Silver Removal from an X-Ray Fixer Solution by means of a Potentiostatically-Controlled Rotating Cylinder Electrode

The removal of silver from a 1 litre batch of an X-ray fixer solution (pH 4.7, 30 °C) is studied using a laboratory rotating cylinder electrode, RCE (diameter 5.0 cm, length 2.5 cm, area 40 cm2) rotating at 175 revolutions per minute. Performance is monitored by measuring the dissolved silver concentration, cell current and cell voltage as a function of time under potentiostatic, i.e., constant potential, control of the RCE (cathode potential -0.500 to -0.650 V vs SCE). The rate of concentration decay is seen to be enhanced by: (i)an increased cathode area, (ii) a roughened cathode surface, (iii) a faster rotation speed, (iv) a higher temperature, and (v) the use of a more negative cathode potential. Potentials more negative than -0.725 V vs SCE resulted in unwanted silver sulphide deposition. The experimental data are used to calculate figures of merit, including the cumulative current efficiency and the apparent mass transport coefficient, which serve to characterize the performance of the RCE.