Metal deposition

If you produce solid metal or a metal layer from a metal compound, this is called metal deposition . In most cases, a metal salt is reduced to the metal . This can be done by a chemical reaction with a reducing agent or by electrochemical reduction during electrolysis . A special case is the thermal decomposition of a compound, for example the deposition of arsenic in the Marsh sample .

Similar processes sometimes also occur in the natural environment, for example in copper deposits (red bed) in Katanga .

Electrolytic metal deposition

Electrolytic metal deposition is of great technical importance, especially for the extraction of many metals, such as aluminum . This is mostly done by fused-salt electrolysis . It is also used in electroplating to produce metallic layers, often for corrosion protection .

Basics

During the electrolysis of an electrolyte that contains various metal ions, the metals that are in a higher position in the electrochemical series - see also redox series - are deposited first . The concentration dependence of the deposition potential on the concentrations according to the Nernst equation and possible overvoltages must also be taken into account.

Technical

Technical electrolysis are often carried out with high currents, which is why high-current transformers are used. In addition, the alternating voltage must be converted into direct voltage for the electrolysis .

Influence of the pH value

The formation of hydrogen can also occur as a competitive reaction to metal deposition from aqueous solution, for example in electroplating . This increases power and electrolyte consumption. In addition, the elemental hydrogen usually arises in the form of gas bubbles that can locally block the metal deposition.

The evolution of hydrogen occurs through the reduction of H ⁺ , which is dissolved in acids as H ₃ O ⁺ :

${\displaystyle {\ce {2H+ + 2e^- -> H2}}}$

The concentration of the H ₃ O ⁺ ions is also given with the aid of the pH value . If you work in an acidic solution, as a first approximation no metals are deposited that are less noble than hydrogen. Then on the one hand overvoltage values ​​(ue) and on the other hand the pH value have to be considered. By inserting it into the Nernst equation , one comes to the requirement that a metal X can only be deposited if the following applies:

${\displaystyle E_{\ce {{X^{n+}} \over X}}^{0}+{0{,}059\,\mathrm {V} \over n}\cdot \lg \;c_{X^{n+}}\geq -0{,}059\,\mathrm {V} \cdot \mathrm {pH} +ue_{\ce {H_{2}}}}$

If you want to separate base metals, you sometimes have to work in the alkaline range in order to meet this requirement.

Chemical metal deposition

An important example is the production of a silver mirror by reducing silver salts.

Web link

• Investigations into electrochemical metal deposition at the Institute for Electrochemistry, Ulm University