I am interested to know what is a reasonable maximum working pressure I could expect against the evolution of oxygen and hydrogen when electrolysing a typical water electrolyte.

My aim is to make small quantities of isolated $\ce{H2}$ and $\ce{O2}$ for a hobby hydrogen torch and I am curious how tall I can make the middle branch of the Hofmann Voltameter before the back pressure stops the production of new gas. Having a predictable gas pressure would allow me to use the collection spaces for gas storage. I do not like the idea of mixing the gases before the torch nozzle like some brave people do so plan to use two collection volumes.

What parameters would be most likely to effect this pressure, electrolyte makeup, temperature, voltage?

Bonus question, is the pressure of $\ce{H2}$ and $\ce{O2}$ expected to be the same in theory and/or practice?

It looks like there is a high pressure electrolysis process out there that operates at many atmospheres of pressure and has been used to pressurise the gas products. I missed it in my first readings on Wikipedia and further research has confirmed it though information is limited, it seems to be a specialised process usually associated with proton exchange membrane systems I think but I am just hoping to use two metal electrodes.


2 Answers 2


The maximum gas pressure you can expect in a Hofmann Voltameter is the atmospheric pressure plus the pressure from the water column in the middle cylinder ($1~\pu{atm} / 10~\pu{m}$).

For a reasonably sized apparatus, you will not reach conditions where you had to consider "back pressure" effects that might stop production of hydrogen or oxygen.

Given that the cross section of all three cylinders is the same, the pressure of hydrogen is slightly higher than oxygen pressure since the water column that adds pressure is higher (as can be seen in the Wikipedia article you've linked.)

  • $\begingroup$ I am trying to find out where the threshold for the reasonably sized apparatus you allude to is reached. What is the maximum pressure that can be achieved in a closed electrolysis apparatus? $\endgroup$
    – KalleMP
    Apr 17, 2017 at 9:19
  • $\begingroup$ 1. Let's assume the middle cylinder is 100 m higher and filled with water. Then the excess pressure would be 10 atm. 2. In a closed electrolysis apparatus pressure could grow very high (you can calculate the theoretical limit using the ideal gas law and the density of water). Such an apparatus will most probably be destroyed. $\endgroup$
    – aventurin
    Apr 17, 2017 at 13:16
  • $\begingroup$ aventurin, you suggest that all the water will turn to gas. Admittedly I have not found any reference to a limit to the pressure that can be reached by electrolysis but that that seems hard to grasp. Is there no reverse reaction that will reform (liguid) water from the evolved gas at the electrode with the free complementary gas in solution due to the high pressure? $\endgroup$
    – KalleMP
    Jul 3, 2017 at 20:06

A point of concern producing hydrogen under high preasure.

Under increasing preasure, both hydrogen and oxygen will increasingly diffuse into the water, then into the opposing columbs, and excape mixing with the gases in the other columb. This will cause it to get harder to keep the respective gas columns purity high, and more importantly, safe.

One would not want to make a potential bomb with explosive amounts of hydrogen in the oxygen columb, nor explosive amounts of oxygen in the hydrogen columb, under normal preasures let alone at high preasures.

Mater of fact, even at low pressures, given enough time, diffusion will cause a mixing of the gases. The relative purity is related to both the rate of production and the rate of diffusion.


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