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A major challenge in the physical chemistry laboratory is development of experiments that are contemporary and affordable. "Contemporary" often implies that computers be involved in the collection and analysis of data (1). It is essential to recognize that computers do not do experiments; yet, appropriate use of computers adds a valuable dimension to the laboratory experience (2). As a step toward effective computer use in the teaching laboratory, we use an experiment in which the pK sub a values of a weak dibasic acid, with closely spaced values for pK sub 1 and pK sub 2 , are determined in an acid-base titration (3). Data are acquired by a computer connected by an RS-232 interface to a pH meter. Instruction in laboratory skills is preserved by requiring students to manipulate a buret, read it, and manually enter volumes into the computer as one measurement for each pH determination. The computer is then used to ease the burden of repetitive calculations and to enable the students to explore the effects of varying important experimental parameters.
This experiment is an extension of a typical quantitative analysis exercise in which students determine the pK sub a of a weak, monoprotic acid by titration with a strong base (4); the titration curve for a monoprotic acid usually is characterized by a distinct equivalence point, readily identified by visual inspection. However, in titration of weak dibasic acids, it is not always easy to observe two equivalence points. If pK sub 1 and pK sub 2 differ by less than about 3 pK units, only the second equivalence point typically is observed (see Fig. 1). (Fig. 1 omitted) Nevertheless, both pK values can be extracted from the data. We describe here the experiment and the techniques for obtaining the desired results.
Theoretical
In the following, alpha sub M is the activity; delta sub M is the activity coefficient; and [M] is the molar concentration of species M. The first and second dissociation constants of a dibasic acid, H sub 2 A, are
(Equations 1 and 2 omitted)
Addition of NaOH(aq) to a solution of H sub 2 A gives a solution containing H sup + , H sub 2 A, HA sup - , A sup 2- , and OH sup...