A Student-Designed Potentiometric Titration: Quantitative Determination of Iron(II) by Caro's Acid Titration

Oxidation–reduction titrations and potentiometric methods provide a convenient experimental means to quantitatively determine compositions of unknown mixtures containing oxidizable/reducible species. Over the past decade, numerous titration methods have appeared in this Journal (1–6) and in standard laboratory manuals (7–10) involving common titrants such as potassium dichromate, iodine, potassium permanganate, cerium(IV) sulfate, and sodium thiosulfate. As representative examples, Kaufman and DeVoe (3) devised a simplified redox method for total iron content by first reducing any iron(III) to iron(II) with metallic zinc, followed by potassium dichromate titration using sodium diphenylamine sulfonate as the indicator. Klett et al. (4) proposed a similar Fe(II)–Cr(VI) titration method, except that titanium(II) chloride was used to reduce all iron to the 2+ oxidation state. Both methods generate waste solutions containing chromium, which should be disposed of by an environmentally safe method (11, 12). Chromium(III) is considered a “high hazard” ion, and may induce toxic effects. Similarly, chromium(VI) is very toxic and is also a suspected carcinogen (11, 12). Harris and Kratochvil (7) described the permanganimetric oxidation of iron and the iodometric determination of copper in brass. Pickering and Monts (6) described a rather novel laboratory experiment involving titration of the nonstoichiometric WO2.223.0 blue oxide, “mineral blue”, with potassium permanganate. This latter method requires that the titration solution be heated, and chloride ions are known to interfere in potassium permanganate titrations.