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The vapor pressure of carbon dioxide in equilibrium with mixtures of solutions of potassium carbonate… Melville, John 1923

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The Vapor Pressure of Carbon Dioxide in Equilibrium with Mixtures of Solutions of Potassium Carbonate and Potassium Bicarbonate. by John I l e l v i l l e , B .A.Sc . A T h e s i s s u b m i t t e d f o r th e Degre e o f UASTER O F APPLIE D SCIENC E i n t h e Depar tmen t of CHELIISTRY THE UNIVERSIT Y O P BRITIS H COLUMBI A APRIL, 1923 . Table of Contents. Introduction. Reviev/ of Researches Relating to This Problem . Preparation and Analysis o f Solutions. Description and Operatio n of Static Apparatus. (3 Types) . Results by Static Methods. Description and Operation of Air Saturatio n Me th od s . Summary, The Yapor Pressure of Carbon Dioxide in Equilibrium with Mixtures of Solutions of Potassium Carbonate and Potassium Bicarbonate. This problem has a direct bearing upon the method commercially used for the production of carbon dioxide; accord-ing to this method, coke is burned to carbon dioxide, an over excess of air being avoided to ensure the partial pressure of the carbon dioxide in the flue gases being as high as possible. The gases are then cooled and passed up through a tower, through which potassium carbonate is trickling. I n the towers, the normal carbonate is converted to acid carbonate, according to the equation: KZ°°3 *  H 2° f  °° 2 =  2  K H G 0 3 The converted carbonate solutio n from the towers is then boiled over the coke burners to effect the reverse change, the carbon dioxide expelled being dried and compressed into cylinders. A knowledge o f the variation in the partial pressure of the G0? over these solutions will therefore sho w the temperature at which the maximum yield of OO2 will be obtained* Review of Be searches Relating to this Problem. McCoy (Am . Chem. J., 2?, I903, Page 436) investigated the "Equilibrium in the System Composed of Sodium Carbonate, Sodium Bicarbonate, Carbon Dioxide and Water." H e states that the experimental problem presented was the determination of the composition of the liquid and vapor phases in equilibrium with 2. one another at constant temperature, for various proportions of their constituents. Th e temperature chosen was 25°. Th e method adopted consisted in shaking 200 or 300 c.c. of the solution of the mixed carbonates, contained in a closed litre bottle, with air, until equilibrium was reached, the whole being kept at constant temperature. A  sample of the gas was transferred to another vessel and its percentage of carbon dioxide determined. Th e composition of the solution was deter-mined by analysis. "Th e main series of experiments was made v/ith solutions of bicarbonate and carbonate of various proport-ions, and of deci-normal strength with respect to sodium. Such solutions of approximately the desired composition were made from separate solutions of bicarbonate and carbonate, each having the same concentration of sodium. In this work he showed that the equilibrium in the system composed of sodium carbonate, sodium bicarbonate, carbon dioxide and water is governed at constant temperature by the formula 2 x20 -  K kP (1 - x) in which the symbols have the following significance; x  is the fraction of the sodiiim in the form of bicarbonate; 1 - x i s the fraction in the form of carbonate; 0  is the concentration of the sodium in gram atoms per litre; k  is the solubility coefficient of carbon dioxide in water at the equilibrium tem-perature; an d P is the partial pressure of the carbon dioxide. 