Correlation Between pH & Electrolysis Rate

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Discussion Overview

The discussion revolves around the relationship between pH and the rate of electrolysis, specifically in the context of various acid solutions. Participants explore the factors influencing electrolysis, including conductivity, charge, and equivalent weight, while examining experimental methodologies and results.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports difficulty finding a correlation between pH and electrolysis rate despite using different acids at constant concentration.
  • Another participant notes that electrolysis speed is influenced by multiple factors, including transport, potential, and solution conductivity, complicating the isolation of pH as a variable.
  • There is a question about whether pH should correlate with solution conductivity, with some arguing that it may not be straightforward.
  • Participants discuss the implications of Faraday's laws of electrolysis, noting that while they govern the amount of substance liberated, they do not directly address the rate of electrolysis.
  • One participant seeks clarification on the role of equivalent weight in determining the amount of electrolyte evolved, particularly in reactions producing different gases.
  • Concerns are raised about defining the "rate of electrolysis" clearly, as different acids may react differently, potentially skewing comparisons.
  • There is speculation about why citric acid might evolve more hydrogen than hydrochloric acid, but clarity on the specific experimental conditions is lacking.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between pH, conductivity, and electrolysis rate. There is no consensus on the role of equivalent weight or the specific factors influencing the rate of hydrogen evolution in different acid solutions.

Contextual Notes

Participants acknowledge limitations in their methodologies and the complexity of isolating pH as a variable in electrolysis experiments. The discussion highlights the need for precise definitions and conditions when comparing results across different reagents.

Incredinoob
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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.
 
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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.
 
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?
 
Last edited:
"Conductivity? Charge? Equivalent weight?" One question at a time.
 
Incredinoob said:
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.
 
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.
 
Incredinoob said:
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.
 
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.
 
Incredinoob said:
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.
 
  • #10
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?
 
  • #11
Incredinoob said:
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.
 

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