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Effect of voltage on composition of plated material

I am working on a projects to recover Indium from Indium Tin Oxide. We end the process by plating the indium for analysis. We also ran a DOE in this project in which one factor we changed was the voltage applied to plating (8V and 15V) in order to see if this had an effect on the purity on the end product. Our results show that we have higher purity when the applied voltage is low and a low purity when it is high. In the plating solution we have a high concentration of In3+ ions and low concentration of Sn4+ ion (we know that the Sn(II) forms an insoluble salt so that will not plate). With what I have researched the standard electrode potential of Sn(IV) is +0.01 and In(III) is -0.34 which, to my understanding, this means that we should be plating the Tin faster and then the Indium. Taking in concentration and the Nernst equation the potentials of Sn is still more noble then In. I am not sure what could be causing this trend. We have theories from researching double layers, over-voltages, and reverse potentials, but I am can't narrow it down. Is this trend similar to kinetics where the voltage is a driving force that has a greater effect on the ions with move positive E values. Some input would be greatly appreciated.

Andrew Righter
Student - San Luis Obispo, California, USA
May 20, 2010

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Hi, Andrew. Interesting topic -- thanks for it. My suspicion is that your theories are right but that you are operating out of the range to which they apply. Somewhat analgous to the idea that the conventional laws of fluid dynamics don't apply once you are into supersonic flow.

Unless you are using some kind of high velocity jets to spray the solution onto the cathode, even 8 Volts sounds quite high and 15 Volts very very high. I think your theories would work fine if the lower voltage was 2 Volts and the higher voltage were 5 volts. But in the range you are in, I suspect that there is no smooth diffusion of favored ions through the boundary layer, no slow growth of a crystal structure, but fluid dynamic / electrochemical chaos. Just a suspicion, not enough to be called a theory; keep us posted :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 21, 2010

Sn(IV) is not directly reduced to metallic Sn but to Sn (II), if you look in your table of electrode potentials the equation should be Sn(IV) + 2e -> Sn(II), the potential is +0.15V,so if the Sn(II) is not soluble , it will precipitate and you will have no Sn deposition

Marcus Hahn
- Zug, Switzerland
May 31, 2010