MadSci Network: Chemistry

Re: why is the convex surface of a spoon only plated during electrolysis?

Date: Tue May 30 21:13:44 2000
Posted By: Charlie Crutchfield, Retired
Area of science: Chemistry
ID: 958702607.Ch

Dear Mr.Khawevh,

In electroplating this kind of problem is common. It is called poor 
"throwing". if you have an encyclopedia available, look up 

The electrical current is what carries the Cu ions through the solution to 
the surface of the cathode electrode, in your case the cathode is a spoon.. 
The current must get equally to all surfaces of the cathode for an even 

Consider an exaggerated situation. Instead of a spoon, you wish to plate 
all surfaces, inside and outside, of a small section of tubing. The piece 
of tubing is hung in the solution and the currrent is applied. The current 
will go to the outer surface of the tubing, but very little will go to the 
inside. The current will always travel by way of the shortest path, that of 
least resistance.

Also, when the copper is plated out of the solution, the solution is 
depleted of copper in that area, and of course no more can plate out. 
Outside the tube, normal circulation. etc. will bring in fresh copper 
solution. Inside the tube, there will be not only very litle electrical 
current, but very little replacement of any copper that had plated out.

One question I have is, what is the exact arrangement of your 
electroplating cell?  How far fron the anode is the cathode [the spoon]?

What side of the spoon faces the anode,is it the convex, or concave sides 
or the edge? If you place the concave side close to the anode you may be 
able to plate it as well as the convex side.  

What is the shape of the anode? Maybe make the anode in the shape of a 
ring, with the spoon [cathode] in the middle of the ring. Then maybe all 
surfaces will then be plated equally.

One experiment may show you what is happening. Obtain an ohm-meter. 
Disconnect the electric current supply from the plating cell. Measure the 
resistance between the anode and various points in the plating bath, 
including the convex and concave areas of the spoon and the electrolyte 
close to these areas.

                        Regards, Charlie Crutchfield 

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