Measurement of pH at elevated temperatures using the optical indicator acridine

Abstract

A new method of measuring the pH of high-temperature aqueous solutions using optical indicators was developed. The absorbance spectrum of acridine changes as a function of pH, allowing its use as an optical indicator for pH. The PK(a) of acridine were experimentally determined in KOH-AcOH pH-buffer solutions at temperatures from 5 to 250-degrees-C at vapor-saturated pressures : pKa = 5.62 + 1158 / T - 0.7168 lnT where T is in Kelvin. The technique is demonstrated by experimentally determining the pKa of benzoic acid at temperatures up to 250-degrees-C where the result was pKa (benzoic acid) = - 60.67 + 3069 / T + 9.5542 lnT