Potentiometric characterization of weak acids by multiple sample addition II. The effect of chemical interferences and the practical performance of linearization methods.
Potentiometric multiple addition of a sample containing a pure weak acid to a solution of supporting electrolyte has been previously shown [C. Maccà and A. Merkoci, Talanta, 41 (1994) 2033] to be formally suitable for the determination of the dissociation constant and concentration of the sample acid. Linear equations have been developed for the treatment of experimental data to yield, simultaneously or separately, the chemical parameters of the acid solution. These equations are now tested on real samples together with analogous equations for titrations, and the results are compared with those obtained with rigorous statistical methods. For the determination of the acidity constant with samples of known concentration, multiple samples addition is comparable with titration. When the sample concentration is unknown and must be determined simultaneously to the acidity constant, the results obtained by linearized multiple sample addition can be seriously affected by impurities present, even at low level, either in the sample or in the supporting solution. Linear equations accounting for the effects of basic or acidic impurities in the sample or in the supporting solution are developed. Sample addition is confirmed to be a useful complement to pH-metric titration for the determination of acidity constants of moderately weak acids by non-linear regression; linearization of data is a convenient technique for screening purposes and a powerful means of detecting and correcting some common pitfalls, interferences and contaminations, whose effects are enhanced in linearized sample addition.
Maccà
C
C
Department of Inorganic and Analytical Chemistry, University of Padua, Via Marzolo 1, 1-35131 Padova, Italy.
Merkoci
A
A
Bombi
G G
GG