Basic analytical chemistry: choosing a redox titration

Abstract

In a previous article published in this Journal, equations that describe the theoretical curves of two different redox volumetric methods that can be used in the determination of iron in solution, namely cerimetry and permanganometry, were deduced. However, the literature describes that the standard procedure adopted for this purpose is the titration of the iron sample with a standard solution of potassium dichromate, using visual detection, even knowing that dichromate has the lowest formal reduction potential of the three oxidizing agents considered. Hence, questions such as: what is the reason for the preference for potassium dichromate? Why use visual equivalence point detection? What are the recommended indicators? How to prepare sample and titrant for a redox titration? To answer these questions, in addition to knowing how the chemical system should behave, which is done through theoretical titration curves, it is also necessary to have a good practical knowledge of the chemistry involved in these procedures, which are determinants to accept the actions to be performed in the laboratory. For this, the analyst's previous experience is an essential factor. In this article, the essential points to be observed will be highlighted, before considering (or accepting) a procedure as adequate for the practice of redox volumetry.