MadSci Network: Chemistry

Re: How do you chrome plate metals?

Date: Wed Jun 9 02:34:24 1999
Posted By: Jonathan Feldman, Undergraduate, Applied Chemistry, University of Western Sydney
Area of science: Chemistry
ID: 928790894.Ch

Hi Jeff

Chrome plating, (and most other types of plating) are part of a branch of 
chemistry known as "Reduction-Oxidation", or Redox for short.

All redox reaction involve a transfer of electrons from one atom to 
another.  If a substance is losing electrons, it is being "oxidised".  If 
it is gaining electrons, it is being "reduced".

Since plating involves moving electrons around, it is better known as 

Electroplating is generally performed by reducing metal ions to solid 
metal. Eg:

Cu2+  +  2e-  --->  Cu(s)	(e- is an electron)

In the case of chrome plating, the object to be plated is electrically 
connected to the negative terminal of a battery, or other DC power source. 
 This is the source of electrons required for the reaction.  The positive 
terminal is connected to an inert electrode (eg, platinum), and they are 
both inserted (not touching!) into an acidified dichromate solution.

When the correct voltage is applied, the following reactions take place:

Cr2O7(2-) + 14 H+  +  12 e- --->  Cr(s) + 7 H2O
2 H20 ---> O2 + 4 H+  +  4 e-

The chromium solid (in the first reaction) is deposited onto the surface of 
whatever is attached to the negative terminal of the power source, 
and oxygen gas is produced at the inert electrode.

There are a lot of good reduction/oxidation resources.  You can start by 
looking in the MadSci library.  Also have a look at How Electroplating 

However, the best place to go is probably your local library.  If you look 
under Dewey No. 537, you'll find chemistry textbooks, which usually have at 
least a whole chapter devoted to Redox and electroplating.

Hope this helps!
Jonathan Feldman, UWS Nepean

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