J. Phys. Chem. 1981, 85, 890-894
89 0
Representation of the Solubility of CuCl in Solutions of Various Aqueous Chlorides J. J. Fritz Depatfment of Chemistry, The Pennsylvania State Unlversky, Unlvers@ Park, Pennsylvania 16802 (Received: September 5, 1980)
Solubility data for CuCl in aqueous solutions of NaCl, KC1, and NH&l have been represented over the temperature range from 0 to 100 "C, using equilibrium constants previously obtained for formation of complex species and selecting virial parameters to optimize the fit for each substance. In the process, values were obtained for the heat of solution of CuCl to form Cu2Clt- (previously not known) and to form triply charged complexes (previously known only very roughly). For solutions in NaC1, sufficient precise data were available to permit evaluation of three virial parameters for each of four complex species. For the other solutions, the number and precision of the data required representation with fewer parameters. A "three-parameter" fit, applicable to systems of limited data, is described. It appears adequate to represent the solubility data to 5 % (or better) whenever the data are consistent to this precision.
Introduction In a previous paper,' solubility data on CuCl is aqueous HC1 solutions were used to evaluate equilibrium constants at 25 "C for the solution of CuCl to form CuC12-,CuC12-, Cu2C1?-, and complexes with triple negative charge (treated as Cu3C12-); these constants are denoted there (and hereafter) as K,, K,,, K24,and K%,respectively. The heats of solution corresponding to K,, and KB3were also determined. In fitting the solubility data, the variation of activity coefficients with concentration was represented by the virial model of Pitzer and c o - w ~ r k e r sin , ~which ~~ the second virial coefficient for each ion pair is calculated as a function of concentration from parameters @(O) and $l), with the coefficient C taken to be independent of concentration. The equations used for the activity coefficients were a simplified version of those of Pitzer and Kim,3 with molarity (M)substituted for molality (m) throughout. They are listed in Appendix I. Their application to calculation of solubility has been discussed in detail previously.' Correlation of the extensive data on solubility of CuCl in HC1 solutions required simultaneous optimization of the values of the three parameters for each of the four ion pairs (HCuCl,, etc.) considered, as well as each of the four equilibrium constants. The availability of equilibrium constants and their temperature coefficients makes possible the correlation of the solubility of CuCl in aqueous solutions of other chlorides after evaluation of the virial coefficients for the ion pairs formed between the various complex species and the appropriate cations. Determination of the virial coefficients can be carried out with data sets of modest size and limited precision, so as to represent the data within the limits of their internal consistency. In addition, knowledge of the temperature dependence of the equilibrium constants makes it possible to combine data at a variety of temperatures into a single correlation. The final results permit the prediction of the solubility at concentrations and/or temperatures where experimental results are not available. This paper describes the application of these procedures to three CuC1-chloride systems. In only one case (NaC1) were there sufficient precise data available to permit evaluation of all 12 parameters. In the others (KC1, "&I), lack of quantity or quality of data required using a simpler set of parameters. The "three-parameter" cor(1) Fritz, J. J. J. Phys. Chem. 1980, 84, 2241. (2) Pitzer, K. S.; Mayorga, G. J. Phys. Chem. 1973, 77, 2300. (3) Pitzer, K. S.; Kim, J. J. J. Am. Chem. SOC.1974, 96, 5701.
relation described below appears quite adequate for situations where a precision of 5% or worse is all that is sought or is attainable from the data at hand. The data set for HC1 solutions extended from 15 to 35 "C and provided reliable heats of solution of CuCl to form CuC12- and CuCl:-. In order to consider data at temperatures far outside this range, we found it necessary to get the heats corresponding to K N and KW This has been done with the aid of less precise data on solutions in HC1 and NaCl extending from 0 to 100 "C, and is described below. Most of the data at temperatures other than 25 "C left much to be desired in terms of precision and consistency. Some data were reported only in the form of graphs, and others showed internal inconsistencies of 10% or more. However, in all cases the range of solubilities reported spanned at least an order of magnitude. We believe that the results obtained are valid within the range of precision of the data.
