Why is silver cyanide used for electroplating

Question

(Partially answered by Safe silver electroplating solutions)

Why is silver cyanide used as an electrolyte in silver plating? (source) There are other (cheaper, less toxic) silver salts, such as silver nitrate. The linked question says that using the nitrate results "makes a fairly rough film that can be somewhat easily removed with abrasion". How does the anion affect the adhesion of the film of silver to the cathode? Why does cyanide result in a strong stable layer of silver, but nitrate (or otherwise) does not?

Answer 1

Note the free cyanide is highly toxic. Cyanide bound to metal ions is less toxic, often much less or even less toxic than table salt. Potassium_ferrocyanide $\ce{K4[Fe(CN)6]}$ has LD50 $\pu{6400 mg/kg}$ (oral, rat), what would be about $\pu{500 g}$ for $\ce{75 kg}$ person, assuming the same LD50. $\ce{NaCl}$ has LD50 $\pu{3000 mg/kg}$ (oral,rat).

Silver cyanide AgCN is hardly used, being insoluble. It would be soluble potassium silver cyanide K[Ag(CN)2](commercially available), with silver strongly bound to cyanide.

Silver (or metal in general) ions too easily available for deposition on cathodes, as are silver ions from silver nitrate, lead to diffusion controlled metal deposition. This prefers exposed metals surfaces which are amplified. This is taken care, aside of solution thickeners, by using metal complexes like $\ce{[Ag(CN)2]-}$ that have to be broken. Additionally, cathodes do not attract anions that migrate to cathodes only to address concentration gradient or to maintain electro-neutrality of solutions.

---

For curiosity, metallic silver dissolves in potassium cyanide solution, evolving hydrogen. Gold needs to be pushed by oxygen to be dissolved in cyanide.

$$\ce{2 Ag(s) + 4 CN-(aq) + 2 H2O(l) \\ -> 2 [Ag(CN)2]-(aq) + 2 OH-(aq) + H2(g)}$$

$$\ce{4 Au(s) + 8 CN-(aq) + 2 H2O(l) + O2(g) \\ -> 4 [Au(CN)2]-(aq) + 4 OH-(aq)}$$

Answer 2

Silver cyanide is used for silver electroplating because of the concentration of the free silver ions in solution which is so low that it is not far from zero. Due to the high values of its complex equilibrium formation constant, the complex $\ce{[Ag(CN)2]^-}$ is practically not dissociated in $\ce{Ag^+}$ ions. If such a complex solution is electrolyzed, only the rare free silver ions are discharged at the cathode. So at the beginning of the electrolysis, these free $\ce{Ag^+}$ ions produce a discontinuous deposit of metallic atoms on the cathode.

Let's consider the mechanism of electrolysis in the microscopic level. As the first metallic atoms produced on the cathode by the beginning of the electrolysis look like small points or bumps, these tiny bumps will attract electric field lines. As a consequence, the next positive ions will get discharged most probably on this bump, making it bigger, if the solution is concentrated enough. More and more positive ions will be attracted and discharged on this point. The metallic deposit will be localized around specific points. The surface of the cathode becomes rough and course.

But if the free ion concentration is extremely low, as with $\ce{Ag^+}$ ions in $\ce{[Ag(CN)2]^-}$ solutions, the first $\ce{Ag}$ atom is too far from the next $\ce{Ag+}$ ion. This metallic ion is not attracted by the $\ce{Ag}$ "bump". It is attracted by the whole cathode. It touches the cathode and get discharged equally on its surface, independently from the position of the previously deposited Ag atom. The Ag deposit makes a smooth layer which looks like a mirror.

To summarize, the formation of a mirror can only occur in a solution of a silver complex where the free (non-complexed) ion concentration is extremely weak, like in $\ce{[Ag(CN)2]^-}$ solutions. Solutions of other silver compounds, like silver nitrate, will only produce rough silver deposits by electrolysis. No mirror !

Answer 3

The problem with electroplating the more noble metals such as copper, silver and gold is that these metals will react with the substrate [cathode] with a competitive immersion plating reaction. This corrodes the surface, causes an uneven, less adherent plate and contaminates the solution. There are two common methods, usually used in conjunction, to combat this: The first is to use a thin electroless coating [or strike plating] of copper or even nickel that will not conversion plate under the conditions. This is also the method to plate a non-conductive surface. [Tollens reagent is an example of an electroless plating method]. The second method is to reduce the chemical activity of the metal ion with an appropriate complexing ion. The complexing agents cause ion transport problems and require good mixing and appropriate anodes, pure silver is best. There are proprietary baths to do this some of which use cyanide as the complexing agent. Electroplating has a high art content and you will have to contact the manufacturers for the actual proprietary mixes and recommended procedures.

The classical method to measure quantitative current flow is the silver coulometer, a cell placed in series with a silver anode, a silver nitrate solution and a platinum cathode. No danger of immersion plating and I understand the plating adhered well. Electrolytic deposition of copper on a platinum cathode can give a quantitative removal of copper [beautifully even and adherent] with appropriate control of voltage and pH.

Answer 4

Silver cyanide is commonly used for electroplating because it is a highly conductive and efficient electroplating solution. When used in electroplating, silver cyanide ions are deposited onto a substrate by passing an electric current through the solution. The silver ions are attracted to the cathode (negative electrode) and are reduced to metallic silver, which is then deposited onto the substrate.

There are several reasons why silver cyanide is a good choice for electroplating:

High conductivity: Silver cyanide is a highly conductive solution, which makes it an efficient choice for electroplating. This is because the higher the conductivity of the solution, the faster the electroplating process can be completed.

High silver content: Silver cyanide contains a high concentration of silver ions, which allows for a thin and uniform layer of silver to be deposited onto the substrate.

Easy to use: Silver cyanide is a stable and easy-to-use electroplating solution, making it suitable for use in a variety of electroplating applications.

Good corrosion resistance: Silver has excellent corrosion resistance, making it a good choice for electroplating applications where corrosion resistance is important.

Overall, silver cyanide is a popular choice for electroplating due to its high conductivity, high silver content, ease of use, and good corrosion resistance.