J. Chem. Eng. Data
1992,37, 433-442
433
Isopiestic Investigation of Water Activities of Aqueous NiC12 and CuC12 Solutions and the Thermodynamic Solubility Product of NiClrGH20 at 298.15 K Joseph A. Rard Lawrence Livermore National Laboratory, Earth Sciences Department, University of California, Livermore, California 94550
Isopiestic vapor-pressure experiments have been performed for aqueous solutions of high-purity NiCl2 from 1.4382 to 5.7199 molnkg-1 (supersaturated) at 298.15 K. The resulting osmotic coefficients were combined with published isopiestic data to yield recommended values of ow,4, and y+. Similarly, the solubility of NiC12*6HzO(cr)was determined by this method, and combined with literature values to yield the molal thermodynamic solubility product Kmo= 1108 f 19. Isopiestic experiments were also performed for aqueous solutions of high-purity CuC12, but the isopiestic molality ratio exhibited hysteresis due to gradual loss of HC1 from the CuClz solutions, for experiments done with both degassed and air-filled chambers. The water activity of a saturated solution in equilibrium with CuCl2.2HzO(cr)was determined to be a,(satd) = 0.680 75 f 0.O00 08.
Introduction Ftard and Platford (1) have given a detailed discussion of possible errors and their influence on the accuracy and precision of isopiestic measurements. They noted that, with favorable conditions and good experimental technique, the resulting osmoticcoefficients4 from independent studies can agree to 0.2-0.3% or even better. Larger differences than this are, unfortunately, not uncommon. They can result from problems such as inadequate purity of the electrolytes being investigated, unreliable chemical analyses for molality, chemical reaction between the electrolyte solutions and the isopiestic sample cups, etc. A knowledge of the thermodynamic properties of aqueous solutions of the transition-metal dichlorides is required for a variety of reasons including the economic importance of the metals and the need to develop better methods for their extraction and separation from ores. In addition, the large variation in the strength and extent of anion-cation interactions makes it more challenging to model their thermodynamicbehaviorthan (forexample)the alkaline-earth-metal dichlorides. For these reasons, we have measured and reported isopiestic data for aqueous solutions of MnC12, NiC12,CdC12, and ZnC12 (2-5) from low to high molalities at 298.15 K; for the fiist three of these electrolytes, the measurements extend well into the supersaturated region. In those reports we critically compared our new isopiestic results to previous values. We found that values of 4 from the variousisopiesticstudies showed a maximum variation of about 0.5% for MnC12, although most 4 values agreed to within 0.3% (2). Similarly, for NiC12 the various studies all agreed within 0.6%, but with better agreement over most of the molality range (3). Comparison of isopiestic 4 values for CdClz solutions (4) indicated agreementto 0.34.4 % ,with a few individualpoints having random deviations of up to 1%. For ZnClz solutions the maximum discrepancy in 4 was about 0.7 % , with most data sets agreeing to within 0.3% (5). Even though these maximum discrepancies are about twice that expected for very favorablesystems,the agreementis stillquite reasonable, given that the degree of purity of the transition-metal dichlorides is generally not known for the earlier studies.
In the last 20 years there has been a resurgence of interest in the thermodynamic properties of aqueous CuC12, and of its mixtures with other electrolytes (6-10). We examined published isopiesticdata for aqueous CuClz solutions at 298.15 K (6,8,11,12) and found that derived values of 4 agree to about 0.5 % at lower molalities,but these differencesincrease to 0.5-1.2% at higher molalities. This maximum discrepancy is about twice as great as observed for the other four transition-metal dichlorides mentioned above. Goldberg et al. (13)have criticallycompared published isopiestic4 values for aqueous CoClz solutions and found a divergence of up to 2.3% at the highest molalities. Because of these rather large discrepancies among the published isopiestic 4 for both aqueous CuCl2 and CoClz solutions at 298.15 K, we decided to perform new isopiestic measurements. In addition we reinvestigated aqueous NiCl2 solutions as a “controlsystem”,to establish how reproducible isopiestic data can be for aqueous transition-metal dichlorides. As noted in the Experimental Section, we abandoned our attempt to study aqueous CoClz because of significant chemical contamination of our supply of “analyticalreagent“ CoClz.6HzO. Experimental Section
Preparation and Analyses of Stock SolutiOms. Stock solutions were prepared of aqueousNiClZ,CuCl2,CoCl2, NaC1, and CaClz, where the last two electrolytes were used as isopiestic reference standards. Water for these solutions was purified by ion exchange followed by distillation. Our NaCl stock solution was prepared by mass from ovendried Mallinckrodt AR NaCl and purified water. The molality of this solution was determined to be 1.9996 f O.OOO6 mol-kgl, by dehydration of samples in triplicate at 473 and 523 K. This value agrees well with the molality calculated from the masses of NaCl and water used for its preparation, 1.9991 mol-kg’, and the average of these two values was accepted. Two separate concentratedstock solutions of aqueousCaCl2 were prepared by reaction of Mallinckrodt primary standard CaC03(differentlots) with aqueousHC1, followed by fiitration to remove excess CaC03 and other insoluble material. We recrystallizedone of these batches of CaCl2 before using it for 1992 American Chemical Society
434 Journal of Chemical and Engineering Data, Vol. 37, No. 4, 1992
stock solution no. 1. In contrast for stock solution no. 2, the filtered reaction mixture was adjusted to the stoichiometric pH of CaClz, as determined by potentiometric-pH titrations of samples of the unadjusted stock solution with dilute HC1. The molalitiesof each CaCl2stock solution were determined by conversion of weighed samples to anhydrous CaS04, resulting from evaporation of these samples to dryness after additionof excess HzSO4. A tare crucibtenot containingCaClz was treated in an identical manner, to allow a correction to be made for nonvolatile residue from the H2S04 solution. The molality of stock solution no. 1 was determined to be 6.6076 f 0.0040 mol-kgl, after firing the CaS04 at 673-773 K. Similarly, the molality of stock solution no. 2 was found to be 6.6058 f 0.0017 mol-kgl after firing the Cas04 to 673873 K, and a second analysis at 673-923 K gave 6.6068 f 0.0018 molskg-I. These two results for stock solution no. 2 were averaged for subsequent calculations. Although reproduciblestock solutionmolalities could be obtained from firing the CaS04at various temperatures at or above 673 K, temperatures of about 825-925 K gave more consistent results. We also determined the molality of CaClz stock solution no. 2 by dehydration of samples at 473 and 498 K, but this method gave a molalityabout 0.1 % above the values obtained by sulfate analyses. We found that the weights of CaC12varied with the humidity of the laboratory. Thus, the dehydration results were rejected as being slightly high due to absorption of moisture. The Cas04 from concentration analysis of stock solution no. 2 was examined for impurities by X-ray fluorescence spectroscopy and was found to contain (in mass fractions) 1 X lo4 Sr,
