Química

200

Chem. Educator 2005, 10, 200–203

The Effect of Structure on the Behavior of Polyprotic Weak Acids in Alkalimetric Titrations
Débora Cassoli de Souza, Andréa Boldarini Couto, and Eduardo Almeida Neves*
Departamento de Química da Universidade Federal de São Carlos. Via Washington Luís Km 235, CEP 13565905, São Carlos, S.P., edneves@dq.ufscar.br Received August 5, 2004. Accepted February1, 2005.

Abstract: The alkalimetric titrations of polyprotic acids provide an interesting didactic discussion for students in analytical chemistry classes. There is a marked decrease in the stepwise ionization constant, normally in the 104 to 105 order-of-magnitude range (e.g., phosphoric acid), due to a progressive increase in the anionic negative charge. With citric acid and some diproticorganic acids, the distance between the carboxylic groups markedly affects the pKn differences, which causes overlapping of the neutralization curves for the intermediate steps. The length of the carbon chain in diprotic acids, for instance, that of oxalic (two carbon atoms) and adipic acid (six carbon atoms) causes a regular decrease in the value of pK2 over pK1. Other interesting pK differencesoccur in compounds with cis–trans isomers, such as fumaric and maleic acid.

Introduction Volumetric methods involving titrations of polyprotic acids with strong bases (e.g., NaOH) provide for interesting discussions with students during analytical chemistry classes [1–3]. It is important to point out the differences in the stepwise ionization constants and how they affect the definition of theintermediate stoichiometric points, which can be clearly seen from potentiometric titration curves. It is well known that the stepwise ionization of polyprotic acids exhibits a continuous decrease in the magnitude of its ionization constants, Kn, and an increase in the corresponding pKn values. Normally a 104 to 105 difference in the stepwise Kn is found in many of the tables available forpolyprotic acids [1–4], which creates a favorable condition for obtaining well-defined neutralization steps during titration of the acids [1–3]. Smaller than 104 differences in Kn yield titration curves showing overlap of the intermediate neutralization steps; thus, the recommendation is that the last Kn should not be smaller than the 10–7 or 10–8 order-of-magnitude range in order to guarantee reliableendpoint detection. In some diprotic organic acids, the length of the carbon chain affects the stepwise ionization constants, causing progressively smaller differences in the ionization constants. The present article discusses this effect and gives some examples of the effect of structure on the magnitude of the available ionization constants. Experimental All reagents were of analytical reagent (AR) quality (Merck, Mallinckrodt, or Carlo Erba). Solutions of citric acid and the others organic acids were prepared by directly weighing the solid chemicals needed to prepare an 0.05 mol L–1 solution. A 0.05 mol L–1 phosphoric acid solution was prepared by diluting 3.5 mL of the concentrated acid (85%) to 1 liter. Distilled water used to prepare all solutions was bubbled with nitrogen before useto remove traces of carbon dioxide.

Solutions of 0.1 to 0.2 mol L–1 sodium hydroxide, carbonate free, were prepared by diluting a concentrated solution (50% NaOH) and standardizing with potassium hydrogen phthalate using phenolphthalein as the end-point indicator [1–3]. The glass electrode was calibrated with 0.0500 mol L–1 potassium hydrogen phthalate (pH = 4.01) and 0.0100 mol L–1 hydratedsodium borate (pH = 9.18) buffers [1–3]. The acids in solution were potentiometrically titrated with 0.2136 mol L–1 sodium hydroxide using a Digimed glass electrode combined with a calomel reference electrode saturated with potassium chloride. An Orion model EA-940 potentiometer was used for pH and potential measurements. The Origin 6.0 program was used to plot the pH and potentiometric data. Two...