JOURNAL OF RESEARCH of the Notional Bureau of Standards - A. Physics and Chemistry Vol. 72A, No.3, May- June 1968 The Thermodynamics of the Ternary System: Water-Potassium Chloride-Calcium Chloride at 25 °C R. A. Robinson I and A. K. Covington 2 Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (January 26. 1968) Iso piesti c va por pressure measure ments have been made on the syste m: water-potassium chl oride­ cal ci um chloride at 25 °C. The osmoti c coeffi cients of the mixed salt solutions and th e activity coeffi ­ cients of e ach salt in the presence of the othe r ha ve been evalu ated. Key Words: Activity coeffi cient, calcium c hloride., isopi esti c measure ments, mixe d salt solutions, os moti c coeffi cients, potassium chloride, vapor pressure. 1. Introduction y~ is the activity coeffi cie nt of potassium c hl uride in a solution containing pot assium chlor­ Isopi esti c vapor pressure meas ure me nts have been ide onl y at the same total io ni c stre ngth reported for the sys te ms: wate r-sodium chl oride­ 1= 17111 + 3mc as the mixe d soluti o n. barium chloride [11 ,3 water- potassium chloride-barium y7· is the acti vity coeffi cie nt of calcium chl oride chloride [2J, and water-sodium c hloride-calcium in a solution cont a ining onl y ca lciu m c hl oride chloride [3j. R esults for the syste m : water-potassium at the same total ioni c s tre ngth , t. chloride-calcium chloride are now describe d. <p = -55.51 In awl (2 m ll + 3mc) =- 55.51 In awl [ (YH + 1) I] is the osmoti c coeffi cient of the mi xed salt solution. 2. Definitions <p71 is th e os moti c coeffi cie nt of the soluti on con­ tain ing po tassium c hl oride only at the same tota l io ni c stre ngth as th at of th e mixed salt Mil is the mola lit y (moles per kil ogra m of water) of solution. a solution containing onl y potassium chl oride; <p~ . is the os moti c coeffi cie nt of the solution con­ it is the " reference" from whic h isopi esti c taining calcium chl oride o nl y at the same total data are calculate d. ioni c stre ngth as the mi xed salt solution. Inll , In( ·, are the molalities of potassium c hloride a nd calci urn chloride, respecti vely, in a solution co ntaining both these salts a nd in isopiesti c 3. Experimental Procedure and Results (va por phase) equilibrium with the potassium chloride solution of molality M/i . The salts were portions of those used in earlier wo rk 111 = 1111< + 1.5 Inc . [2 , 31. Isopiesti c va por pressure meas ure me nts we re R = Mli lm is the iso piesti c ra ti o. made in the ma nne r describe d previous ly [11 . The Xc = 1. .') mr-/ m. results are give n in tabl e 1. 1 = mfj + 3mc is the total ioni c stre ngth of the solutio n. YII = m./lil . 4. Discussion Yc = 3mclt . YII , Ye, are the activity coeffi cie nt s of potassium The immedi ate result of a n isopi estic measure me nt chlori de a nd calcium c hloride , respecti vely, is the information that a solution containing two salts in the mixed salt solution. of known molality has the same aqueous vapor pres­ I Present address: De part ment of Che mi stry. S iale Un ive rsi ty of Ne w York al Bing­ s ure as a solution of one of the salts, again at a known hamt on. N .Y. 139 01. Z (:; ues l wo rk er (1966) on leave from th e Uni vers it y of Newcastle u pon Tync, England. molality. Such informati on is readily transformed into 3 Figures in brac kets indicate the lit erature refere nces at the end of't hi s paper. osmoti c coeffi cient d ata. Acti vity coeffi cie nt data, 239 TABLE 1. /sopiestic data for the system: potassium chloride (B)- calcium chloride (el Set M" m 'H mc 'P(ob s.) 'P(ca le.) Set Mil mil m e 'P(obs.) c,t'(calc.l 1 0.7560 0.5219 0.1564 0.8964 0.8942 0.6726 1.2065 1.1193 1.1182 .2903 .3087 .9002 .8990 ............... 1.5264 1.2136 1.2109 .1347 .4072 .9095 .9055 2.9694 1.8591 0.6052 1.0044 1.0048 ............. .4948 .9141 .9140 1.0832 1.0040 1.0733 1.0728 0.7577 0.6384 0.0789 0.8987 0.8944 0.3690 1.3528 1.1588 1.1570 .4168 .2267 .8980 .8957 .1929 .3723 .9047 .9031 7 3.4262 2.8230 0.3250 0.9815 0.9811 1.7945 .8579 1.0545 1.0552 2 1.1535 0.9288 0.1440 0.9051 0.9041 0.8574 1. 