Precise coulometric titration of cerium(iv) as an oxidising agent with electrogenerated iron(ii) and reliability in cerium(iv) standardisation with sodium thiosulfate

Abstract

Cerium(IV) is a useful strong oxidising agent and is often standardised against thiosulfate. In the standardisation, triiodide liberated by Ce(IV) in an acidic potassium iodide solution can be titrated with thiosulfate to determine the concentration of Ce(IV). The accuracy of the standardisation has not been established because the iodine liberation process is not well-understood. In the present study, to evaluate the iodine liberation process, Ce(IV) was assayed through two different methods: precise coulometric titration with electrogenerated ferrous ions and gravimetric titration with thiosulfate independently determined by precise coulometric titration with electrogenerated iodine. The reliability of Ce(IV) standardisation with thiosulfate is discussed on an absolute basis by coulometric titrations of both Ce(IV) and thiosulfate solutions. Finally, the accurate determination of Ce(IV) was achieved by both methods. Information on the stability of Ce(IV) solutions is also presented.

Reliability in standardization of sodium thiosulfate with potassium dichromate

“…The experimental set-ups and the procedures of coulometric titrations for potassium dichromate and a sodium thiosulfate solution were described previously [7,[13][14][15]. As described in the previous report [13], high purity potassium dichromate as a certified reference material (CRM) was established by coulometric titration with electrogenerated Fe(II).…”

“…The current had good proportionality to the amount of liberated iodine [7]. A dual platinum-plate electrode (each 5 mm × 5 mm) was used in gravimetric titration to increase sensitivity as described previously [7,[13][14][15].…”

“…Coulometric titration absolutely yields an accurate oxidimetric-reductometric assay based on Faraday's Law. The accuracy of the iodine liberation process was examined by comparing two assays of potassium dichromate using different Microchemical Journal 123 (2015) [9][10][11][12][13][14] titrations based on coulometric titration. Firstly, potassium dichromate was directly assayed by coulometric titration with electrogenerated Fe(II) [13].…”

Potassium bromate assay by primary methods through iodine liberation reaction

“…12,31 The authors tried to apply the coulometric titration with electrogenerated Fe(II) to the assay of bromate in the same manner as that for potassium dichromate or cerium(IV) sulfate tetrahydrate. 24,32 The titration curve could be clearly drawn with a combination of Pt and Ag/AgCl electrodes as the end-point detection; potentiometry and constant-potential amperometry were efficient as the end-point determination methods. However, the assays of potassium bromate with electrogenerated Fe(II) had an inferior measurement repeatability compared with that by gravimetric titration with thiosulfate and were much lower than that assayed under an appropriate gravimetric titration (section "Uncertainty for potassium bromate assay").…”

Reliability in Standardization of Iron(III) and Titanium(III) Solutions in Volumetric Analysis

“…The process, therefore, plays a key role in the accuracy of standardization of iron­(III). One of the authors has discussed several appropriate standardization procedures through the iodine liberation reaction for strong oxidants such as iodate, cerium­(IV), bromate, periodate, dichromate, and osmium­(VIII). − The accuracy of the titrimetric procedure for iron­(III) would be lower than that for these strong oxidants because the lower oxidizing ability of iron­(III) would lead to a smaller oxidation rate of iodide ions. Longer experimental time would lead to larger biases due to side reactions such as the oxidation of iodide ions by atmospheric oxygen and the volatilization of generated iodine.…”