= Solution
<nickel> is less noble than <copper> and can dissolve anodically, but its <reduction> requires a much more negative cathodic <electrode potential>. It therefore normally accumulates in solution rather than codepositing with <copper>. <lead> can also oxidize, but sulfate can precipitate the resulting <lead> <ions> as poorly soluble <lead sulfate>, contributing to <anode slime> or <passivation>. <gold> normally remains in the solid residue. <silver> is normally recovered predominantly in <anode slime>, but its path can include dissolution and reprecipitation or cementation: a permanent refusal of every <silver> atom to dissolve is not the mechanism. Experiments with controlled Cu–Ag–Pb <anodes>, a fixed sulfate <electrolyte> and SEM/EDS plus <diffraction> analysis explicitly find metallic <silver> and sulfate-containing slime phases.<The Role of Lead in Suppressing Passivation of High Silver-Containing Copper Anodes During Electrorefining by Labone L. Godirilwe and others>[printed pp.137–138 and 141, experimental design and identification of silver/lead-containing slime]{ref}
Selective <electrode potential>, solution management and physical exclusion of slime permit purer <copper> deposition. However, \b[the unusually large printed anodic slope prevents an unconditional silver-retention claim here]. The preceding calculation places the <anode> near $0.34+0.6788=1.0188$ V versus the <standard hydrogen electrode>, above the tabulated $\mathrm{Ag}^+$/Ag value. <silver> <oxidation> is then thermodynamically possible; any <silver> <ions> reaching the <cathode> can be reduced preferentially to <copper>. Actual activities, precipitation, cementation and transport determine contamination. The requested 99.999 wt% purity cannot be established from the listed standard potentials alone. Monitor <anode> potential against a reference, assay the <electrolyte> and deposit, and prevent slime entrainment; do not silently replace the printed 0.295 V/decade with a smaller number to remove this issue.
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