SPECTROPHOTOMETRIC DETERMINATION OF PH AND ITS APPLICATION TO DETERMINATION OF THERMODYNAMIC-EQUILIBRIUM CONSTANTS

Highly accurate pH estimations are conveniently made by evaluation of the spectra of an acid-bass indicator suitable for the pH range of interest, if the pK(a,ind) is accurately known. Stoichiometric pH values wore to determine pK(a,ind) to an accuracy of +/-0.008 or better, utilizing a neutralization titration of a reference standard weak acid or bass. When acetic acid (HOAc) and aqueous ammonia are adopted as reference substances and the best literature values are taken for their pK(a) values and activity coefficients as arbitrary constants, the calculated pH values are more accurate than those given by a glass electrode/pH meter combination. In turn, those pH values can be used for calibration of the glass electrode/pH motor combination. Thus, the pK(a) for thymol blue was determined at 25-65-degrees-C and ionic strength = 0-3.2 M, at 20-30 pH values for each set of conditions with an overall error of +/-0.002 log unit. Similarly, the pK(a) of bromocresol green was determined at 20-30 pH points during titration, at 25-degrees-C, and at ionic strength = 0-0.4 M, with an overall error of +/-0.008 log unit. The use of a diode array spectrophotometer and computer statistical calculations made possible analyses of photometric errors at all useful UV-vis wavelengths. Statistical analyses of such measurements permit the measurement of pK(a) values of unusally high reliability, limited by errors in pH evaluation, not by those of spectrophotometric measurements.