The Measurement of the Activity Coefficient Ratios of Protonated and Neutral Forms of Hammett Indicators in Several Strong Acids

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.