Dibasic Acids, Ampholytes, and Diacidic Bases

The terms Ca and Ct are defined by equations (3 9a) and (3-9) respectively. The hydrogen and hydroxyl ion activity terms have been placed in the equation to emphasize that these are assumed to be the measured quantities derived from the pH measurements. Whilst it is correct to use the hydrogen ion activity in equation (3-22), the hydrogen ion concentration is required for equation (3-22a). It is apparent, therefore that mixed constants would result from the solution of equation (3 -22). A further complication is the ionic strength which, in this case, cannot be calculated directly from the stoicheiometric concentration used, for example, in equation (3-19). To obtain the thermodynamic values, K and KI, the activity functions must be calculated from estimates of the ionic strength. How this can be achieved will now be described for dibasic acids followed by a similar method for ampholytes and diacidic bases. 

This nomenclature describes all three possible cases diacidic bases, dibasic acids, and ampholytes. The observed extinction coefficient, e at the analytical wavelength for a given pH value, when all three species are present, is... 

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