Development of instrumentation for coulometric titrations

This contribution is focused on the development and testing of coulometric analyzer, which was employed for in situ generation of analytical agent. The goal of the project was construction of a new coulometer, which ensures the exact and precise automatic coulometric titration. Coulometry is one of the basic electroanalytical methods. Two main approaches are direct and indirect coulometry, based on monitoring of the current required to convert the substance of interest into a precisely defined product (direct), and/or the amount of current required to produce an equivalent amount of reagent which takes place in the reaction with selected analyte (indirect). The main advantage of indirect coulometry is the possibility of in situ generation of reactive unstable ions that may serve as titration agents. The newly constructed device is based on an integrated circuit with a connected screen and electrode connection inputs. The measurement set-up consists of a beaker in which the coulometric titration takes place, and a pair of generator Pt electrodes connected to a controllable DC source for coulometric analyzer. Reference and Pt indicator electrodes are utilized for potentiometric detection of equivalence point. The titrations in this work are based on oxidization of analyte by iodine, which was coulometrically generated from alkaline potassium iodide. Other additives in solution were oxalic acid, which ensures the stability of analyzed ascorbic acid, and starch, which serves as an indicator forming blue-colored inclusive compounds with iodine. Firstly, the sample solution of ascorbic acid was titrated and its oxidation over time was recorded. Oxidation of ascorbic acid occurs due to oxygen in the air, even in solution, where metal ions have the same effect. All titrations of ascorbic acid for different concentrations were evaluated by observing the blue color of starch-iodine inclusion complex. However, the color of titration solution is very weak at the equivalence point, and thus the end of titration is subjective. Secondly, the purity of sodium thiosulphate was checked using two ways of equivalence point estimation – (i) again using visual detection with starch, and (ii) by potentiometric indication using the second-derivative curve, which allowed to achieve significantly improved results under the optimized experimental conditions.