FUNDAMENTAL THERMOCHROMIC PROPERTIES OF BUFFERED PH INDICATOR SOLUTIONS AND THE FORMULATION OF ATHERMOCHROMIC SYSTEMS

A thermodynamic basis is derived and experimentally verified for predicting the temperature-dependent spectral changes in solutions containing a pH indicator and any number of pH-buffering components. Absorbance by the acidic and basic species of the pH indicator is measured at two different wavelengths. A plot of the logarithm of the ratio of absorbances at the two wavelengths vs reciprocal temperature is a straight line with slope that varies with solution composition. The slope is used as a thermochromicity parameter to determine proton dissociation enthalpy of pH indicators and pH buffers and to calibrate solutions in ''optical thermometers''. The parameter is independent of optical path length, concentration of pH indicator, temperature range, and, within limits, the two wavelengths selected. We also have used the parameter to formulate solutions with temperature-independent spectral properties. Such ''athermochromic'' solutions remove temperature error from optical acidimetric measurements.