Solubility in NaCl Solutions The only precise and reliable data for the solubility of CuCl in aqueous NaCl solutions are those of Ahrland and Rawsthorne4at 25 "C. These extend from 0.01 to 5.0 M chloride ion concentration, all at a constant (nominal) ionic strength of 5.0 M maintained with NaC104. There is a limited set of data at 19 "C, due to Fedotieff? extending from 0.8 to 2.1 M in "pure" NaCl (no inert electrolyte). Finally, there are the data of Utkina et al.: who measured the solubility up to about 6 M NaCl at 0, 25, 50, 75, and 95 "C. Of the three sets of data, only those Ahrland and Rawsthorne4 are sufficiently extensive (32 points) and precise (- 1%) to permit evaluation of all of the virial coefficients. However, since all of their data were for a single concentration of (Na+),they did not permit separation of the second and third virial coefficients for the various ion pairs. A complete set of virial coefficients was obtained by optimizing the fit of the Ahrland and Rawsthorne data, while at the same time adjusting the third virial coefficients (C)so as to get the best possible fit to the Fedotieff 19 O C data and the Utkina 24 "C data (5 points each). By this procedure, the Ahrland and Rawsthorne data could be fitted to about 1.4%, and the other data to about 10%. (4) Ahrland, S.; Rawsthorne, J. Acta Chim. Scand. 1974, 24, 157. (5) Fedotieff, P. P. Z . Anorg. Allg. Chem. 1928, 173, 81. (6) Utkina, I. N.; Kunin, T. I.; Shutov, A. A. Izu. Vyssh. Ucheb. Zaved, Khim. Khim. Technol. 1965, 12, 706.
0022-3654/81/2085-0890$01.25/00 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 7, 1981 801
CuCl Solubility in Chloride Solutions
TABLE I : Virial Coefficients for Complexes Formed from CuCl in NaCl Solutions species CuC1,CUCI, c u c142c u 3 c1,3-
,
,-
p(0)
0.0837 0.0896 -0.0169 0.1677
p(')
0.1595 1.1778 0.6372 3.4014
TABLE 11: Virial Coefficients for CuCl Complexes in KCl Solutions
C 0.0098 0.0329 0.0154 0.0209
species CuCl, Cuc1,*Cu,C1,2CU,CI,~-
p(0)
p(')
C
0.2837 0.0480 -0.1719 -0.1123
0.4180 0.9540 -0.7000 2.1333
0.0098 0.0114 0.0076 0.0219
Unfortunately, the quality of fit was highly insensitive data used were those of Bronsted' at concentrations where to the partition of doubly charged species between CuC12CuCl was observed to be in the solid phase. These data and Cu2Cld2-. The formation constants deduced by were reported in weight units, and the necessity for conAhrland and Rawsthrone4give about equal fractions of the verting them to molarities limited the usable data further copper in these two ions, with about 50% uncertainty in to those where the KC1 concentration was less than 4.0 M. the distribution. The optimum fit of the present work The Bronsted data appeared consistent to about 5% and gives over 90% of the doubly charged species as C U ~ C ~ , ~ - . were fitted to an average (rms) deviation of 4.1% except However, the quality of fit [root-mean-square (rms) defor three points below 1.4 M which were extraordinarily viation] varied very little with the partition (1.4% at 57% low (deviations of up to 60%). Because of the limited vs. 1.3% a t 93%). For the present purposes, the paramnumber of usable data points (twelve), only the p(O)s and eters giving 57% present as Cu2C1t- will be used. The Cs were varied independently. The p(l)swere taken from virial coefficients giving the best fit for this distribution standard curves representing typical variation of p(l)with are given in Table I. Compared with values previously p ( O ) as given by Pitzer and Mayorga.2 obtained for HC1, the ps are all substantially lower. In fact, The virial coefficients for the best fit of the data are the value of p(O) for NaCuC12 is 0.1100 lower than for given in Table 11. The p(O)s and p(l)sfor the doubly and HCuC12,compared with a similar lowering averaging 0.108 triply charged species are less positive (or more negative) for NaC1, NaBr, NaI, NaClO , and NaN03 with respect than those for NaC1, reflecting the higher solubility of CuCl to the corresponding acids.2 ptl) for this species is likewise in solutions at higher molarities of C1-. The large positive about 0.1 lower than for HCuCl,, although P(l) for