mike!—EA 43/5-6 (ISSUE) (839304)—MS 1661  Pergamon Electrochimica Acta, Vol. 43, Nos 5–6, pp. 449–455, 1998  1997 Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain 0013–4686/98 $19.00 + 0.00 PII: S0013-4686(97)00126-6 The effect upon limiting currents and potentials of coupling a rotating disc and cylindrical electrode with ultrasound J. P. Lorimer,* B. Pollet, S. S. Phull, T. J. Mason and D. J. Walton School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry CV1 5FB, U.K. (Received 17 February 1997) Abstract—The reduction of the silver cation, Ag + , was studied in dilute aqueous Na2S2O3/NaHSO3 solution at a platinum and stainless steel rotating electrode in the absence and presence of ultrasound. Under silent conditions, electrode potential measurements indicate that silver deposition is the main reaction occurring at the cathode during the electrolysis of a model photographic processing solution. Under insonation conditions at 20 kHz the reduction wave shifts anodically with increasing ultrasonic power. Similarly the discharge of hydrogen shifted anodically with increasing power. It is also shown that below 1000 r.p.m. insonation leads to improved limiting currents when compared to the silent system. As the rotation speed of the electrode increases above 1000 r.p.m., the limiting current decreases as the ultrasonic power increases.  1997 Published by Elsevier Science Ltd Key words : sonoelectrochemistry, electrodeposition, rotating disc electrode, rotating cylindrical electrode. INTRODUCTION Sonochemistry, the chemical effects of intense sound waves travelling through a liquid, has been known since 1927. Various workers have demonstrated that simultaneous ultrasonic irradiation of electrochemi- cal systems can modify limiting parameters and significantly improve electrochemical reactions [1, 2]. These improvements include enhanced diffusion processes, increased yields and current efficiencies, lower overpotentials, suppressed electrode fouling and alteration of reaction mechanism. One well-characterised effect of ultrasonic ir- radiation is the generation and subsequent collapse of cavitation bubbles within the electrochemical cell which can be important both within the electrolyte medium and near to the electrode surface. The electrode surface causes asymmetrical collapse of a bubble which in turn leads to the formation of a high velocity jet of liquid being directed toward the surface. This jetting, together with microstreaming, is thought to lead to the disruption of the mass transfer boundary layer [3] at the electrode. This improves the overall mass transfer of the system and, as a consequence, the electrode reaction rates. Our interest was in the electrochemical removal of silver from photographic processing solutions, which consist mainly of sodium thiosulphate, sodium bisulphite and silver halides. In the photographic industry, this type of solution is often called ‘‘wash’’ solution. This is an important commercial process which provides a route to the recovery of high-purity silver and also the possibility of recycling the remaining media [4]. Since photographic processing solutions have a low metal content, the deposition is mainly diffusion controlled. Thus to achieve a suitable space-time-yield in the electrochemical cell it is necessary to provide either high mass-transfer conditions to the electrode or increase the specific electrode area [5]. This paper reports the effect of applying simul- taneous ultrasound at different powers upon both a rotating disc electrode (rde ) and a rotating cylindrical electrode (rce ) for Ag + /Ag. In our conditions, Ag + /Ag is an irreversible system as shown in Fig. 1. The paper also describes the effect on several parameters important in rotating elec- *Author to whom correspondence should be addressed. 449