SILVER REMOVAL FROM AN X-RAY FIXER SOLUTION BY MEANS OF A POTENTIOSTATICALLY-CONTROLLED ROTATING CYLINDER

The removal of silver from a 1 litre batch of an X-ray fixer solution (pH 4.7, 30 degrees C) is studied using a laboratory rotating cylinder electrode, RCE (diameter 5.0 cm, length 2.5 cm, urea 40 cm(2)) rotati ng at 175 revolutions per minute. Performance is monitored by measurin g the dissolved silver concentration, cell current and cell voltage as a function of time under potentiostatic, i.e., constant potential, co ntrol of tile RCE (cathode potential -0.500 to -0.650 V vs SCE). The r ate of concentration decay is seen to be enhanced by: (i) an increased cathode area, (ii) a roughened cathode surface, (iii) a faster rotati on speed, (iv) a higher temperature, and (v) the use of a move negativ e cathode potential. Potentials more negative than -0.725 V vs SCE res ulted in unwanted silver sulphide deposition. The experimental data ar e used to calculated figures of merit, including the cumulative curren t efficiency and the apparent mass transport coefficient, which serve to characterize the performance of the RCE.