NaCl values for CuClf are required to fit the low solubilities is nearly as large as for HC1. No similar comparisons can reported for the region 1.2-2.0 M, which may be spurious; be made for the doubly or triply charged species for lack however, since CuC12- appear to contribute only slightly of other 1-2 and 1-3 salts to compare with. The third virial to the solubility above 1.4 M (about 5% a t 2.2 M), this coefficients, C, are of the same order of magnitude as for should not introduce any substantial uncertainty in the those in HCl, two being smaller and two larger. other virial coefficients. Of the double charged species, Ahrland and Rawsthorne4evaluated formation constants Cu2Clz- appears to make up about 40% of the total at 1.7 for CuC12-, CUC~,~-, C U ~ C ~and ~ ~cu3c163-, at a nominal M, rising to about 70% at 2.9 M. ionic strength of 5.0 M, assuming that these constants Refinement of Heats of Solution depended only on the ionic strength. In terms of the Ks for formation of these species from CuCl(s), their values In earlier work,l the data of Hikita et al.1° on HCl soare KBl= 0.0405, K , ' = 0.0415, K24/= 0.021, and K34 = lutions at 15,25, and 35 OC were used to obtain the heats 0.0021. For low (el-), our results are in reasonable of the processes agreement with theirs; viz., Ksl = 0.0405, K8; = 0.0326, CuCl(s) + c1 = cuc12(1) KB1= 0.0238, and K,' = 0.00226. However, each K'except K8iincreases with increase in chloride ion concentration; CuCl(s) + 2c1- = cuc132(2) a t 5.0 M (Cl-), Ks3/ and K24/ have each increased nearly 30%, and K34 has about quadrupled. An extrapolation and a rough value for the average heat of solution to form method such as used by Ahrland and Rawsthorne can at triply charged complexes, represented as Cu3Cb3-. In that best get the Ks appropriate to very small values of the work the equilibrium constants for the four solution procomplexing agent and may obtain only an order-of-magcesses involved were determined independently at each of nitude average. the three temperatures so as to give the best possible fit to the solubility at that temperature. The data at 15 and Solubility in KCl Solutions 35 "C were relatively insensitive to the equilibrium conThere are no data for the solubility of CuCl in aqueous stants for c u 2 c 1 ~(K2J and cU3c1G3- (KS6);the resulting KC1 solutions of the quality of those available for NaCl constants did not follow a van't Hoff relationship for and HC1 solutions. Data have been reported by Bronsted,' Cu2C14and followed one only roughly for c U 3 c & - . Valeton and Fromel: and Malik et d9 Of these, only the In order to obtain more reliable values for those latter Bronsted data (at 22 OC) proved consistent enough for use heats, we made more extensive examination of the effect in evaluation of virial coefficients. All three authors agree of temperature on solubility, using not only the Hikita that the solubility in KCl solutions is substantially higher data, but also the data of Morosov and Ustavshikovall on than in NaCl solutions, but the Malik data are extremely solubility in HC1 solutions from 0 to 100 "C and those of high and irregular, while the Valeton data are both high Utkina, et ale6on solubility in NaCl solutions from 0 to 95 and require formation of complexes with fewer than two "C. Although the latter two sets of data were much less chlorine atoms per copper, a sort of complex definitely not precise and consistent (- 10-20%) than the Hikita data, observed in either NaCl or HC1 solutions. All three authors they covered a temperature range such that the solubility agree that a double salt becomes the stable solid phase at varied by factors of 3-4 for a given molality of chloride, high KCl concentrations. For present purposes, the only far beyond the uncertainty in the data. ~~
(7) Bronsted, J. N. 2. Phys. Chern. 1912, 80,206. (8) Valeton, J. J. P.; Fromel, W. 2. Anorg. Allg. Chern. 1924,137,91. (9) Malik, W. U.; Fazlur Raman, S. M.; Anwar Ali, S. 2. Anorg. Allg. Chern. 1959, 299, 322.
(10) Hikita, H.; Ishikawa, H.; Esaka, N. Nippon Kagaku Kaiishi 1978,
1, 13.
(11)Morosov, I. S.; Ustavshikova, G . V. Isuest. Akad. Nauk. SSSR 1944, 451.
