Correlation Between pH & Electrolysis Rate

[Incredinoob]

Hi,

I have been performing an experiment to determine whether there is any correlation between pH and the rate of electrolysis. I have performed it using monobasic, dibasic and tribasic acids of constant concentration. However, I can't seem to find any correlation. Is there something I should be looking for? I tried utilising Faraday's laws of electrolysis, but it doesn't seem to be leading me in the right direction because it doesn't take account of pH.

Thanks in advance.

 

[Borek]

Electrolysis of what?

In general electrolysis speed depends on many factors - transport, potential, solution conductivity. Designing an experiment in which pH changes while all other things are kept constant is quite difficult.

 

[Incredinoob]

Electrolysis of water. I used a couple of acid solutions - citric, sulphuric, acetic, hydrochloric at constant concentration (0.1M). Shouldn't pH be directly related to the to solution conductivity? and if the same amount of charge is supplied to a given solution, shouldn't the amount of substance liberated at the cathode and anode be the same? Is it dependent on the equivalent weight?

 

[Bystander]

"Conductivity? Charge? Equivalent weight?" One question at a time.

 

[Borek]

Shouldn't pH be directly related to the to solution conductivity?

In general - no. It may easily happen for some solutions, as H⁺ has the highest limiting molar conductivity of all ions.

and if the same amount of charge is supplied to a given solution, shouldn't the amount of substance liberated at the cathode and anode be the same?

That's an easy question, answered by the Faraday's law of electrolysis. However, it doesn't say anything about rate.

 

[Incredinoob]

Thank you very much Borek. As for equivalent weight of the acid: will it play a definitive role in determining how much of the electrolyte is evolved at the electrodes? Keeping in mind that most of my reactions evolved Hydrogen and Oxygen whereas some (such as HCl and NaCl) evolved Hydrogen and Chlorine, I am not sure what plays a role as of now.

Thanks once again.

 

[Borek]

Keeping in mind that most of my reactions evolved Hydrogen and Oxygen whereas some (such as HCl and NaCl) evolved Hydrogen and Chlorine, I am not sure what plays a role as of now.

You have to define precisely what you mean by the "rate of electrolysis", otherwise you will be comparing apples and oranges. If your different reagents react already differently, looking for a dependence between the rate and pH is a waste of time and won't lead to any reasonable conclusion.

In other words: you are checking how the speed of the car depends on the drag coefficient, ignoring different engines and transmissions.

 

[Incredinoob]

I'm defining rate of electrolysis as the amount of hydrogen evolved at the cathode. I am measuring this as a mass defect in the electrolytic apparatus. Since Hydrogen and Oxygen are evolved in all reactions except those of Hydrochloric acid and sodium chloride, I have adjusted for the amount of hydrogen evolved in each, as a fraction of the total mass defect. I too have realized that pH and rate of electrolysis do not mesh. I was wondering if there is any reasonable relationship between rate of electrolysis and equivalent weight. Also I realize that the methodology is flawed to some extent but I just wish to know if in theory there may be a correlation.

Thanks once again.

 

[Borek]

I'm defining rate of electrolysis as the amount of hydrogen evolved

I was wondering if there is any reasonable relationship between rate of electrolysis and equivalent weight.

Equivalent weight for hydrogen is constant, no matter what the solution contains.

 

[Incredinoob]

So despite different acids having different equivalent weights, there will be no role played by their individual equivalent weights?

Secondly: What could be a possible explanation that citric acid evolves more hydrogen in this scenario than hydrochloric acid?

 

[Borek]

So despite different acids having different equivalent weights, there will be no role played by their individual equivalent weights?

Please elaborate on the scenario we are talking about. We started with electrolysis, Faraday's law is clear here - as long as we are talking about number of moles of hydrogen produced the only thing that matters is the charge and number of electrons involved.

Secondly: What could be a possible explanation that citric acid evolves more hydrogen in this scenario than hydrochloric acid?

See above - it is not clear what is the scenario involved, so it is not clear what are the things being compared.