MEDIATED, THIN-LAYER CELL, COULOMETRIC DETERMINATION OF REDOX-ACTIVE TRON ON THE SURFACE OF ASBESTOS FIBERS

Redox-active iron on the surface of asbestos fibers was detected and q uantified using a thin-layer cell, coulometric method with soluble med iators to shuttle electrons between the mineral fibers and the solid e lectrode. The working and counter electrodes consisted of gold films o n a glass slide with reference electrodes of silver. Asbestos fibers w ere entrapped in a thin-layer cell of 25 mu m thickness. Hexaamminerut henium(II) or o-dianisidine (dication) was used as the reducing or oxi dizing mediator, respectively. Hexaammineruthenium(III) undergoes a on e-electron reduction, and protonated o-dianisidine undergoes a sequent ial 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 sho wed 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, re spectively. This corresponded to a surface coverage of accessible redo x-active iron of approximately 4.3 x 10(-11) mol/cm(2) for crocidolite and 9.5 x 10(-11) mol/cm(2) for amosite. Furthermore, Fe(II) constitu ted 76% or 25% of the accessible redox-active iron on the surface of c rocidolite or amosite, respectively. The method may be applied to othe r types of solid materials with redox-active species on their surfaces .