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  • Author: Heinz Gamsjäger x
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Solubility equilibria between solid salts, salt hydrates, and water play an important role in fundamental and applied branches of chemistry. The continuous interest in this field has been reflected by the 15th International Symposium on Solubility Phenomena as well as by the ongoing IUPAC-NIST Solubility Data Series (SDS), which by now comprises close to 100 volumes. Three typical examples concerning solubility phenomena of ionic solids in aqueous solutions are discussed: (1) sparingly soluble, simple molybdates; (2) sparingly soluble ionic solids with basic anions; and (3) hydrolysis of inert hexa-aqua-M(III) ions, where M is Ir, Rh, or Cr. In the first two cases, essential experimental details are discussed, an outline of thermodynamic analyses is given, and theoretical aspects are emphasized. In the third case, an educational suggestion is made.

Phenomena related to the solubility of solids, liquids, and gases with one another are of interest to scientists and technologists in an array of disciplines. The diversity of backgrounds of individuals concerned with solubility creates a potential for confusion and miscommunication and heightens the need for an authoritative glossary of terms related to solubility. This glossary defines 166 terms used to describe solubility and related phenomena. The definitions are consistent with one another and with recommendations of the International Union of Pure and Applied Chemistry for terminology and nomenclature.

Abstract

Standard molar quantities of molybdate ion entropy, Sm0, enthalpy of formation, ΔfHmo, and Gibbs energy of formation, ΔfGmo, are key data for the thermodynamic properties of molybdenum compounds and complexes, which are at present investigated by an OECD NEA review project. The most reliable method to determine ΔfHmo of molybdate ion and alkali molybdates directly consists in measuring calorimetrically the enthalpy of dissolution of crystallized molybdenum trioxide and anhydrous alkali molybdates in corresponding aqueous alkali metal hydroxide solutions. Solubility equilibria of sparingly soluble alkaline earth molybdates and silver molybdate lead to trustworthy data for ΔfGmo of molybdate ion. Thereby the Gibbs energies of the metal molybdates and the corresponding metal ions are combined with the Gibbs energies of dissolution. As reliable values are available for ΔfGmo of the relevant metal ions the problem reduces to select the best values of solubility constants and ΔfGmo of alkaline earth molybdates and silver molybdate. There are two independent possibilities to achieve the latter task. (1) ΔfHmo for alkaline earth molybdates and silver molybdate have been determined by solution calorimetry. Entropy data of molybdenum have been compiled and evaluated recently. CODATA key values are available for Smo of the other elements involved. Whereas Smo(CaMoO4, cr) is well known since decades, low-temperature heat capacity measurements had to be performed recently, but now reliable values for Smo of Ag2MoO4(cr), BaMoO4(cr) and SrMoO4(cr) are available. (2) ΔfHmo(BaMoO4, cr), for example, can be obtained from high temperature equilibria also, but the result is less accurate than that of the first method. Once Gibbs energy of formation, ΔfGmo, and enthalpy of formation, ΔfHmo, of molybdate ion are known its standard entropy, Smo, can be calculated.

The IUPAC-NIST Solubility Data Series (SDS) is an ongoing project that provides comprehensive reviews of published data for solubilities of gases, liquids, and solids in liquids or solids. Data are compiled in a uniform format, evaluated, and, where data from independent sources agree sufficiently, recommended values are proposed. This paper is a guide to the SDS and is intended for the benefit of both those who use the SDS as a source of critically evaluated solubility data and who prepare compilations and evaluations for future volumes. A major portion of this paper presents terminology and nomenclature currently recommended by IUPAC and other international bodies and relates these to obsolete forms that appear in the older solubility literature. In addition, this paper presents a detailed guide to the criteria and procedures used in data compilation, evaluation, and presentation and considers special features of solubility in gas + liquid, liquid + liquid, and solid + liquid systems. In the past, much of this information was included in introductory sections of individual volumes of the SDS. However, to eliminate repetitive publication, this information has been collected, updated, and expanded for separate publication here.

The aqueous solubilities of La(OH)0.8(CO3)1.1.0.1 H2O, basic lanthanum carbonate of ancylite type, have been investigated as a function of ionic strength at 25.0 °C. The stoichiometric solubility constants defined by log* KI ps0 = log [La3 +] + 1.1 log pCO2 - 3 log [H+] have the values 11.10, 11.32, 11.42, 11.63, and 11.70 for I = 0.1, 0.25, 1.0, 2.0, and 4.0 mol kg -1(Na)ClO4 respectively. The extrapolation to infinite dilution using the Pitzer equations resulted in a "thermodynamic" solubility constant, log* K°pz0 = 10.48 ± 0.08. This in turn led to the Gibbs energy of ancylite formation: Δf G 298 {La(OH)0.8(CO3)1.1 · 0.1 H2O} = - 1.531.5 kJ mol-1.