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topic 59398

Electroplating: Is it possible to have deposition mass greater than theoretical values of Faraday's law?

September 5, 2013

Q. Dear Madame/Sir,

I am a Year 12 Chemistry student from Australia in desperate need of your assistance!
I've recently conduced an experiment on the effect of current on electrodeposition mass, appearance and adhesiveness. For each of the amperage levels I've tested (0.25A, 0.5A, 0.75A, and 0.1A), the deposition mass is somewhat greater than the corresponding theoretical values (obtained using Faraday's law). After having plotted my results in excel, I find that there is certainly a clear trend in my results in that, as current doubles, so does the mass deposited. However, I'm still incredibly confused as to why these practical values are greater than their corresponding theoretical ones. This must be due to a source of error, right? Considering Faraday's law predicts the maximum amount of metal that can be deposited (for any given current and electrolysis period).



Kind Regards,

P.S. Just to clarify, I used copper electrodes for both the anode and cathode.

Gaitee-Ara Taseer
student - Canberra, Australian Capital Territory (ACT), Australia

First of two simultaneous responses -- September 6, 2013

A. Based on the limited amount of information provided, I will guess that the amp meter was not properly calibrated.
As things plate, they tend to form a somewhat rough surface and or nodules on the edges. The efficiency of the plating bath will vary as the ASD (ASF) changes. It will never get 100% efficiency!! SO. more plate than theoretical is not possible.
Did you do a good job of rinsing and drying the plated part??

James Watts
- Navarre, Florida

Second of two simultaneous responses -- September 7, 2013

A. Hi Gaitee
The answer to the headline question is yes. It is possible to have a non electrolytic reaction going on at the same time as the electrolysis and give a high result. But this is unlikely.
Long before the availability of accurate ammeters, current was measured with either a silver or a copper electrometer which depends on the fact that the mass deposited was so accurately correlated with the current.
There are caveats. Evolution of hydrogen or reduction of organic species (brighteners) can use some of the available electrons leading to a lower than expected mass deposited. However the sum total of all reactions always obeys Faraday. The ratio of expected to actual mass is known as the Faradic Efficiency and is very important to the economics of the plating process.
So, assuming that you are using a simple pure acid copper solution, how to explain your results?
I would suggest that you should look first at your power supply. Is it smooth DC current and is the ammeter traceably calibrated? You might also check that your deposit is fully dried, especially if it has a rough surface.

geoff smith
Geoff Smith
Hampshire, England

September 10, 2013

A. The probability of an autocatalytic (non electrolytic) deposition is very very near zero with a copper anode, cathode and solution. Not much to serve as a potential for electron movement.
If the cathode was steel and you did not go in hot and were slow in bringing up the current, then you could have some non electrolytic weight gain.

James Watts
- Navarre, Florida

September 11, 2013

Q. So you guys think that it may be an issue associated with the current? In that the multimeter I used to detect the current in the circuit was not properly calibrated?

Gaitee-Ara Taseer
- Canberra, Australian Capital Territory (ACT), Australia

September 12, 2013

That's about it.
Meters go wrong, Faraday does not.

geoff smith
Geoff Smith
Hampshire, England

September 17, 2013

A. It is possible, but unlikely, that in addition to electroplating that follows Faraday's Law of Electrolysis, you are getting some "immersion" plating. This could happen with an acid copper plating solution and steel, or with silver plating solution and steel, etc. This is a displacement reaction where for every copper atom that immersion plates out, an atom of iron is dissolved.

Lyle Kirman
consultant - Cleveland Heights, Ohio

September 18, 2013

A. What is your electrolyte? Have you considered the possibility of depositing a Cu(1) ion and using the common value of Cu(2) ion for calculation of equivalent weight?

Hamilton Solidum
- Mays Landing, New Jersey, USA

December 4, 2013

A. Using a copper anode and cathode strongly suggests to me that the ammeter is not correctly calibrated. The chances of getting an immersion deposit form the system described is pretty well zero. As Geoff says, Faraday's Laws do not go wrong, except when the actual deposition process is not fully understood!

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

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