892
The Journal of Physical Chemistry, Vol. 85, No. 7, 1987
TABLE 111: Equilibrium Constants and Heats of Solution of CuCP species formed
notation for K
AH cuc1, 0.0604 6653 (6610r 50) Ks, cuc1,20.0128 3320 (3170f 110) KS3 0.00082 6100 c u *c1,2K24 CU,Cl, 3K36 0.000034 1420 combined 2- K S 3+ 2 K , , 0.0144 3750 a K s are for concentrations in molarity; AHS are calories per mole of complex. K29,
With the values of the equilibrium constants at 25 "C from previous work,l the AHs were varied to obtain the best (rrns) fit to the Morosov and Utkina data without impairing the fit to the Hikita data. In this process all virial coefficients were held at their 25 "C value for the less precise data; a small variation with temperature in the Ps for CuC1, was used with the Hikita data to improve the fit, as bef0re.l The final fit to the Hikita data was excellent (1.1% rms). The fits to the Utkina data varied from 6% (at 75 "C to 19%, (at 0 "C), 13% overall. The Morosov data, which had the least precision, were fitted to an average of 16% (variation 11-25%). The results for AH2 and AH3 were insignificantly different from those obtained previously; the result for AH36 was substantially lower. Table I11 gives the results for the heats of solution, along with the best values, from previous work,l for the corresponding equilibrium constants at 25 "C. In addition, the value of (K3+ 2KZ4)is given along with the "heat" describing its variation with temperature for use as described in a later section. The values reported previously' for the heats are given in parentheses. A number of features of these heats of solution should be noted. First, for lack of necessary data, no attempt was made to account for possible temperature variation of AH. Thus all results represent averages over the temperature range covered. Second, the heats were required to absorb any efforts due to the unknown variation with temperature of the virial coefficients. Finally, because of the lack of precise data at the higher concentrations, the heats for CUC~,~and Cu3ClG3-are rather uncertain, possibly to 25 or 30%; it is especially probable that one of them is high and the other low. Application of these heats of solution requires the equilibrium constant for formation of CuClL to increase more than 25-fold between 0 and 100 "C, and that for CuzC1t- by ZO-fold, while those for CuC13- and Cu3C12increase only 5-fold and 2-fold, respectively. An immediate consequence of this fact is that the solubility at low halide concentration increases more rapidly with temperature than that at high halide concentrations, as can be seen directly in the available data. An indirect, but not surprising, consequence is that the simpler ionic species become more important at high temperatures and the more complex ones at low temperatures. In any case, the heats and the equilibrium constants at 298 K given in Table I11 can be used to obtain reasonably reliable equilibrium constants at any temperature between 0 and 100 "C. These can then be used with known or estimated virial coefficients for any chloride to predict solubilities of CuC1. Solubility in NH&l Solutions The only data obtainable on the solubility of CuCl in NH4C1solutions were those of Morosov and Ustavshikova.ll They were of particular interest because the solubilities given are the highest reported for monovalent chloride solutions. The data range between 0 and 100 "C, with only a few points at each temperature, all given only
Fritz
graphically on a weight percent basis. Selection of only those points at sufficiently low concentrations to permit reasonable conversion to molarities provided a total of 13 points over the entire range, at a variety of NH4Cl concentrations ranging from 1.35 to 4.5 M. Because of the scarcity of points, only a few parameters could be evaluated. The number of parameters was limited as follows: 1. The species with doubly negative charge were represented at CuCls2-. The corresponding equilibrium constant was taken as K 3 + 2K2,, with its temperature variation represented by an effective "heat of solution" intermediate between those of the two individual species; both are given in Table 111. 2. The third virial coefficients, C, were taken as those for HC1 (note that those for NaCl were of similar magnitudes) for the three species considered [CuCl;, CuCl:-, and CU~C&~-]. 3. All P(l)swere derived from the corresponding as described earlier for KC1 solutions. 