Mediated, thin-layer cell, coulometric determination of redox-active iron on the surface of asbestos fibers

Redox-active iron on the surface of asbestos fibers was detected and quantified using a thin-layer cell, coulometric method with soluble mediators to shuttle electrons between the mineral fibers and the solid electrode. The working and counter electrodes consisted of gold films on a glass slide with reference electrodes of silver. Asbestos fibers were entrapped in a thin-layer cell of 25 microns thickness. Hexaammineruthenium(II) or o-dianisidine (dication) was used as the reducing or oxidizing mediator, respectively. Hexaammineruthenium(III) undergoes a one-electron reduction, and protonated o-dianisidine undergoes a sequential two-electron oxidation. The measurement involved determination of the total charge for the oxidation or reduction of surface-immobilized Fe(II) or Fe(III) on the asbestos fibers. Analysis of the results showed that crocidolite and amosite have 4.3 +/- 0.7 and 3.3 +/- 0.7 nmol/mg of total redox-active iron that is accessible to the mediators, respectively. This corresponded to a surface coverage of accessible redox-active iron of approximately 4.3 x 10(-11) mol/cm2 for crocidolite and 9.5 x 10(-11) mol/cm2 for amosite. Furthermore, Fe(II) constituted 76% or 25% of the accessible redox-active iron on the surface of crocidolite or amosite, respectively. The method may be applied to other types of solid materials with redox-active species on their surfaces.