Chris E. Baldwin1 ond James S. Evans
Lawrence university Appleton, W~sconsin5491 1
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Determination of the Ionization Constant of Sparingly Soluble Weak Acids A n experiment f o r analytical-organic courses
Experiments based on the titration of weak monoprotic acids seem invariably to occur somewhere in undergraduate chemistw curricula. ~ r o h a h l vbecause of the nice combination of topics invoked in t h e determination and interpretation of ionization constants: quantitative lahoratory techniques, principles of ionic equilihria, and structure-function correlations. In some curricula. these exneriments are especially valued for their contribution td developing quantitative laboratory techniques; in other curricula, they are also being exploited to illustrate the principles of ionic equilihria; in "integrated" curricula, these same experiments can take on increased significance when the analytical work assists the student in the structural characterization of acid derivatives related to his syntheses. For this latter application, however, the range of acids studied ought not to he restricted to those which are moderately water soluble. The purpose of this article is to discuss the feasibility of p H titrations of sparingly soluble acids and to show how the titration curves of such acids illustrate a particularly simple example of simultaneous equilihria. An acid that is only slightly soluble in water presents difficulties in the ordinary potentiometric-titration method for determining pK, values (1). The estimation of pK, from the midpoint of a titration curve becomes both inaccurate and imprecise when the solution is so dilute that the curve does not exhihit an inflection point. The proposed alternative approach is to make potentiometric measurements on a set of samples having undissolved acid in equilibrium with an aqueous phase of the acid and its s a k ~ h e s eparallel meas&emenis on several equilibrated samples are formally equivalent to a "titration" where a solid phase of undissolved acid would be present nearly up to the endpoint. The data from such a "titration" determine the value of a parameter that is a function of the ionization constant and the molar solubility of the acid. Thus the ionization constant can he evaluated after the solubility has been measured by a suitable method on a sample of saturated solution that has been filtered free from undissolved solid. Consider a system where solid acid, HA(s), is in equilibrium with a saturated aqueous solution containing the undissociated acid, HA(aq), and its conjugate hase, A-(aq). This system can he described by the solubility and ionization equilihria given in eqns. (1)-(4)
'Junior independent project, winter-spring 1972. Mr. Baldwin is now attending Washington University Medical School.
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[A-Ix 103 ( M I Figure 1. Calculated titration curves of p H versus molar concentration of conjugate base at a constant volume of 100 mi for 0.001 mole of a weak acid with PK, = 4.47. The solid line is the titration curve calculated from eqn. (5) assuming pK, = 2.90 for this acid. The vertical dotted line marks the right-hand limit of the region where solid acid is present under these conditions. The dashed line is the titration curve calculated from eqn. ( 6 ) assuming instead that this acid would be soluble under these conditions (corresponding topK,