4. All virial coefficients were taken to be independent of temperature. This procedure left only three parameters (the P(O)s) to be evaluated. At optimum, the data were fit to 19% rms, with all points included, and to 13% when only the points below 100 "C were used. The latter fit appeared quite consistent with the uncertainty of reading the graphical data plus that of converting the higher concentrations used from weight to volume basis. The results at 100 "C are questionable in any case, since, at concentrations higher than any used in this work, Morosovll reports mole ratios of CuC1/NH4C1in greater than unity in his solutions, which would require complexes with Cl/Cu less than two (e.g., Cu2C1J. Sukhova et al.12 invoked the presence of such complexes to explain their potentiometric study of CuCl in 6.5 and 1 M NH4C1, but do not present actual data. There is no evidence for complexes of this sort in any other system, particularly not for NaCl solutions, where Ahrland and Rawsthorne4examined both solubility and potentiometric data and concluded that these sorts of complexes did not contribute measurable. It is, of course, possible that the especially high solubility of CuCl in high concentrations of NH4C1 comes about partly because conditions permit such additional complexes to be formed in appreciable concentrations. For the representation used here, all P(O)s were negative. They are given in Table V, along with similar sets for the other systems considered. Three-Parameter Representation of the Various Solubility Data Where a sufficient number of data points of adequate precision are available for a particular system, it is certainly desirable to evaluate and use all of the virial coefficients, as done here for NaCl and earlier for HC1.l Since this requires 12 parameters in all, at least 20 data points are needed, and a precision (or consistency) well under 5% is required for unambiguous choices. As can be seen from the various sources of data cited, such a set of data may not be available, or the end use may not require representation even as good as 5%. In such cases, a simpler procedure may well be adequate. To examine this possibility, the available data for HC1, NaC1, and KC1 were subjected to the same type of 3species, 3-parameter fit as described above for NH4C1. Where more than one data set was available, the parameters were selected so as to optimize the fit to the set with (12) Sukhova, T.G.;Temkin, 0.N.; Flid, R. M.; Kaliga, T. K. Rum. J. Inorg. Chem. 1968, 13,1072.
The Journal of Physical Chemistry, Vol. 85,No. 7, 198 1 093
CuCl Solubility in Chloride Solutions
TABLE IV: Evaluation of 3-Parameter Procedurea data set used
no. of points
av % dev 3 parameters 1 2 parameters HCl
Hikita,'O "pure HCl" only Hildta," all points Chang,1325 "C Morosov,ll 0-100 "C
16
1.20
0.88
104 10 27
2.75 4.03 19.3
1.20 3.45 16.5
NaCl ~hriand,4 f i 0 =.5 M F e d ~ t i e f f 1, ~9 "C Utkina,, 0-95 "C
32
1.47
1.42
5 23
10.30 13.0
10.20 12.20
Bronsted '
12
a
KC1 5.40
4.0815
Reference 14.
TABLE V : Virial Coefficients for 3-Parameter Procedure chloride used
species
p(1)
C
HCl
cuc1,cuc1,zcu,c1,3-
0.2097 0.2749 0.4473
0.3556 1.5878 4.5590
0.0107 0.0254 0.0215
NaCl
CuCl,' cuc1,zCU,Cl, 3-
0.0747 0.0619 0.1294
0.2052 0.9999 3.1185
0.0107 0.0254 0.0215
KC1
cuc1,CuCl ,2cu,c1,3-
0.3221 - 0.0503 -0.1821
0.4635 0.4495 1.9210
0.0107 0.0254 0.0215
NH4Cl
CuC1,c u c1,2c u , c1,3-
-0.1422 -0.0677 - 0.1775
0.0545 0.2610 1.893
0.0107 0.0254 0.0215
p(0)
the best precision. Table IV compares the quality of fit for the 3-parameter system with that obtained from the more extended set of parameters for each of the data sets examined. In addition to data from works previously cited, those of Chang and Cha13 for solubility in aqueous HC1 are included in the comparison. As would be expected, the fit of the 3-parameter procedure is inferior to that of the full 12-parameter treatment, but not drastically so. For the relatively imprecise data of Morosov, Fedotieff, and Utkina, there is little to choose between them. The 3-parameter system should be quite adequate for correlation or prediction of solubilities to the order of 5%, provided there is a data set of at least this precision available for the selection of the parameters to be used. Table V gives the virial parameters used for each of the three species considered for HCl, NaC1, KCl, and NH4C1. Only the were varied independently in optimizing the fit to the data; the ,8% are given to define the curves used to obtain the PC1)appropriate to a given P(O). Except for low concentrations of aqueous KC1, these parameters, along with the equilibrium constants and heats given in Table 111, should be adequate to predict the solubility of CuCl (13) Chang, K. S.; Cha, J. T. J . Chinese Chem. SOC.1939, 2, 298. (14) For HC1, the 12-parameter set was optimized for the full 104-point set, composed mainly of points for which mixtures of HCI04 and HC1 were used to produce solutions at six constant nominal ionic strengths. The 3-parameterset was optimized for the 16 data points in which HCl alone was added. For NaCl, both sets of parameters were optimized for the Ahrland data, in which NaCl-NaC104 mixtures were used in constant nominal ionic strength of 5.0 p. (15) For solutions in aqueous KC1, because of the limited number of data points, only 8 of the 12 parameters were varied independently (see above).
