t are g e n e r a l l y i l l u s t r a t i v e of s e p a r a t i o n s which c o u l d be c l a s s i f i e d as r e l a t i v e l y poor. These are i l l u s t r a t e d i n P l a t e s 5 and 6. Since Guinea Green i s a. con-g e n e r i c dye w i t h L i g h t Green SF Y e l l o w i s h i t i s to be expected that a mixture of the two w i l l not show any d i s t i n g u i s h i n g features;. R e c a l l i n g that the "peak" f o r L i g h t Green i s 634 m i l l i m i c r o n s and that f o r Guinea Green i s 621 m i l l i m i c r o n s , i t i s not s u r p r i s i n g to f i n d the mixture showing a peak between these p o i n t s , namely at 630 m i l l i m i c r o n s . F u r t h e r i l l u s t r a t i o n of t h i s important p o i n t that a peak i s not by any means d i a g n o s t i c i s o f f e r e d i n P l a t e 7. Here, on a l a r g e s c a l e , are shown curves f o r Guinea Green and F a s t Green i n c o n c e n t r a t i o n Page 12. of 1/100,000. At the bottom of Plate 7 i s shown the curve for a mixture of equal parts of each dye i n concentration of 1/3,004 000. I t is clear that th is l a t te r i s a simple curve with a maximum at the expected wave-length (624 millimicrons) and with no suggestion that i t is not the curve of a definite compound. This misleading s impl ic i ty of many curves produced by mixtures i s a condition to be judged with extreme caution. The curve for Naphthol Yellow S i n Light Green SF Yellow-ish shows l i t t l e modification of the major curve except at 236 millimicrons - 250 mill imicrons, where the Naphthol Yellow mixture curve i s at variance with the Light Green curve. Referring to Plate 6, Amaranth shows a d is t inc t though small in f lec t ion at 521 millimicrons ( i ts absorption maximum), while the Erythrosine mixture shows a d is t inc t maximum at 525 mill imicrons. Uhese two red dyes are easi ly determined i n ten percent, mixtures with Light Green SF Yellowish. Plate 8 shows five binary mixtures with Sunset Yellow FCF as the major constftuent (ninety percent.). These curves i l l u s t r a t e two important points. The f i r s t is that i f the minor constituent i s absorbed strongly i n a region where the major constituent shows l i t t l e or no absorption, then the curve for the mixture closely follows that of the di luted pure dye present to the smaller degree. This fact i s shown i n the curves for the three green dyes and for B r i l l i a n t Blue, The second important point i s i l l u s t r a t ed i n the ourve for the mixture of Erythrosine i n Sunset Yellow FCF. Here, Page 13. d e s p i t e the f a c t t h a t the curve c l o s e l y f o l l o w s the p a t t e r n f o r Sunset Yellow FCF, n e v e r t h e l e s s the unique ( f o r these t h i r t e e n dyes) presence of the double maximum of E r y t h r o s i n e a s s e r t s i t s e l f , and c l e a r l y i d e n t i f i e s E r y t h r o s i n e as the minor c o n s t i t -uent • The data i n P l a t e s 9 and 10 are o f p a r t i c u l a r p r a c t i c a l v a l u e . T a r t r a z i n e i s the dye most commonly found i n foods, and, because o f i t s y e l l o w c o l o u r , i s o f t e n present i n much g r e a t e r c o n c e n t r a t i o n that t h e other admixed dyes. Moreover, i f the admixed dye i s of the triphenylmethane type, and present i n r e l a t i v e l y s m a l l q u a n t i t y , i t w i l l be de s t r o y e d and l o s t by the s t r o n g c o n c e n t r a t i o n of h y d r o c h l o r i c a c i d r e q u i r e d to d r i v e i t i n t o the amyl a l c o h o l . T h i s i s n o t a b l y