A test of the universality principle: The solubility of metal salts and oxides in isobutyric acid plus water near its consolute point

A mixture of isobutyric acid + water has a critical point of solution at a composition of 38.8 mass % isobutyric acid and a temperature near 300 K. Under these conditions, the pH = 2, which makes the mixture a ready solvent for metal salts and basic/amphoteric oxides. Measurements of the temperature dependence of the solubility of a solid in the critical region of such a mixture can be used as a test of the Principle of Critical Point Universality. This inductive generalization, which is thought to govern all critical phenomena, predicts that there will be a critical effect in the solubility of a solid when at constant temperature and pressure the position of equilibrium is specified by no more than three fixed variables. We report herein a determination of the temperature dependence of the solubility of gallium (III) oxide in a mixture of isobutyric acid + water, and in a separate experiment the determination of the temperature dependence of the solubility of titanium dioxide in this solvent. In a third experiment, we report the effects of the critical point of this mixture on the simultaneous dissolution of calcium sulfate and barium sulfate. In each of these experiments, there are three fixed variables. We carry out a fourth experiment involving the solubility of solid lead (II) iodide in isobutyric acid + water doped with completely dissolved sodium sulfate. In this case, four variables are fixed. Independent of the chemical details, and consistent with the universality principle, we find a critical solubility effect in each of the first three experiments but none in the fourth.