The activity coefficient ratio for several indicators has been estimated for the protonated and neutral forms of the same indicator molecule over a large range of acidity in H2SO4, HClO4, and HCl systems. The activity coefficient (fB) of the neutral form of a number of Hammett indicators was determined via solubility measurements. The observed linear relationship between log fB and [Formula: see text] then permitted estimate by extrapolation of log fB not experimentally accessible for the more basic indicators. Solubility studies of the pentacyanopropenide (PCP−) salts of these more basic protonated indicators were referred to a reference standard, tetraethylammonium pentacyanopropenide (TEA+PCP−) according to the method developed by Boyd to obtain the activity coefficient ratio [Formula: see text] By synthesizing the term [Formula: see text] the approach to a meaningful value of [Formula: see text] is believed to be facilitated. This function is uniquely related to the water activity of the various acid systems examined to support the suggestion by Hammett that [Formula: see text] is a constant for primary aniline indicators at a specified acid concentration.
