Anomalies in the Titrimetric Determination of Chromium and total Iron in Chromium Compounds

Abstract

Potassium dichromate is a powerful oxidizing agent (Eo =1.36 V) which is used for a variety of purposes in analytical chemistry. Because of it's high purity (AR or GPR) and the easiness of it's preparation K2Cr2O7 was used as primary standard. In the present work the traditional titrimetric method between the dichromate and iron(II) which have been used in the determination of chromium and iron in both industrial potassium dichromate and iron(II) sulfate, was studied; Four redox indicator are used to detect the colour change of the indicator at the end point in presence of the background colour of chromium(III) produced as the result of the reduction of K2Cr2O7. The precise observation of this colour change is very critical in the determination of chromium content in the analyzed sample. Precise result were obtained in the case of pheny1 anthranilic acid and pheny1 a mine dicarboxylic acid indicators which have high Eo 1.08 and 1.12V respectively. Low result were obtained in the case of diphenylamine and sodium dipheylamine indicators, which have low Eo. 0.76 and 0.98V respectively. This result agreed well with the theory of Kolothoff for the choice of a suitable redox indicator. In comparison of the results obtained by N-phenyl anthranilic acid and phenyl amine dicarboxyle acid indicators and those obtained by potentiometric and spectrophotometric methods, it was found that relative errors was much decreased. These indicators have great advantage that they can be used without the addition of phosphoric acid during the titration.