The Effect of Structure on the Behavior of Polyprotic Weak Acids in Alkalimetric Titrations

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 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.