Neue Methoden zur spektrophotometrischen pK-Wertbestimmung, III Dreistufige Titrationssysteme / New Methods for Spectrophotometric Determination of pK-Values, III Titration of Trivalent Protolytes

The spectrophotometric titration of three-step sequential dissociation equilibria is analyzed by means of two dimensional absorbance diagrams. The evaluation of pK1 and pK3 as well as of the absorbance coefficients of both ampholytes is made by the uniform segments at the beginning and end of the titration. New methods of evaluating the value of pK2 precisely and without difficulty even at small pK-differences are described. By means of several independent criteria the pK-values and the absorbance coefficients of the ampholytes can be examined. The titration of (1,2,4)-benzenetricarboxylic acid illustrates the new method.