Implementation of the Theoretical Coulometric Titration Curve in the Determination of the Amount of Substance of Potassium Hydrogen Phthalate: the Search for a Better Metrological Approach

In metrology, the certification of potassium hydrogen phthalate (KHP) as a reference material is made using the potential primary method of coulometric titration. Usually, this titration is performed in three steps at constant current, where two endpoint (EP) times are determined from the nonlinear regression that fits the empirical W-function (eWf) to the experimental data. As an alternative, we propose the implementation of the theoretical coulometric titration curve (TC). The TC allowed us to compute the KHP amount of substance, the influence of CO2 in the system, the acid dissociation constants for carbonic and phthalic acids, assuming that those species are the only acids present during titration, and the EP times. The amount of substance of KHP estimated with the EP time and obtained from the TC was compared with the results of the eWf, and no statistical difference was found, while the amount of substance, when estimated directly as a parameter of the nonlinear regression of the TC, was lower. Therefore, the traditional method finds the total acidity of the dissolution, and our method finds the KHP purity. In addition, the acid dissociation constants for H2CO3 and phthalic acid estimated in this work agreed with the data reported in the literature. Finally, the description of the coulometric system using the theoretical TC has a solid and well-known chemical support that is not present in the eWf; this fact is essential for the uncertainty budget and the scientific support for coulometry as a potential primary method.

Automated summary

This study proposes an alternative method for metrology by implementing the theoretical coulometric titration curve. The results show that the proposed method provides similar results to the traditional method, and the experimental data agrees with the literature. The theoretical coulometric titration curve method has a stronger chemical support compared to the traditional method.