Apparent stability constants of proton and metal ion complexes of glycine, iminodiacetic acid, nitrilotriacetic acid, and triethylenetetramine in aqueous methanol

Abstract

Conditional formation constants were determined at 25 °C and with a background electrolyte of 0.1 mol dm−3 NaNO3 for the proton, nickel(II), copper(II), and zinc(II) complexes of glycine, iminodiacetic acid, and nitrilotriacetic acid in 0, 20, 50, 80, and 95 volume percent methanol mixed with water. Similar measurements were made for triethylenetetramine in all but 95% methanol, and the formation constants so determined were used to determine KMNTA by a competitive method.The variation of all constants with solvent composition was examined from the point of view that complex formation is accompanied by solvent displacement. Accordingly it should be possible to correlate the observations by means of an over-all formation constant, valid for any solvent concentration. It was concluded that for two solvents as similar as water and methanol, both of which appear to be involved in solvation or desolvation in the reaction, the correlations sought could not very precisely fix the amounts of solvents displaced or taken up. The goodness of fit to the attempted correlations suggests that they would be useful for interpolations to solvent compositions other than those studied.