true w i t h small q u a n t i t i e s of I n d i g o t i n e , and t h i s l a t t e r s e p a r a t i o n i s d e f i n i t e l y u n s u c c e s s f u l by chemical methods. However, by means of the spectrophotometer, since at 611 m i l l i m i c r o n s (the maximum f o r I n d i g o t i n e ) T a r t r a z i n e shows p r a c t i c a l l y no absorp-t i o n , t h i s s e p a r a t i o n proved to be a s i g n a l success. Three c o n c e n t r a t i o n s of I n d i g o t i n e i n T a r t r a z i n e are i l l u s t r a t e d . I n P l a t e 10, curve (a) i s one p a r t of I n d i g o t i n e i n nine of T a r t r a z i n e , (b) i s one i n t w e n t y - f i v e , and (c) i s one i n f i f t y . A l l s o l u t i o n s used were one p a r t o f dye i n 25,000 p a r t s o f water. The s o l u t i o n i n curve (c) i s q u i t e yellow, with no v i s i b l e g r e e n i s h hue, yet the d e n s i t y curve shows unmistakably the presence of I n d i g o t i n e , Page 14. As expected the three green dyes and B r i l l i a n t Blue ( P l a t e s 9 and 10) are e a s i l y d e t e c t e d when present w i t h T a r t r a z i n e . P l a t e 11 i l l u s t r a t e s the ease w i t h which the presence of the green and blue dyes i s d e t e c t e d when mixed w i t h even a n i n e - f o l d c o n c e n t r a t i o n o f Amaranth, P l a t e 12 demonstrates an i n t e r e s t i n g experiment of p r a c t i -c a l s i g n i f i c a n c e . A sample of grape-coloured b o i l e d candies was brought to the l a b o r a t o r y f o r a n a l y s i s of food c o l o u r s p r e s e n t . The p u r p l e dye passed completely through the scheme without apparent change, and gave a l l signs of being a pure dye. As a f i n a l t e s t mixtures of Amaranth w i t h B r i l l i a n t B l u e and of Amaranth wi t h I n d i g o t i n e were made to match, by eye, the c o l o u r of the "unknown" s o l u t i o n . D e n s i t y curves of the three s o l u t i o n s were then prepared. A glance at P l a t e 12 shows a t e l l t a l e "break" i n the curve of the dye being t e s t e d at e x a c t l y the same wavelength as the i n f l e c t i o n p o i n t of the known Indigotine-Amaranth m i x t u r e . Thus i t i s shown beyond doubt t h a t the "unknown" must be a mixture of a l i t t l e Indlgo= t i n e i n a l a r g e q u a n t i t y of Amaranth. I t seems c l e a r , then, i n the l i g h t of t h i s r e s e a r c h , that the spectrophotometer w i l l be o f great v a l u e i n i d e n t i f y i n g many dye-mixtures which u n t i l now have been d i f f i c u l t or impossible to a n a l y s e . The w r i t e r proposes to continue t h i s i n v e s t i g a t i o n , p a r t i c u l a r l y along the f o l l o w i n g l i n e s : Page 15 (1) A study of the dynamics of the density curves of the various mixtures, with a view to e s t a b l i s h i n g a s p e c i f i c scheme of analysis for t h e i r i d e n t i t y . (2) Quantitative measurements of the dyes. This should be r e l a t i v e l y easy i n simple solutions, but w i l l present d i f f i c u l t i e s when two or more dyes are present together, (5) A study of the variations i n the density curves brought about by changes i n pH. There i s every reason'to believe that several of the mixtures at present d i f f i c u l t to define may be resolved i n t h i s way. REFERENCES (1) "Methods of Analysis of the Association of O f f i c i a l A g r i c u l t u r a l Chemists", Sixth E d i t i o n , 194-5. Chapter XXI. (2); Holmes, W.C.; Scanlan, J.T.; Peterson, A.R.; "The V i s u a l Spectrophotometry of Dyes." (United States Department of Agriculture, Technical B u l l e t i n No. 510, June, 1952.) ERRATA (with reference to Plates) (1) The abscissa units are to be read "^nfl" for "millimicrons" (2) The ordinate scales are i n "density" units, i . e . , the logarithm of the r e c i p r o c a l of the percentage transmission. / 4 i BLUE DYES. Plote I 3.0-2 0 -. 