Temperature and salinity dependence of molar absorptivities of thymol blue: Application to the spectrophotometric determination of pH in estuarine waters

Abstract

Molar absorptivities of thymol blue have been determined in synthetic solutions of ionic strength ranging from 0.1 to 0.7 M_w. The solutions were buffered to pH 11.5 in order to characterize the absorption spectrum of the basic form of the indicator and to pH 5.0 to characterize the HI^– form. The extinction coefficients corresponding to the maximums of absorption of these forms of the indicator, as well as the molar absorptivities that describe the residual absorbances at the same wavelengths, were determined through linear regression between the indicator concentration in these samples and the absorbances of the maximums after successive additions of 2 mM thymol blue. A linear dependence of the molar absorptivities was found with temperature between 5ºC and 30ºC, and with salinity only for ₅₉₆ε (I^2–). The application of the expressions of the molar absorptivities for the calculation of the spectrophotometric pH provided results in accordance with the potentiometric pH measurements in the total hydrogen ion concentration scale