in solutions of these chlorides from 0 to nearly 100 "C, for chloride concentrations between about 0.1 and 6.0 M. Again, except for KC1, all PSs decrease with increasing overall solubility of CuCl, the effect being most marked Cu3C12-. The high value given for CuC12- in the case of KC1 may be spurious, as discussed in an earlier section.
General Discussion Applications described above indicate that the methods employed previously to fit solubility of CuCl in HC1 solutions can be used to represent data on its solubility in other aqueous chloride solutions merely by evaluation of the necessary virial coefficients for the ion pairs formed between the complex species and the cation involved. If sufficient data points of good precision are available, all three parameters [$O), @(I), and C] can be obtained for each of the four major complex species. If the data points are limited in number or of low precision (more than 5 % uncertainty), a single parameter [,B(O)] can be used for each type of charged complex (singly, doubly, and triply negative), without serious deterioration in precision. In this case the are determined from the P(O)s, and a standard set of values for C is used. Prediction of the solubility in the presence of "inert" electrolytes can be made as well as those for "pure" chloride solutions, provided that the virial coefficients are known for the added ion pairs introduced by the inert electrolyte. The heats of solution and equilibrium constants given in Table 111 can be used to predict solubilities at temperatures where measurements are not available, within the range 0 to 100 "C. These give excellent results for the effect of temperature between 15 and 35 "C, where data of about 1% precision are available. Outside this range, they fit the relatively imprecise data uniformly to about the consistency of the data (10-15%). The constants P(I)and PCo)which define the second virial coefficients all follow the trends of solubility as a group, all being lower (or more negative) for chlorides of the heavier (or bulkier) cations for which the solubility of CuCl is greater. In the two cases (HC1, NaC1) where it was possible to get good values for the third virial coefficient, C, these values for the complexes were similar for both salts, although substantially higher than those for simple ion pairs, such as HC1 and NaC1.2 This fact contributes significantly to the rapid increases in solubility at high concentrations of the various chlorides. Although the simple complexes CuC1,- and CuC132-account for most of the solubility at low chloride concentrations in all cases, at high chloride concentrations a large portion of the solubility appears to be due to polynuclear complexes such as CuzC1z-and Cu3Cle3-. This effect becomes more pronounced in those chlorides in whose solutions CuCl is most soluble. The KC1 and NHJl systems pose interesting problems for future study. In both these systems, past investigators have reported regions of concentration where the stable phase is not CuCl, but a double salt of CuCl with the other halide. These equilibria can certainly be treated by the same procedures. The same virial coefficients for the solutions could be used as before, with the relevant solubility data used to evaluate the equilibrium constants for the new set of solution equilibria. However, the data available on solutions at equilibrium with a double salt are scant, and no attempt has been made to correlate them at this time. The parameters and thermodynamic data given above can be used to predict the solubility of CuCl in aqueous solutions in HCl, NaC1, KCl, and NH4CIat concentrations up to about 6 M and at temperatures from 0 to 100 "C. Near 25 O C an uncertainty near 1% is to be expected for
894
J. Phys. Chem. 1981, 85, 894-901
the solubility in aqueous HC1 and aqueous NaC1. This uncertainty rises to about 10% for temperatures far from 25 "C for these systems and for all temperatures for aqueous KC1 and NH4C1,where the available experimental data are scant and of comparatively low precision. The procedure can also be used to calculate the solubility of CuCl in mixtures of soluble chlorides over the same ranges of temperature and overall concentration. Somewhat greater uncertainties are to be expected in calculations for the mixtures, since the simplified model used contains no terms to account for interactions between the cations. Practical application of these correlations can be made in any situation where CuCl is processed in solution, as for example in the preparation of CuCl for use as a catalyst. Another example is the Cymet process of receiving of copper from its ores, in which the copper is isolated as CuCl from aqueous solution, then reduced chemically to metallic copper. In both these examples, the effects of temperature and of salt concentrations on the solubility of CuCl are of major importance in the procedure. In the second example, rather complex mixtures of cations are encountered, but they should be amenable to treatment once the necessary virial parameters are established for ion pairs consisting of the cuprous complexes and the individual cations.