3132 1.1494 1.1510 .5890 .3579 .9204 .9207 3.4611 2.2892 0.6221 1.0196 1.0193 .2704 .5525 .9427 .9434 1.2082 1.1605 1.1142 1.1151 ... ........... .7140 .9675 .9691 0.3930 1.5399 1.2156 1.2153 ....... ...... .. 1. 7164 1.2761 1.2742 3 1.5135 1.3037 0.1310 0.9108 0.9117 0.9350 .3559 .9302 .9327 8 3.7176 2.7254 0.5244 1.0125 1.0128 .5999 .5518 .9571 .9569 1.6537 1.0560 1.0983 1.0982 .............. .8952 1.0175 1.0180 *0.5957 1.5476 1.2190 .. .. ... ....... 1.5240 1.0898 0.2681 0.9223 0.9240 3. 7221 3.1867 0.2866 0.9845 0.9848 0.7744 .4569 .9427 .9448 2.2269 .7784 1.0489 1.0492 .4029 .6734 .9740 .9759 1.1 373 1.3036 1.1513 1.1529 .. .. ........... 1.8128 1.3094 1.3089 4 2.0809 1.2658 0.4755 0.9613 0.9641 0.7063 .7865 1.0087 1.0118 9 4.3263 3.7758 0.2844 1.0031 1.0018 .2483 1.0229 1.0672 1.0600 *2.6197 .8587 1.0784 ......... .. ... 2.1 530 1.7491 0.2373 0.9370 0.9383 *1.6439 1.3151 1.1653 .............. 1.0520 .6320 .9862 .9881 ............... 2.0262 1.3871 1.3859 0.4806 .9403 1.0431 1.0434 4.3543 3.2196 0.5776 1.0394 1.0380 ............. 1.1896 1.1050 1.1041 *1.9778 1.1736 1.1360 . ............. *0.5997 1.7831 1.2970 ... ....... ... 5 2.7872 1.9415 0.4654 0.9833 0.9813 0.6796 1.1226 1.0982 1.0954 10 4.6033 3.8770 0.3749 1.0188 1.0217 .3588 1.2791 1.1397 1.1349 *2.4736 1.0524 1.1161 ........... ... 2.7986 2.3180 0.2674 0.9589 0.9585 *1.0844 1.6724 1.2607 .. ... .. .... ... 1.4805 . 7194 1.0185 1.0178 ............... 2.1195 1.4226 1.4206 1.0515 . 9406 1.0586 1.0567 4.6201 *3.1511 0.7405 1.0656 .............. *1.5840 1.4625 1.2022 .............. 6 2.9689 2.4640 0.2806 0.9632 0.9645 *0.5200 1.9134 1.3396 .............. 1.5330 .7747 1.0310 1.0307 *Not used in computing values of {Jo in eq (26). how ever, have to be obtained indirectly. One method was adequate, it would be difficult to handle the inte­ achieves this objective by means of the McKay­ gration needed in the McKay-Perring method. Perring procedure [4J. It was shown earlier [1] that if An alternative, but equivalent, approach, based on the isopiestic ratios, R , can be expressed in terms of the work of Scatchard [5] , is now presented. As this the ionic fraction, xc, by means of the equation should have wide application to the study of mixed salt solutions, it will be described in some detail. R = l + axc, (1) Consider, first, a solution of two salts each of the 1: 1 charge type. Let the activity coefficients of the the application of the McKay-Perring method is very salts in solutions of constant total ionic strength simple. Even if an additional term is needed, (I = mB + me if the salts are of the 1: 1 charge type) R = 1 + aXe+ bx't·, (2) be capable of representation by the equations, it is still possible to use the method. In 'YB = In 'Y~ + QBYe + RByt. (4) Figure 1 shows some of the isopiestic data ob­ tained in the present work_ It will be observed that there is considerable curvature in the plots, particu­ In 'Yc = In 'Y~+ QCYB + R e~ . (5) larly for those corresponding to solutions of low total , I ioni c strength. It was found that eq (2) was inadequate Here YB = mBII , Yc= melI for salts of this charge type. to re present the experimental results and even the QB , Qe, R B, and R e may be functions of I but not of addition of a further term in x~ ga ve only slightly better YB, Yc individually. agreement. However , even .if the equation It has been shown, by the use of the Gibbs-Duhem equation, [2, 6, 7] that, if eqs (4) and (5) hold, the R = 1 + aXe + bx~ + cx~ (3) osmotic coefficient of the solution is given as 240 Combining e qs (6) and (7) , we get SET 10 1 cp = )'IICPYi + YC CP~ + "2 YHYI(QII + OC) 1 1. 4 +"3 YIJ yr(YIJ - yc) (Rc - R II ) ' (8) S in ce CPYi and cpV· depe nd only upon properties of solu ­ ti ons containing a single salt, the departure from id eal­ ity on mixing the single salt solutions can be repre ­ sented as /).