3. The error in HcCoy's work soerra to be the assumption that the solubility of oarbon dioxide in his solution is the same as the solubility in water, and this error would become more apparent in solutions of the strength used i n this work, namely 1 and 2 normal. Cameron and Briggs (J. Phys. Chem., 5, 537, 1901) studied the "Equilibrium between Carbonates and Bicarbonates in Aqueous Solution." The y drew air through aqueous solutions of the bioarbonates of sodium, potassium and magnesium of various oonoentrations, and at four different temperatures and determined by analysis the proportions of oarbonate and bioar-bonate after equilibrium had been reached. McCoy'8 demonstration of the equilibrium existing between carbonate, bicarbonate, oarbon dioxide and v/ater is applicable to this study of the vapor pressure of oarbon dioxide in equilibrium with mixtures of solutions of potassium oarbonate and bicarbonate* The hydrolytio dissocation of potassium bioarbonate is represented by the equation KECCj •  Hg O =  EO H f  HgCOj . This action would naturally be expected to reach a state of equilibrium by reason of the reverse reaotion. However , the beoarbonate is itself an acid and can aot upon the free potass-ium hydroxide, forming water and the normal oarbonate thus: KHCOj f  KO H -  KgCO j *  H O . 4. From the reaction KHCO, +  H 20 =  KO H f  HgOO, . Gl ° 3 ° 4 where C., etc., represent the concentrations in gram molecules per litre, we have (1) and from 2H00* °i we have G3C4 f ZO H °3 =1°3 -= -K1G1 H20 f K2G2 X2G03 G2 From (1) and (2) 0 ^ K 2c2 0,0 3G4 ¥ l or G- L G2G4 (2) where K  is the equilibrium constant for the system under consideration. % Preparation and Analysis of Solutions, The mixed solutions were obtained by first preparing separate solutions of potassium carbonate and potassium bicarbonate of equivalent strength with respect to carbonate content and then mixing these separately prepared solutions. The solutions were analysed by the method of Winkler as out-lined by HcCoy (Loc. cit). "Thi s method consists in adding in excess a known voliime of standard alkali, free from carbonate, precipitating the carbonate by addition of barium chloride and finally titrating the excess of alkali with hydrochloric acid using phenolphthalein as indicator" — "Th e net amount of alka-li used (that is, the whole amount used less that neutralized by acid in the final titration) is a measure of the carbon dioxide in excess of that existing as normal carbonate. Th e solutions used ranged from 1 to 2.5 normal with respect to carbonate. The determination of the vapor pressure of these solut-ions was carried out by two methods: (1 ) a  static method (j5 modifications, described below) which gave the combined vapor pressure of carbon dioxide and water and (2) a n air saturation method, which gave the vapor pressures of carbon dioxide and water separately. A n attempt was also made to determine the separate partial pressures of carbon dioxide and water by isolating and analyzing a sample of the gas in equilibrium with the solution. Th e addition of the separate values obtained by 6 . t h i s l a t t e r metho d o r b y th e a i r s a t u r a t i o n metho d t o giv e th e same valu e a s obtaine d b y th e s t a t i c method , fo r correspondin