Appendix I. Equations for Mean-Ion Activity Coefficients
BNX
= @Nx(O)+ (2@NXc1)/cy2r) [I -(I
BNX'
(2@NX"'/cy212)[-1
+ a N 2 ) e~p(-cyl/~)]
+ (1+ C Y P+/ ~1/2cy2r) exp(-W2)]
(CMZ) = CMg, C
= 2.0 These equations are simplified versions of eq 12-15 of Pitzer and Kim,3 with molarity (M) used instead of molality (m)throughout. The symbolism is otherwise that of Pitzer and Kim.3 cy
Solute-Solvent Interactions in Ion-Pair Extraction of Tetraalkylammonium Iodides. 1. A New Approach to the Extraction Constant' Etsuro Iwamoto, Karuaki Ito, and Yuroku Yamamoto" Department of Chemistry, Faculty of Science, Hiroshima Universlty, Hiroshima 730, Japan (Received: September 22, 1980)
The extraction equilibria of tetraalkylammonium iodides R4NI(R= Me, Et, n-Pr, n-Bu, and i-Am) between water and organic solvents (nitrobenzene (NB), 1,2-dichloroethane(1,2-DCE), 1,l-dichloroethane(1,l-DCE), o-dichlorobenzene (o-DCB), dichloromethane (DCM), trichloromethane (TCM), chlorobenzene (CB), and l-chlorobutane (CBu)) were studied at 25 OC. The overall extraction constant was defined as KO = [R~N+]~[I-]~[R4NI]~02/[R4N+]w[I-]w[R4NI]wf,2 which consists of two extraction constants: one is the ionic part Ki = [R4N+],[I-]~~/[R4N+]w[I-]wf,2 and the other the neutral part K,, = [R4N1],/[R4NI],, where the subscripts o and w are ascribed to the organic and aqueous phases, respectively. These extraction constants were calculated by combining the distribution ratios of the iodides determined by use of 1311as a tracer with the ionic association constants obtained by conductivity measurements. Thereby the influences of solvent on the extractability of R4NI were split into the two parts for ionic and neutral species. The extractability of neutral species was successfully explained in terms of regular solution theory. The contribution of triple ion formation in organic solvents was also discussed. Introduction Solute-Solvent interactions in electrolyte solutions have received continuous attention in various fields of chemi ~ t r y . ~ Solvent -~ extraction6-8 is a useful technique for (1)Presented in part in the ACS/CSJ Chemical Congress, Hawaii, 1979. (2)J. F. Coetzee and C. D. Ritchie, Ed., "Solute-Solvent Interactions", Marcel Dekker, New York Vol. 1,1969; Vol. 2, 1976. (3)A. J. Parker, Chem. Rev,, 69,1 (1969). (4)A. K. Covington and T. Dickinson, Ed., "Physical Chemistry of Organic Solvent Systems", Plenum Press, New York, 1973. (5)"Ion-Ion and Ion-Solvent Interactions", Faraday Discuss. Chern. SOC.,No. 64 (1977). (6)H. L.Friedman and G. R. Haugen, J. Am. Chem. Soc., 76, 2060 (1954). (7)C. Hansch, J. E. Quinlan, and G . L. Laurence, J. Org. Chem., 33, 347 (1968). 0022-3654/81/2085-0894$01.25/0
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0 1981 American Chemical Society
