[Zensation]

I understand the two are tied together, I notice a reoccurring theme were many electroorganic synthesis seem to regard overall current density as the key determining factor in an electrolytic reaction. While, in nonorganic situations or electroplating situations, a steady voltage seems to be more important.

Can someone explain this? If current density, ie, the amount of electrons flowing at any one point on an electrode, is the most important aspect when it comes to electro-organic reductions, then why is voltage and voltage potentials so commonly discussed in various texts?

If a person had a power supply capable of either holding voltage steady, or current steady, which one would be most desirable to hold steady to keep a specific reaction going?


Edit:

One particular curiosity I have is regarding the metal Vanadium. It can be electrochemically reduced, as well as plated. If a solution of Vanadium is being stirred and electricity applied, how can reaction be prevented from electroplating onto the cathode, and instead reducing, or vice versa in the case that you do want to electroplate it?

[Borek]

Redox reactions occur at well defined potentials, so you have to control potential to be sure you know what reaction takes place.

Current is tied to the speed of the reaction - the higher the current, the faster the reaction. However, you can't increase the current just by changing the voltage, as you could get a completely different reaction.

[Zensation]

Thanks for the reply, Borek.

I spoke to some electrical engineers a few weeks back in depth and they were assuring me that if resistance of the system is unchanging, then changing the voltage will definitely effect the current, and vise versa. Are you saying this doesn't happen? Some videos I've seen of small variable dc switch power supplies being used to electrolysis, would have two knobs for current and voltage. You could set one of them, and the other would automatically adjust in accordance to any changing resistance in the system.

Now, I'm no electricity expert, to be clear. When you talk about well defined potentials, how does this relate to voltage and current? Certain reactions happen at certain current densities? Or certain reactions happen at certain voltages?

I read some literature somewhere that stated reduction of a specific organic molecule(which I cannot for the life of me remember), occurs readily at 200mA per cm^2 of cathode area, where as at 100mA per cm^2  the reaction does not occur at all. You're saying that current doesn't effect the reaction itself, but only the speed?

Is there a specific 'potential' that electroplating can occur, and a reaction not, or a specific potential where a reaction occurs, but electroplating doesn't? Will a metal ion automatically plate to the cathode if it is reduced regardless of the potential?

Sorry for all the questions... I have been reading into this topic for several weeks on a daily basis and still am having trouble forming a clear picture of the mechanisms behind these reactions on an electron/voltage/current/etc level. Many of the sites I have been reading only seem to give very incomplete and partial information regarding these explanations

[Borek]

I spoke to some electrical engineers a few weeks back in depth and they were assuring me that if resistance of the system is unchanging

If.

Plenty of methods of changing the resistance of the system.

Some videos I've seen of small variable dc switch power supplies being used to electrolysis, would have two knobs for current and voltage. You could set one of them, and the other would automatically adjust in accordance to any changing resistance in the system.

To some extent it can work, but as I explained earlier, when you change the voltage you risk reactions that occur won't be the ones you want. In the case of some simple systems (like water electrolysis) increasing voltage can speed up the reaction, in other systems it will start another, unwanted reactions.

Now, I'm no electricity expert, to be clear. When you talk about well defined potentials, how does this relate to voltage and current? Certain reactions happen at certain current densities? Or certain reactions happen at certain voltages?

Which reaction takes place depends on the voltage, not on the current density.

I read some literature somewhere that stated reduction of a specific organic molecule(which I cannot for the life of me remember), occurs readily at 200mA per cm^2 of cathode area, where as at 100mA per cm^2  the reaction does not occur at all. You're saying that current doesn't effect the reaction itself, but only the speed?

Hard to comment on not seeing the context. In general current density should not matter, 100 mA/cm² should be just twice slower than 200 mA/cm². But perhaps to increase the current density they had to change the potential applied and for some reasons it was easier to control the system measuring the current, not the potential.

Is there a specific 'potential' that electroplating can occur, and a reaction not, or a specific potential where a reaction occurs, but electroplating doesn't? Will a metal ion automatically plate to the cathode if it is reduced regardless of the potential?

Good recipes should contain information about the potential that should be applied. Some recipes will not give this information, but the solution composition is selected in such a way side reactions are not interfering (much), so as long as the voltage is high enough to keep the current flowing and low enough gases don't evolve, the main reaction is the one dominating the solution. Recipes are often designed in such a way they are relatively fool proof, but even then quality of the surface can depend on the potential applied.

Sorry, this is a very wide subject, and while I know a little bit I am far from being an expert. There are thick books on the subject, but I suggest you at least try to assimilate Phys101 and GenChem101 level of electricity and redox reactions.

[Zensation]

There are indeed plenty of recipes.... though I have a main interest in understanding mechanisms

Here's a curious idea you may or may not know about.. if current density is only relevant to the speed, then theoretically should the same reaction occur with a 20cm^2 cathode, as would at a 200cm^2 cathode? Or would swapping these out automatically cause other values in the system to adjust? Current density with regard to electrode surface area seems to be an incredibly key detail stressed in many discussions of electrochemistry. I do suppose that with many simple molecules, the type of reaction that can occur is somewhat limited anyways.

http://forum.caswellplating.com/electroplating-questions/8140-voltage-electroplating.html

Here is a large discussion about electroplating being mostly current dependent. There also is debate there that some types of plating are current dependent, and others are voltage dependent. o.O

[Borek]

Changing geometry of the conductor (cell) changes the conductor (cell) resistance. This is Phys101.

Current density can change properties of the surface produced (so it has to be controlled), but it is voltage that defines reactions taking place. Too fast and too slow deposition can end in a dull surface, or in one that flakes out, and that's where the current comes in. That's what they refer to.

Not my fault that people know practical side of the process, but have no idea about underlying chemistry. I have explained teh details several times, so I am not going to repeat it again.

[Zensation]

Yes Yes no need to repeat yourself. I am infinitely greatful that you have taken the time to respond. With the above links I was not trying to prove you wrong. I was just pointing out what I had read.

I read a lot of electrochemistry Journal papers and one thing that strikes me curious is some of them have a focus on current density, and others are voltage.  I suppose it may have some degree of influence of the power supply that they are working with and how it behaves.. who knows. I plan to do some experimenting in a few weeks so hopefully that will shed some light on my desire to understand this at a molecular level