Factors affecting the precisions of analyses,by potentiometric titrimetry,of solutions containing two weak acids

This paper deals with potentiometric titrations in which mixtures of two monobasic weak acids are titrated with a strong base, and in which weighted non-linear regression analysis is used to find the concentrations and values of pK_a for both acids. The precisions of the resulting values of the concentrations depend on the difference between the values of pka, the ratio of the initial concentrations, and on the standard errors of measurement of both the pH and the volume of base. For any given values of the ratio of concentrations and the standard errors of measurement, the precision with which the concentration of the stronger acid can be evaluated is, in general, poorest when the difference between the pK_a values (Δ) is approximately 1.5, and improve if Δ is either larger or smaller than that value. Even if Δ is as small as 0.1, the precision that is attainable is very much better than it is generally believed to be. Surprisingly, there are conditions under which the concentrations of both acids can be determined with fair precision even though titrimetric data will not reveal that two acids are present in the sample.