Electrode kinetics in cyanide silver-plating electrolytes at different surface coverages by lead adatoms

Abstract

INTRODUCTIONIn a previous work and in accordance with [1][2][3] it was demonstrated [4] that the silver deposition in cyanide solutions accelerates in the presence of microscopic (~10 -5 to 10 -4 M) quantities of lead compounds. Hydroxy compounds of lead affect the gold electrodeposition similarly [5][6][7], which was linked with electrocatalysis of this process by lead adatoms. Lead atoms undergo adsorption on silver as well [8]. Therefore, one could expect that the effect of lead compounds on the silver deposition has the same nature that in the gold deposition. Approaches to studying the deposition and dissolution while monitoring the surface coverage θ by adatoms of catalytically active metals [7,9,10] are used here to examine how microscopic quantities of lead compounds affect the electrode kinetics in cyanide silver-plating electrolytes. EXPERIMENTALObviously, kinetic parameters of an electrochemical process in the presence of a catalyst are dependent on its concentration in the reaction zone, i.e. at the electrode surface. Therefore, to determine these, it is necessary at the very least to monitor the constancy of θ when taking measurements. In our case the catalyst is lead atoms, which form at the silver surface out of anions H ê b present in the solution. The adsorption of atoms of "foreign" metals at a metal substrate surface is known to begin at potentials that are by several hundreds of millivolts more positive O 2 -than the equilibrium value of this indicator, i.e. at an "undervoltage" [8]. The equilibrium potential of the reaction HPb(1)Here, c 1 and c 2 are concentrations of hydroxy compounds of lead and hydroxide ions. In most of the solutions under study ( c 1 = 10 -5 -10 -4 M), which contained 0.05 M AgNO 3 , 1.0 M KCN, and 0.3 M KOH as basic components, the equilibrium potential of silver is close to -0.5 V. Thus, as lead ions form no strong compounds with cyanide ions, E eq Ӎ -0.65 V at c 2 Ӎ 0.3 M. Hence, lead adatoms might affect both the initial portion of an anodic polarization curve and the entire cathodic silver deposition curve, for some atoms of lead remain on the surface despite its co-deposition with silver.The surface density of lead atoms may be determined from the balance of supply rates of hydroxy compounds of lead (it is limited by, more likely than not, diffusion) and the inclusion of lead atoms into the deposit [7,9,12] and described by the equation(2)Here, D is the diffusion coefficient for lead ions in solution; δ = 0.01 cm is the effective diffusion layer thickness [9]; Γ lim Ӎ 3 × 10 -9 mol cm -2 is the limiting silver surface coverage by lead atoms; b Ӎ 0.09 cm 2 mA -1 s -1 is a "capture" coefficient [12]; and i is the current density, in mA cm -2 [9].Abstract -Effective values of exchange current i 0 , cathodic and anodic transfer coefficients α and β , and cathodic and anodic reaction orders with respect to cyanide ions ( P c , P ‡ ) are measured in cyanide silver-plating electrolytes at different surface coverages by lead adatoms θ . With increasing coverage, i 0 and α...

Investigation of the electroreduction of silver sulfite complexes by means of electrochemical FFT impedance spectroscopy

“…Considerable attention has been paid in the literature to the process of silver electrodeposition from cyanide electrolytes [2,3] and till today this process is widely used in different fields of industry and science [4][5][6][7][8][9][10][11][12][13][14]. There are many publications dedicated to the application and mechanism of silver electroreduction from cyanide baths.…”