Abstract: Method of and apparatus for the amperometric determination of the concentration of arendiazonium salts in a diazocoupling reaction mixture. The apparatus has an external body in the bottom part of which cleaning material is disposed, a shaft, an inner body mounted on the shaft, at least two electrodes located in the inner body, a commutator, each of the electrodes being independently connected with one segment of the commutator. The commutator forms a component part of the shaft of the inner body; an electric motor is connected with the shaft to drive the inner body, compression bearings with a spring and an adjusting screw being disposed for adjustment of the compressive force between the inner body with its electrodes and the cleaning material. Brushes cooperate with the commutator for alternately connecting spaced pairs of electrodes on the inner body of the measuring circuit and disconnecting them therefrom.