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The dissociation constant for a weak monoprotic acid may be calculated in aqueous alkaline solution by straightforward pH measurements, before and after the addition of the acid, and with the advantage of not having to standardize the solutions. The proposed experimental procedure is particularly interesting in the case of organic acids that are slightly soluble in water.
The conventional procedures for the determination of the acid dissociation constants in aqueous solutions, Ka's, use potentiometric, conductometric, or spectrophotometric techniques (1-3). In the case of organic acids slightly soluble in water, such techniques are often inapplicable. In the present work, we show how these constants may be calculated by simple pH measurements (in alkaline solutions) before and after the addition of the selected acid.
Procedure
The aqueous dissociation reaction for a monoprotic acid, HA, can be obtained as the sum, FORMULA OMMITED where Ka represents the apparent dissociation constant, in which the activity coefficients for each ionic specie are included.
If the ionic strength, I, is maintained constant, those coefficients, yi's, may be calculated through the DebyeHuckel theory, and in this way, a thermodynamic value for Ka is obtained. Ka = K.Kr FORMULA OMMITED FORMULA OMMITED In the laboratory we just calculate Ka by working...