On the Commonality of Speciation of Inorganic Solutes in Superacids, Strong Bases, Molten Salts, and Water

When dealing with inorganic solutes in protic solvents such as superacids, ammonia, and water, and especially in the case of molten salts, the authors of most textbooks and specialised reviews treat solution processes in each solvent or class of solvent separately, and rarely, if ever, draw the separate sections together to indicate the commonality of the solution processes from a generalised acid-base viewpoint. Initially in this review three chemically-simple solution media, H2O, anhydrous HF, and molten chloroaluminates, are used to illustrate the generality of the behaviour of CrIII in the three solvents. In neutral media “Cr(OH)3”, CrF3, and CrCl3 are sparingly soluble, whereas in basic media the solvo-anions Cr(OH)4−, CrF63−, and CrCl63− are the dominant species. In acidic media Cr3+ cations, solvated to a greater or less extent, are formed. In highly acidic media, transition metal cations in very low oxidation states can be generated, e.g. Ti2+, U3+, and Sm2+, as well as homopolyatomic cations, such as I2+, Se82+, Te64+, N5+, and Hg32+. Each of these cations would disproportionate in aqueous solutions and, indeed, they do so on addition of sufficient base, even in HF or chloroaluminates. Under oxidising conditions solvo-anions in high oxidation states, e.g. CrO42−, AgF4− and NiF62−, are stable in basic media. Solute speciation in sulfur acids and ammonia is dealt with briefly because most investigation of homopolyatomic cations and anions has been carried out in these solvents. The overall aim of this review is to demonstrate the generality of inorganic solute speciation in all ionising solvents. It is proposed here that the primary determinant of speciation in all media is the level of acidity or basicity of the solvent rather than its chemical composition or its temperature domain.