The aerobic acidophilic bacterium Acidithiobacillus ferrooxidans oxidizes Fe²⁺ and S₂O₃^2– ions by consuming oxygen. An amperometric biosensor was designed including an oxygen probe as transducer and a recognition element immobilized by a suitable home-made membrane. This biosensor was used for the indirect amperometric determination of Cr₂O₇^2– ions owing to methods based on a mediator (Fe²⁺) or titration.

Using the mediator, the biosensor response versus Cr₂O₇^2– was linear up to 0.4 mmol L^–1, with a response time of, respectively, 51 s (2 × 10^–5 mol L^–1 Cr₂O₇^2–) and 61 s (6 × 10^–5 mol L–1 Cr₂O₇^2–). The method sensitivity was 816 μA L mol^–1. Response time and measurement sensitivity depended on membrane material and technique for biomass immobilization. For example, their values were 90 s—200 μA L mol^–1 when using a glass-felt membrane and 540 s—4.95 μA L mol^–1 with a carbon felt one to determine a concentration of 2 × 10^–5 mol L^–1 Cr₂O₇^2–.

For the titration method, the biosensor is used to determine the equivalence point. The relative error of quantitative analysis was lower than 5%.