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
posted on 2022-12-16, 00:34authored byJessica
L. Smith-Osorio, Henry Torres-Quezada, Andrea P. Sandoval-Rojas, Jesús A. Ágreda
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.