0 -30TTp brilliant blue GREEN DYES. Rate 2 light green SF yellowish YELLOW & ORANGE DYES. P | a t e 3 3.0-sunset yellow 20-, naphthol yellow S. orange I. 1.0-200 u 400 600 .ponceau 3R RED DYES. Plate 4 erythrosine ponceau SX 1.5-1.0-0.5-amaranth 200 p 400 600 18 » LIGHT GREEN & 1 0 % BINARIES p, d t e 5 GUINEA GREEN & FAST GREEN. Plate 7 iast green .80-guineo: greer .55^ .30- 50% mixture 6 0 0 M 6 2 0 6 4 0 SUNSET YELLOW & 10% BINARIES. Plate8 sunset yellow 'erythrosine I.O-y brilliant blue 2L0-&. TARTRAZINE and 10% BINARIES. P l a t e 9 TARTRAZINE, BRILLIANT BLUE & INDIGOTINE. Plate 10 450 p 550 1.5 AMARANTH & 10% BINARIES. Plate 11 amaranth brilliant blue fast green light green I 450 u 650 GRAPE COLOUR MIXTURES. Plate 12 0.6T 0.4-0.2-580 p "unknown" brilliant blue with amaranth 620 660 a 2.. SUBJECT TO THE ACT OR REGULATIONS THE FOLLOWING COLOURS MAY BE USEU IN OR UPON FOOU Natural Colours, being cochineal and vegetable colour extractives A r t i f i c i a l Colours, being caramel and s p e c i a l l y p u r i f i e d vegetable and sugar charcoals, Coal Tar Dyes, being AMARANTH, the trisodium sal t of 1-(4-sulpho-l-naphthylazo)-2-naphthol -3 , 6-disulphonic acid; PONCEAU 3R, the disodium salt of l-pseudocumylazo-2-napththol-3, b-disulphonic acid, the b o i l i n g range of the crude, pseudocumi-dine obtained by reduction of which s h a l l be between 220"C and 245°C; ERYTHROSINE, the disodium s a l t of 9-o-carboxyphenyl-6-hydroxy-2, 4 , 5 , 7-te craiodo-3-isoxanthone; PONCEAU SX, the disodium sal t of 2-(5-sulpho-2, 4-xylylazo)-1-naphthol-4-sulphonic acid; OIL RED X0; l-xylylazo-2-naphthol, of which the xy l i d i n e obtained by i t s reduction s h a l l contain not more than 30.0 per cent of m-xylidine, and 95 per cent of such x y l i d i r a s h a l l d i s t i l between 212°C and 232°C; ORANGE 3. the mono sodium sal t of 4-p-sulphophenylazo-l-naphthol; ORANGE .SS, l-o-tolylazo-2-naphthol; NAPHTHOL YELLOW S, the disodium or dipotassium salt of 2,4-dinitro-l-naphthol - 7-sulphoniC' acid, that s h a l l not contain more than 0.03 per cent of Martius Yellow; OIL YELLOW AB, l-phenylazo-2-naphthylamine; OIL YELLOW OB, l-o-tolylazo-2-naphthylamine; TARTRAZINE, The trisodium sal t of 3-carboxy-5-hydroxy-l-p-sulpho-phenyl-4-p-sulphophenjilazopyrazole; SUNSET. YELLOW FCF, the disodium s a l t of-l-p-sulphophenylazo-2-naphthol-6-sulphonic acid; LIGHT GREEN S.F. YELLOWISH, the disodium s a l t of 4-( (4-(N-ethyl-p-sulphobenzylamino)-phenyl)) - (4-sulphoniumphenyl)-methylene) -(l-(N-ethyl-N-p-sulphobenzyl)-delta 2 , 5 , - cyclohexadienimine: GUINEA GREEN B, the monosodium s a l t of 4-(4-(N-ethyl-p-sulpho-benzylamino) -diphen^rl-methylene )-(l-(N-ethyl-N-p-sulphoniumbenzyl-delta 2 , 5 , cyclohexandienimine); FAST GREEN FCF, the disodium sal t of 4-((4-N-ethyT-p-sulphobenzylamino)-phenyl)-(4-hydroxy-2-sulphoniumphenyl)-methylene)-(1-(N-ethyl-N-p-sulphobenzyl) delta 2 , 5 , cyclohexadienimine); INDIGOTINE, the disodium s a l t of indigotine--5,5 1-disulphonic acid; BRILLIANT BLUE FCF, the disodium s a l t of 4-((4-N-ethyl-p-sulpho-benzylamino)-phenyl) - (2-sulphoniumphenyl).- methylene) -(l-fN-ethyl-N-p/sulphobenzyl) -delta 2 , 5 , cyclohexadienimine). 

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