Substitution titration: a new approach to automatic titration : Determination of Acidity in Precipitation Samples

In substitution titrations, a small portion of the sample is repeatedly replaced by an exactly equal volume of titrant, by means of a rotary injection valve. The titration curves obtained in this manner can be linearized by using suitable functions, and hence equivalence volumes can be calculated. The substitution process means that there is no increase in volume during a titration. It is therefore possible to use a completely closed system, which is advantageous for samples that are sensitive to the ambient atmosphere. The principle is demonstrated for the potentiometric titration of hydrochloric acid containing varying amounts of weak acid (acetic acid or ammonium ion) with sodium hydroxide solutions. By the substitution titration, it is possible to determine the strong and weak acid in HCl/NH₄Cl mixtures with an accuracy of ca. 2%, but acetic acid is difficult to determine in the mixture because of partial protolysis at the concentration level examined. Sample concentrations are chosen in the range 125–1000 μM total acidity, to conform with the concentrations normally found in precipitation samples. The development of a procedure for determining the acidity in rain-watet and throughfall samples is described. The substitution titrations are easily automated and provide adequate sensitivity for routine work.