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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 10 Issue 3 (2005) pp 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 13565-905, São Carlos, S.P.,
Received August 5, 2004. Accepted February 1, 2005.

Published online: 27 May 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 to105 order-of-magnitude range (e.g., phosphoric acid), due to a progressive increase in the anionic negative charge. With citric acid and some diprotic organic 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 differences occur in compounds with cis–trans isomers, such as fumaric and maleic acid.

Key Words: Laboratories and Demonstrations; general chemistry; polyprotic acids; titration; pK; potentiometry

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Issue date: June 1, 2005

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