A little more about why there is a rotating drum cathode on these units. A little electroplating 101.
I'm no chemist, but I'm pretty sure this is how it works. The silver in these solutions is in the form of a negative complex silver thiosulfate ion. Since this ion is negative, it is attracted to the anode and repelled by the cathode. All plating occurs only from the thin solution layer right next to the cathode and this layer's thickness is probably measured in nanometers. Once this layer is somewhat depleted, other reactions must occur, as long as there is current flow. In this case, the next cathode reaction, in the order of things, is the reduction of the thiosulfate ion to the sulfide ion. The sulfide ion precipitates silver sulfide, a black smelly material.
The rotating cathode continually puts fresh solution into the cathode film and thus prevents the formation of sulfide. You can not plate out a sound silver deposit from a static fixer plating system. You must have some sort of agitation present that will continually refresh the cathode film. Other methods of agitation, such as ultrasonics or the impingement of solution on the cathode, can be successful.
Even with a rotating cathode, another thing that will cause sulfide to be produced is when the silver concentration in the overall solution becomes depleted beyond a certain level. In practice, they often plate, until the silver reaches a certain level, and then polish the solution by removing the remaining silver in a bucket of steel wool - a cementation reaction.
In most non-cyanide plating solutions, such as those used normally used to plate nickel or copper, the metal ion is positive and is attracted to the cathode. Therefore, agitation is not as critical in these cases.
For some reason, although gold or silver form negative complex ions in most solutions, here again, agitation is usually not very critical. Probably because there is no secondary reaction (such as the sulfide production in the fixer solution), except for water splitting, that has a high enough cross-section (probability) to occur. The solution keeps plating gold, even though no agitation at all can cause the gold to plate at a lower rate. Water will then split to adjust the difference and maintain the set current flow.
For each valence form of each metal ion, there is a rate of deposition (plating) that will occur when 100% of the amps applied deposit metal, with no side reactions. This is called 100% "efficiency". For silver, which always has a valence (I realize this term is antiquated, but, so am I) of +1, it plates, at 100% efficiency, 4.02 (I think) grams per amp per hour. You can never plate silver faster than that. Normally, the lower the valence, the faster it will plate. In a cyanide solution, gold (valence of +1) will plate 3 times faster than out of a chloride system (valence of +3), at 100% efficiencies.
In practice, no metal plates at 100% efficiency. 98+% is about as high as you get from any metal. Bumper-type chrome plates at about 40% efficiency. The reduction in efficiency is always caused by the kicking in of a secondary reaction (or, is it the other way around?), of which water splitting at the cathode, (OH- ion and H2 gas are produced from the water) is the most common , which, in general, causes few problems. I could probably list 20 things that will effect plating efficiency - anode/cathode ratio, anode/cathode spacing, metal concentration(s) in solution, contamination (both organic and inorganic), condition of the cathode surface, agitation (or, lack of), etc., etc.
The anode efficiency (rate of dissolving) can also come into play. The rate of dissolving, at 100% efficiency, is exactly the same number as 100% efficiency plating, which is 4.02 g/amp-hour. For example, if the cathode efficiency is lower than the anode efficiency, the pH will climb. If the anode efficiency is lower, the pH will go down. This is due to the H+ ion (and O2 gas) produced from the water at the anode. If the efficiencies are equal, the pH won't change. The H+ and OH- neutralize each other and form H2O = water.
Sorry I got off on a tangent.
Chris
