Balancing redox activity allowing spectrophotometric detection of Au(I) using tetramethylbenzidine dihydrochloride

Aqueous, acid solutions containing balanced amounts of a strong reductant (formaldehyde, HCHO) and a strong oxididant (N-bromosuccinimide, NBS) allow the first sensitive spectrophotometric analysis of monovalent gold ion, Au(I), using oxidation of color reagent 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB). This new method enables various oxidation states of Au ion to be quantified by balancing reduction potential in a Au solution. At low reductant levels, NBS oxidizes Au(I), which linearly suppresses subsequent oxidation of TMB by NBS to its blue charge-transfer complex of diamine and diimine to 2.00 mg L^-1 of Au, resulting in reduced color formation. The linear range of Au(I) quantitation was increased substantially relative to existing methods: from 0.005 to 1.00 mg L^-1 (R² = 0.988). For this range, the limit of detection was 0.0025 mg L^-1, which is comparable to the best reported spectroscopic method to analyze Au(III). At relatively high reductant levels, Au(I) is reduced to Au(0), then subsequently oxidized from Au(0) to Au(III) by addition of NBS. TMB is oxidized to its blue charge-transfer complex via the reduction of the reoxidized Au(III) to Au(0). Balancing redox conditions of HCHO/NBS at a molar ration of 22.7 allows quantitative measurement of Au(I) across a linear concentration range of 0.05-2.00 mg L^-1 (R² = 0.997). This balancing redox condition could allow sensitive, quantitative, spectrophotometric analysis of other metal ions besides Au by targeting the metal ion's reduction potential with an associated redox-sensitive color reagent.