g temperatures shoul d serv e a s a  chec k o n th e work . Descr ip t ion an d Operatio n o f S t a t i c Apparatus . The so lu t io n t o b e i n v e s t i g a t e d wa s containe d i n th e f l a sk A  (P la t e 1 ) whic h wa s containe d i n a  wate r bat h B l Connection wa s mad e b y mean s o f th e three-wa y stop-cock , 3 , wi th th e Manometer , M . Change s i n p r e s su re , brough t abou t b y varying th e temperatur e o f th e ba th , wer e rea d of f fro m th e manometer b y mean s o f a  cathetomete r o r a  metr e s t i c k place d behind th e manometer . I t wa s foun d t h a t whe n th e f l a sk , A , was connecte d d i r e c t l y t o th e manometer , withou t th e f l a s k F, there wa s excessiv e condensatio n i n th e connectin g tubes . The apparatu s wa s cleaned , dr ie d an d the n evacuate d b y an oil-pump , th e three-wa y stop-cock , S , bein g i n p o s i t i o n ( 1 ) . The ope n v e r t i c a l ar m o f th e three-wa y stop-coc k wa s the n f i l l e d wit h so lu t io n an d th e stop-coc k turne d t o p o s i t i o n (2 ) so t h a t th e so lu t io n ra n in t o f l as k A . Th e stop-coc k wa s turned bac k t o p o s i t i o n ( l ) befor e a l l th e so lu t io n ha d ru n ou t of th e ope n ar m s o t h a t n o a i r entere d th e appara tus . Thi s procedure o f f i l l i n g th e ope n ar m wit h so lu t io n an d allowin g i t to tu n in t o f l as k A  was repeate d u n t i l ther e wa s approximatel y 100 c . c . o f so lu t io n i n Flas k A . Th e bat h wa s the n heate d over a  rang e o f fro m 10 ° C . t o 9.5 ° C . an d th e p ressu re s note d FLRTE T 7. on the manometer. I n order to make sure that equilibrium was reached between the vapor and solution, the temperature was raised slowly and maintained at the various temperatures for several minutes before the pressure corresponding to that temperature v/as read off. Results by Static IJethod. The curves of vapor pressures on a temperature base for three solutions are shown in Plates 11, 111 and IV, the data from which they were plotted being given in Table 1. To prevent condensation in the connection between the solution and manometer of the static apparatus, the stationary manometer was replaced by one having one arm maintained at constant level in the bath by raising or lowering the open arm. However with the solution at 120° C (paraffin bath), thi s necessitated a mercury column of approximately 2.5 metres high so that reliable data could not easily be obtained. gable 1, Solution No. 1, Wo . 2, No . 3, 1.38 Hormal, I.3 4 Formal, 2.2 3 Formal. Tenrpj Pressur e in mm: Pressur e in mm. Pressur e in mm; 8 12 Jl 16 18 20 21 23 24 25 28 32 33 35 37 40 43 45 46 48 50 32 33 55 56 57 60 62 U 70 72 75 77 80 81 82 £3 84 Si 22 88 90 91 92 93 94 _ -15.0 -17.4 18.2 -26.6 -29.8 ---48.8 -58.4 a* 74.4 --9 1 . 4 --1 1 2 . 8 --140 .0 -1 7 6 . 4 -212.8 -279.6 320 .0 350 .0 3 5 9 . 6 378 .8 -428.0 -----*• --12 16 --m _ 20 _ -26 3 1 m 38 41 45 51 58 67 _ 78 85 -97 112 -139 149 167 196 212 235 263 292 325 338 351 363 387** 439 455 491 513 536 549 567 „ — m 23.6 M _ w m 29 .8 M 3 4 . 6 40 .6 _ 4 6 . 4 53 .0 5 9 . 2 _ 7 8 . 8 83 .0 91 .2 98 .6 — 113 .0 — 123 .0 136 .6 1 5 1 . 8 1 7 2 . 1 196 .4 216.6 239.2 267.4 297.6 332 .0 3 6 4 . 0 3 8 2 . 4 412 .6 _ „ _ — — _ — PLFITt 77 1 4 soo 4O0 1 JSOO zoo /oo ynmm* GT. mefcu.ro _ ^ /ofaf fapor Prendre  of •/Y''Ae.c£ Ca.r  donccfe. So/cci Ay S/snJi/e.  /yTxryo^rj  €7^ ^ So/ution- /i< 5 /form <*/ . '•n degrees C. VMU / / / / 0 Co  s a / < »o /PLfiTb nt . i son i 400 300 ZOO . /OO rretiu-re jr> m/i?  of tyercuiy • Tofa./ 1/a.oor  Preau-rc  of f*f/Kcci Carbonate.  So/u.ti<. dy S//r?p/e  Wano/nete-r. an /•£>#-/Yof~sr>a-/--.r^A^ ""> degrees  C. f* •> 6 >a. / ) < 3 0 A 3 \ \ i o PLFITL , / v soo 4O0 300 ZOO /OJ) /n msn. of Trercartf 1 7 5 / W fa?  or  Pressor*  of •rf/xCci Ca.rhona.fc . 3o/aTio Au S>mp/e  /vfan  o/mefer. So/u. t,on.  ZZ3  /form  a  /. ^ n. I f 1 r / f i i ! 1 L . i ... -Tenr,^era.turt ZO  40  io  io  /oO \t*» Ueortes  Ci i 9. The determination of the total vapor pressure of the solutions was then attempted l>y  means of the isoteniscope or differential manometer (Plate V). Thi s consists of two glass bulbs of 200 c.c. capacity provided with stop-cocks and connected through a manometer. Th e apparatus is evacuated through the oj)en arms, the solution run into one bulb and dis-tilled water into the other bulb. Th e open arms are then sealed off below the stop-cocks. Th e apparatus is immersed in an oil bath provided with a glass window so that the manometer can be observed. Th e differences in the vapor pressures of the pure water and the solution, as the bath is heated, are read off from the manometer by means of a cathetometer and scale attached immediately behind the manometer. Th e vapor pressure of water and the density of the mercuty column at different temperatures are obtained from tables. The curves of vapor pressures on a temperature base for two solutions obtained by this method are shown in Plates Vll and Vlll, the data from which they were plotted being given in Table II. — JHLATZM. Table 1 1 . S o l u t i o n 1 . Norma l w i t h r e s p e c t t o X . Temp. °G 13 27 ••7 70 81 100 no 125 145 133 158 160 165 167 169 Uanometer D i f f e r e n c e C o r r e c t e d D i f f e r e n c e Differanee XV.P. of H20. + 1.54 c™ 1.48 1.68 2.11 2.82 3.41 3.76 4.36 5.40 3.50 3.11 2.40 .84 .37 .03 +-1, 1, 1, 2, 2, J. 3-4, 3 3. 3, 2, 339 479 665 085 78o 330 670 260 250 410 030 333 815 33? 029 C f* v Ha <A  O » C , 2.806 4 .126 9.372 25.416 3 9 . 7 3 1 79 .350 112.207 178.948 317 .805 412.27 444.98 467.50 528.27 554.50 581.92 Solution 1.5 Normal with respect to 19 31 46 34 72 p 86 *2 98 102 109 117 120 123 126 130 132 133 137 139 140 - . 3 1 - . 6 2 -.33 - . 1 6 +-.22 +-.91 + 1.3 9 1 .33 1.53 1.72 2.29 3 .06 4 . 4 5 5 .44 6.52 7 .75 8.02 7.96 6.48 4 . 2 5 3 . 3 4 - . 5 0 8 - .61 7 _ .34 7 - . 1 3 9 + .21 8 + .93 7 + I . 5 6 5 1.524 1.512 I . 6 8 9 2.250 2 .933 4 .365 3.320 6.360 7.560 7.840 7.760 6.315 4 .145 3 .455 1.124 2.720 7.166 11 .03 8 25.648 35.030 46.612 54 .071 72 .225 83.290 106 .215 138 .421 153.493 169.216 186.195 210.588 223.343 243.13 255.74 268.29 275.22 PL fire ss PL/9T£ ¥7' 1 -' i 1 i 1 zoo zzs /SO 75 Pre Hurt in cm.  of-rnercury 4tflm. 34f~, T&trr) Ifi+rr, 1 I 1 7o/"a/ l / a ^ o / PresSou-*  a/-Sf/%.ccL C(XrA>c,na.fe.  S'o/cc/'/  on. by J  So/e.nr  scope. 5o/u.f, on.  /'S~/forma./ femferafccrt 30  <o / / / / 1 / f / / 0 9 o n ~ to / . so 1 1 . Description and Operation of Air Saturation Apparatus* The solution was contained in the towers I (Plate IX) filled with glass beads and immersed in the constant temper-ature water bath. (Belo w 30° and above 70°, difficulty was experienced in keeping the temperature constant to within 1° since the heating was effected by means of a Bunsen burner and no temperature regulator was available). Ai r was drawn through the solution by means of the aspirator A and the water vapor absorbed in U-tubes (1) and (2) containing concentrated sulphuric acid on pumice stone and the carbon dioxide absorbed in the Griessler bulb G, containing concentrated (40^) ICO H solution and in U-tube (3) . Betwee n the towers, U-tubes, Giessler and aspirator, close rubber connections were used and the cork of the aspirator was sealed in with paraffin. Whe n closed at the inlet end of U-tube (1 ) and exit from the aspirator, the system did not leak appreciably with a pressure of 60 num. of mercury less than atmospheric in the aspirator. In all the runs, the pressure in the aspirator was never more than 55 m.m. of mercury less than atmospheric so that no error could have entered in the measurement of the volume of the air which was determined from the volume of water run out. This air saturation method is essentially similar to that used by Wiedermann, Stelzner and Uiedershulte (Verh . phys. Ges., 3, 159) an d by G.P. Baxter (J. Am. Chem. Soc, 2?, 1907, page 127) i n the determination of the vapor pressure of iodine, r~ PL/iTL *F» 12. and by Schumb (J. Am. Ghem. Soc, 43, 1?23, page 342) i n the determination of the dissociation pressures of certain salt hydrates. Schumb's method (loc. cit) consiste d in drawing a measured amount of dry air, free from carbon dioxide, through an intimate mixture of the pair of hydrates and collecting the water vapor thus taken up by the air in weighed U-tubes con-taining phosphorus pentoxide. Th e normal length of his runs was 18 hours during which time 6 to 8 litres of air were aspirated. Als o the temperature of his system never varied more than two degrees from room temperature so that he would not encounter the condensation difficulties which developed in this work. Baxter's method (loc. cit) wa s to pass a measured volume of pure dry air, first over the pure iodine, and then into a solution of sodium sulphite to absorb the volatilized iodine. From this sulphite solution the iodine was precipitated as silver iodide, which was collected and weighed. Hi s method of calculating the vapor pressure of the iodine from the v/eight of silver iodide was applied to this problem as given in the following example. Increase in U-tub e (l ) U-tub e (2) Giessle r U-tub e (3 ) Weight of .178 3 gm. .002 ? gm. .027 1 gm. .00?2 gm. .1814 gm. HO .036 3 gm. CO„ 2 2 Temp, of bath 6 0 Baromete r 73 ? m.m. Temp, of air I8.3 0 -4 3 m.m. in Aspirator. 1 3 . Volume o f a i r 130 3 c . c . -2 9 num . i n Tower . Vapor t ens io n o f wate r a t 18.5 ° i s 15. 8 m.m . Therefore th e a i r i n th e Asp i ra to r wa s unde r a  pressur e o f 759 -  4 3 -  15. 8 o r 698. 2 m.m . The volum e whic h t h a t a i r woul d occup y i n th e tower s i s 1305 x  m  x  6^8. 2 =  142 7 c . c . Wtt ~7To ~ .1814 gm . H2 O a t t h e t emp , an d p r e s s u r e i n th e tower s would occup y .181 4 x  2240 0 x  33 3 x  76 0 -  286. 2 c . c . T 8 ~ TT3  7J0 " " .0363 gm . G O a t t h e temp , an d p r e s s u r e i n th e tower s would occup y .036 3 x  2240 0 x  ^3^  x  76 0 -  23 . 3 c . c . 4T" "2"7 J 1JU T o t a l : 1736. 7 c . c . P a r t i a l p r e s s u r e o f H O -  286. 2 x  73 0 -  120. 3 m.m . d 1736. 7 P a r t i a l Pressur e o f G0 2 =  23. 3 x  73 0 =  9.9  m.m . To ta l : 130. 2 m.m . Total b y a i r s a t u r a t i o n metho d :  140. 