1532
Wood et al.
Heats of Mixing Aqueous Electrolytes. XII. The Reciprocal Salt Pair Na+, Li+l(CI-, S042R. H. Wood,' Danne E. Smith, H. K. W. Chen, Department of Chemistry, University of Dela ware, Newark, Delaware 197 1 1
and P. T. Thompson Department of Chemistry, Swarthmore College, Swarthmore, Pennsylvania 1908 1 (Received June 21, 1974; Revised Manuscript Received February 27, 1975) Publication costs assisted by the National Science Foundation
The heats of mixing aqueous solutions of all combinations of the reciprocal salt pair Na+, Li+ IICl-, S042have been measured at 25'. In one set of experiments, the initial solutions had the same molal ionic strength ( I = 1,3, or 6 mol/kg). In another set of experiments, the initial solutions had the same number of equivalents per kilogram of solvent ( E = 1, 3, or 6 mol/kg). For charge-asymmetric mixtures, the magnitude of constant E mixings is about the same as that of the constant I mixings. Young's cross-square rule is quite accurate a t I = 3 mol/kg and E = 3 and 6 mol/kg, marginal at I and E = 1and fails definitely a t I = 6 mol/kg. The constant E mixing process is preferred because the cross-square rule holds a t higher concentrations. The predictions of the cross-square rule based on both constant I and constant E mixings are surprisingly accurate a t these high concentrations in view of the fact that sodium and sulfate as well as lithium and sulfate ions have rather strong interactions.
Introduction According to Young's cross-square rule,l for a reciprocal salt pair, the sum of the heats of mixing involving a common ion equals the sum of the heats of mixing not involving a common ion. This rule has been shown to be quite accurate in predicting heats of mixing aqueous solutions of charge-symmetric ele~trolytes.l-~For charge-asymmetric mixtures appropriate weighting factors should be used for constant ionic strength m i x i n g ~ .Wood ~ and Ghamkhar5 suggested that for charge-asymmetric mixtures, mixing a t a constant number of equivalents per kilogram of water ( E ) might be preferred to mixing a t constant molal ionic strength ( I ) ,especially at higher electrolyte concentrations (>0.2 ional). Reilly and Wood6 recently measured the heats of mixing of aqueous solutions of the reciprocal salt pair Mg2+, Naf IICl-, Br- at both constant E and constant I . Their results indicated the accuracy of Young's rule for both types of mixing even at very high concentrations ( I , E = 6 mol/kg). The heat of mixing with a common ion was lower for the constant E mixings indicating the potential usefulness of the constant E mixing process. The present investigation was undertaken in order to study the relative advantages of the E concentration scale when strong cation-anion interactions are present. Experimental Section Because the complete experimental procedures and results for both the constant I7 and constant E* mixings have been given elsewhere, only a brief description will be given here. The solutions were prepared from reagent grade salts and deionized distilled water. All impurities recorded for these salts were less than 0.1%. The sodium salts were dried under vacuum for 24 hr a t 100'. Lithium chloride was dried under vacuum for 24 hr a t 200°.9 The lithium sulfate used for the constant ionic strength runs was dried under vacuum at 200' for 24 hr.9 The lithium sulfate used in the conThe Journal of Physical Chemistry, Vol. 79. No. 15, 1975
stant equivalents per kilogram measurements was standardized by conductance measurements on solutions that were diluted to 0.1 m. The conductance of 0.1 m lithium sulfate was first determined by drying two separate samples a t 500' in silica crucibles for 15 hr.loJ1 Solutions with m = 0.1000 mol/kg were prepared from these samples and the conductivity measured a t 25 f 0.02'. The conductivity for m = 0.1000 mol/kg Li2SO4 is 141.63 f 0.04 g mol-' ohm-' cm-l. The conductance cell was calibrated using potassium chloride standards. The pH was adjusted to 9 for the sulfate solutions and to 8 for the chlorides by the addition of negligible amounts (
