Identification of iron(III) oxides and hydroxy-oxides by voltammetry of immobilised microparticles

In this paper, we propose the use of an electroanalytical technique based on the direct oxidation or reduction of the electroactive components of the sample on the surface of the working electrode, called voltammetry of immobilised microparticles (VMPs). The sample is easily deposited on the electrode by abrasion and then the electrode is transferred to the electrochemical cell where the square wave potential scan is performed. Electroactive species showed peaks whose peak potential is related to the standard formal potentials. We applied this technique to the identification of iron oxides and hydroxy-oxides in cosmetics. To characterise and identify the iron(III) oxides and hydroxy-oxides VMP was performed in two different media: oxalic acid and hydrochloric acid, that is, a complexing and a slightly complexing media. Two electrode processes were observed. They were influenced by the media and the synthesis procedure of the oxides. The reduction peak at negative potentials (−0.50 V in hydrochloric acid and −0.60 V in oxalic acid) is related to the direct reduction of the iron(III) oxide and it does not appear in the case of the more reactive phases (hydroxy-oxides). The peak at positive potentials (0.90 V in hydrochloric acid and 0.60 V in oxalic acid) involves the reduction of iron(III) in solution. The same electrode process were observed for binary mixtures but the peak potentials are shifted from the pure components peak potentials. This allowed us to distinguish between their mixtures. Finally, VMP was used to characterise iron oxides in cosmetic powders.