Abstract Isopiestic measurements have been made at 55 compositions of the {yK2HPO4 + (1 − y)KH2PO4}(aq) system at T = (298.15 ± 0.01) K, 11 for each of the limiting binary solutions and 33 for mixture compositions at K2HPO4 stoichiometric ionic strength fractions y = (0.23330, 0.47671, and 0.73177), using KCl(aq) as the reference standard. Model parameters for the binary subsystems were evaluated at this temperature for an extended form of Pitzer’s ion-interaction model and also for the Clegg, Pitzer and Brimblecombe model based on the mole-fraction-composition scale, using the present isopiestic results along with critically-assessed osmotic coefficients for both of these aqueous electrolytes as extracted from the published literature. The thermodynamic models for KH2PO4(aq) extend to slightly above the saturated solution molality at T = (298.15 ± 0.01) K, whereas those for K2HPO4(aq) extend to m = 9.7429 mol·kg−1, which is the molality of the saturated solution, also at T = (298.15 ± 0.01) K. These results yield the CODATA-compatible standard Gibbs energy of formation Δ f G m o ( K 2 HPO 4 · 3 H 2 O , cr , 298.15 K ) = - 2367.70 ± 1.60 kJ · mol - 1 . The 33 osmotic coefficients for the ternary mixtures were likewise represented with these models, using both the usual Pitzer mixing terms and also Scatchard’s neutral-electrolyte model mixing terms for the extended ion-interaction model. Two mixing parameters are needed for each of the three models for {yK2HPO4 + (1 − y)KH2PO4}(aq), and both of these ion-interaction models give similar high-quality representations of the experimental results. However, the Clegg, Pitzer and Brimblecombe model had more difficulty in representing the osmotic coefficients of K2HPO4(aq), especially below 3 mol·kg−1, and consequently the corresponding mixture model with two mixing parameters is slightly less accurate for representing the osmotic coefficients. The maximum difference in calculated values of the mean molality-based activity coefficients for the two recommended extended Pitzer models with the different types of mixing terms are 0.0061 for the trace activity coefficient of K2HPO4(aq) in KH2PO4(aq) but with much better agreement at most mixture compositions.

中文翻译：

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{yK2HPO4 + (1 − y)KH2PO4}(aq) 在 T = 298.15 K 时的渗透系数和活性系数的等压测定

摘要 在 T = (298.15 ± 0.01) K 时，对 {yK2HPO4 + (1 − y)KH2PO4}(aq) 系统的 55 种成分进行了等压测量，对每种限制性二元溶液进行了 11 次测量，对 K2HPO4 下的混合成分进行了 33 次等压测量化学计量离子强度分数 y = (0.23330、0.47671 和 0.73177)，使用 KCl(aq) 作为参考标准。在此温度下，针对 Pitzer 离子相互作用模型的扩展形式以及基于摩尔分数组成标度的 Clegg、Pitzer 和 Brimblecombe 模型评估了二元子系统的模型参数，使用当前的等压结果以及批判性地-评估了这两种水性电解质的渗透系数，如从已发表的文献中提取的。KH2PO4(aq) 的热力学模型扩展到略高于 T = (298. 15 ± 0.01) K，而 K2HPO4(aq) 的那些扩展到 m = 9.7429 mol·kg−1，这是饱和溶液的摩尔浓度，也在 T = (298.15 ± 0.01) K。这些结果产生了 CODATA-兼容的标准吉布斯生成能 Δ f G mo (K 2 HPO 4 · 3 H 2 O, cr, 298.15 K) = - 2367.70 ± 1.60 kJ · mol-1。三元混合物的 33 个渗透系数同样用这些模型表示，使用常用的 Pitzer 混合项和扩展离子相互作用模型的 Scatchard 中性电解质模型混合项。{yK2HPO4 + (1 − y)KH2PO4}(aq) 的三个模型中的每一个都需要两个混合参数，并且这两种离子相互作用模型都提供了类似的高质量实验结果表示。然而，克莱格，Pitzer 和 Brimblecombe 模型更难表示 K2HPO4(aq) 的渗透系数，尤其是低于 3 mol·kg-1，因此具有两个混合参数的相应混合物模型表示渗透系数的准确度略低。对于 KH2PO4(aq) 中 K2HPO4(aq) 的微量活度系数，使用不同类型混合项的两个推荐的扩展 Pitzer 模型的平均摩尔活度系数计算值的最大差异为 0.0061，但具有更好的一致性最多混合成分。