POTENTIOMETRIC CHARACTERIZATION OF WEAK ACIDS BY MULTIPLE SAMPLE ADDITION .2. THE EFFECT OF CHEMICAL INTERFERENCES AND THE PRACTICAL PERFORMANCE OF LINEARIZATION METHODS

Potentiometric multiple addition of a sample containing a pure weak ac id to a solution of supporting electrolyte has been previously shown [ C. Macca and A. Merkoci, Talanta, 41 (1994) 2033] to be formally suita ble for the determination of the dissociation constant and concentrati on of the sample acid. Linear equations have been developed for the tr eatment of experimental data to yield, simultaneously or separately, t he chemical parameters of the acid solution. These equations are now t ested on real samples together with analogous equations for titrations , and the results are compared with those obtained with rigorous stati stical methods. For the determination of the acidity constant with sam ples of known concentration, multiple samples addition is comparable w ith titration. When the sample concentration is unknown and must be de termined simultaneously to the acidity constant, the results obtained by linearized multiple sample addition can be seriously affected by im purities present, even at low level, either in the sample or in the su pporting solution. Linear equations accounting for the effects of basi c 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 mode rately weak acids by non-linear regression; linearization of data is a convenient technique for screening purposes and a powerful means of d etecting and correcting some common pitfalls, interferences and contam inations, whose effects are enhanced in linearized sample addition.