Manual Differential Spectrophotometric Titrations: Application to Aqueous and Nonaqueous Acid-Base Titrations and to Chelometric Titrations

A differential end point technique based on the use of spectrophotometry has been used for aqueous and nonaqueous acid-base titrations, and chelometric titrations. A suitable indicator is added to the unknown and the change in indicator absorbance on adding a titrant increment is determined using the unknown solution as its own high absorbance reference standard, i.e., employing the method of differentia! spectrophotometry. ΔpH/ΔV (or ΔpM/ΔV) can then be calculated from the observed change in absorbance and other easily determined absorbances. Phosphoric acid (1st and 2nd end point, 0.08M), acetic acid (0.001 M), L-phenylalanine (0.03M), L-leucine (0.03M), and DL- alanine (0.05M) were titrated with sodium hydroxide in aqueous solution, and sodium acetate (0.001M) and o-chloroaniline (0.09M) were titrated in glacial acetic acid as solvent with perchloric acid obtaining results superior to those found with a potentiometric titration using a glass electrode. Also, dilute aqueous calcium solutions (0.001 M) containing no magnesium were titrated with EDTA using Erio- chrome Black T as indicator with an accuracy of 0.4%. Under optimum conditions the error in this method is equivalent to a differential potentiometric procedure with an electromotive force error of ±0.1 mv.