0 m.m . Such c o n s i s t e n t l y lo w r e s u l t s b y th e a i r s a t u r a t i o n method, a s i n th e abov e example , le d t o th e conclusio n t h a t there wa s condensatio n o f wate r befor e absorp t ion . I f suc h wer e the case , i t woul d b e reasonabl e t o assum e t h a t wit h a  l a rg e volume o f a i r a s p i r a t e d , th e amoun t o f wate r remainin g unabsorb -ed woul d becom e a  neg l ig ib l e f r ac t io n o f th e t o t a l quant i t y o f water . Consequentl y a  l a rg e numbe r o f run s wer e mad e eac h 14, extending ove r a  per io d o f 9  t o 1 2 hour s i n whic h tim e 3  t o 3 l i t r e s o f a i r wer e a s p i r a t e d . I t wa s foun d howeve r t h a t , eve n wi th t h i s p recau t io n th e a i r s a t u r a t i o n metho d s t i l l gav e r e s u l t s cons iderabl y lowe r tha n th e t o t a l obtaine d b y th e s t a t i c method. The followin g f igure s fo r t h r e e run s sho w th e wid e v a r i a t i o n i n th e weight s o f CO ? an d wate r vapo r absorbed , Temp, o f Bath . 60 ° 6c r 60 ° Vol. o f a i r a s p i r a t e d 320 0 c . c . 32.5. 5 c . c . 328 0 c . c . Wt. o f H 20 absorbe d .136 4 gm . .421 0 gm . .2.51 9 gm . Wt. o f C0 2 absorbe d .128 3 gm . .130 8 gm . .093 8 P a r t i a l Pressur e o f H^O . '47, 6 mm , 113.7mm . • "  "  0 0 2 - 13, 9 "  14. 7 " Total b y a i r s a t u r a t i o n . 63. 3 "  130. 4 " Total b y s t a t i c . I49. O "  149. 0 u Boswell an d Gantel o (Can . Gh . an d Mat. , Yol . 7 1 , Ho.3 , May 1922 ) a l s o ha d troubl e wi t h t h i s metho d an d conclude d t h a t even wit h a l l poss ib l e car e "th e bubblin g metho d i s s t i l l sub -j e c t t o a n e r r o r s o l a rg e an d unce r t a i n a s t o mak e i t e n t i r e l y un t rus twor thy . " Attempts wer e the n mad e t o determin e th e p a r t i a l pressur e of th e carbo n dioxid e b y a  d i f fe ren t method , us in g apparatu s shown i n P la t e X . Thi s cons is te d o f a  f las k connecte d b y mean s of a  wire d rubbe r connectio n t o a  tub e provide d wit h a  stop-coc k a t eac h en d o f th e enlarge d p o r t i o n . Th e apparatu s wa s evacuat -ed, the n so lu t io n ru n i n t o th e f l a sk , stop-cock s bein g open . ?*£.J9T£ U> £ 15. The apparatu s wa s the n immerse d i n a  constan t temperatur e bat h for 3  t o 4  hour s wit h intermit ten t agi ta t ion , afte r whic h tim e stop-cock S  wa s closed , thu s i so la t in g a  sampl e o f th e vapo r in equilibriu m wit h th e solutio n a t tha t temperature . O n cooling, t h i s sampl e o f ga s wa s analyze d b y standar d method s for i t s carbo n dioxid e content . Owing t o th e di f f icul t y i n keeping ' th e stop-cock s t igh t in eve n a  moderatel y war m bat h an d i n closin g stop-coc k 3 without exposin g th e connections , thu s disturbin g th e equi l ib -rium, re l i ab l e dat a coul d no t b e obtaine d b y thi s method . 16. Summary* (1) Stati c methods have been described for obtaining the total vapor pressure of carbonate solutions, and results by simple and differential manometer methods have been tabulated and plotted. (2) Ai r saturation methods, for determining separately the partial pressures of carbon dioxide and water vapor from mixed carbonate and bicarbonate solutions have been described and difficulties in their operation pointedout. In conclusion I desire to express my indebtedness to the Advisory Research Council and to Dr. E.H. Archibald for encouragement and assistance. 


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