Volumetric properties of, and ion-pairing in, aqueous solutions of alkali-metal sulfates under superambient conditions

Volumes of MgCl2(aq) at temperatures from 298 K to 623 K and pressures up to 30 MPa

“…(39,40) The predictions for this reaction (41) are based on the potentiometric results of Majer and Stulik (42) between T = 288.15 K and T = 358 K and the data of Saccocia and Seyfried (36) between T = 573 K and T = 673 K, and are consistent with the calorimetric results of Gillespie et al Table 4 contains the association constant K°i p and degrees of ion pairing a for formation of MgCl + calculated for three molalities via equation (6). In our previous communication, (17) we estimated association in superambient Na 2 SO 4 (aq) based on the SUPCRT92 prediction scheme using parameters newly determined by Pokrovski et al (43) Comparison of K°i p and a values suggests that ion pairing is much stronger in Na 2 SO 4 (aq) than in MgCl 2 (aq) between T = 298.15 K and T = 573 K.…”

“…In contrast to the alkali metal sulphates where B V was molality independent, (17,21) i.e., B V = b (0)V , inclusion of the b (1)V coefficient was necessary to describe quantitatively the molality dependence of V f,m . This is almost certainly a reflection of the much greater range studied for MgCl 2 (aq) rather than any significant difference between the two electrolytes.…”

“…(16,26,32) It is of interest to compare the present data with those of related salts. Figures 4 and 5 compare the volumetric behaviour of MgCl 2 (aq) with three other electrolyte solutions having a common ion: a 1-1 electrolyte NaCl(aq), (24) a 2:1 electrolyte CaCl 2 (aq), (33) a 2:2 electrolyte MgSO 4 (aq), (34) and with a typical 1:2 electrolyte Na 2 SO 4 (aq) (17) consisting of different ions. Apparent molar volumes at infinite dilution for MgSO 4 (aq) cannot be obtained by the usual extrapolation to infinite dilution as the salt ion-pairs strongly and the molality dependence of V f,m deviates markedly from the Debye-Hu¨ckel theory, even in relatively dilute solution (see figure 4).…”

“…The correction of ''stoichiometric'' volumetric data for the effects of ion-pairing for unsymmetrical (1:2 and 2:1) electrolytes has been discussed in detail in our previous paper. (17) Unlike symmetrical electrolytes, (16) where neutral species result from the ion-pairing reaction, the procedure is more complicated and, in the absence of extensive auxiliary data, can be only approximate. This is because the ion-pairing equilibrium, in this case:…”

“…Such effects become especially important at higher temperatures (T q 473 K), where even strong electrolytes tend to be significantly associated, and also for more highly charged ions. Accordingly, a semiquantitative approach developed previously (16,17) was adopted to correct the present results for ion-pairing.…”