Abstract
Spectrophotometric studies at temperatures between 288K and 473K in aqueous solution have been undertaken to determine the association constants for 2-, 3- and 4-nitrobenzoate, 4-chlorophenolate, 4-bromophenolate ions with hydrogen ion. Both association constants with hydrogen ion have been determined in each case for o-phthalic acid, nicotinic acid and picolinic acid. For 8-hydroxyquinoline the effect of temperature on the protonation of the anion was studied and previous work on the protonation of the heterocylic nitrogen atom was extended to higher temperatures. Conditions of constant ionic strength were employed. Non-linear plots of pKa against reciprocal temperature were found for 3-nitrobenzoic acid, 4-nitrobenzoic acid, o-phthalic acid (both stages of association), 8-hydroxyquinoline (both steps) and 4-bromophenol; linear relationships were found for 2-nitrobenzoic acid, nicotinic acid and 4-chlorophenol within the estimated experimental error and over the experimental and temperature range. The changes in standard enthalpy, entropy, Gibbs free energy and heat capacity for each reaction (except picolinic acid) have been calculated based on results from a wider temperature range than previously available. In addition, the association constants for the first copper(Il)-chloro, iron(III)-chloro, and iron(III)-sulphate complexes have been measured at temperatures up to 363K in the first case and 34 8K in the last two. Spectrophotometric techniques were used as were constant ionic strength conditions except for the iron(III)-sulphato complex. A criticism of the commonly-used method for determining the constants is offered. Finally, a vapour equilibration cell was designed and used to determine the equilibrium constant for the formation of triiodide anion from iodine and iodide ion between 313K and 460K; for comparison the system was studied using the same spectrophotometric technique employed for the transition metal complexes.