I want to carry out a photocatalytic water spliting reaction. This reaction will generate Hydrogen and Oxygen gas in 2:1 ratio according to the following reaction -

2H2O + energy (light) → 2H2 + O2

So, if I collect the produced gas over water in a test tube, should I be able to determine the volume of hydrogen gas in that mixture? From literature I came to know gas chromatography is used to determine the volume and evolution rate of hydrogen. Is that necessary for a small lab experiment?

  • $\begingroup$ It depends on your desing. You could use a gas burette or even a graduated test tube uf you are working with a small set-up and small volumes of gas. $\endgroup$
    – PAEP
    Commented Feb 21, 2023 at 10:13
  • 1
    $\begingroup$ Collecting any amount of the combined gasses would be an explosion hazard. $\endgroup$ Commented Feb 22, 2023 at 8:05

2 Answers 2


If you collect oxygen and hydrogen in a single tube, the volume reflects both. If you know that the molar ratio is 1:2, you can calculate the volume of hydrogen gas by multiplying the total volume by 2/3. If you want to verify the molar ratio without expensive instrumentation, you might consider a chemical reaction that only reacts with oxygen or only reacts with hydrogen.

There is a standard lab to determine the oxygen content of air. It uses steel wool to react with the oxygen (but not the nitrogen) in air, turning it into "rust". The volume change observed corresponds to the oxygen that reacts. A mixture of oxygen and hydrogen is explosive (more so at a ratio of 1:2), so I would keep it at a small scale and avoid sparks.

I'm sure there are also convenient reaction to use up the hydrogen while leaving the oxygen, but the most common search result on the internet is the reaction with oxygen, which would remove both oxygen and hydrogen from the gas phase rather than isolating one of them.


With electrolysis, H2 bubbles up from the cathode. O2 from the anode. The apparatus is H shaped glass tubing with Pt electrodes on the bottom and measuring burettes on top.

Around 1.23 V is required to split water. This can easily be provided from a solar panel. Salt, such as KOH, improves the conductivity of water. It will help improve yields of both gasses.

Should you wish to continue on to a scale-up, it is possible to pressurize your H2 and O2 yields by simply having the water intake from a height above the apparatus. Every 20 feet above gives 8 psi. No need for a compressor after the gasses are made.

  • $\begingroup$ Electrolysis powered by a solar panel could be called "photocatalytic", but it is far from what plants (like biology, not like chemical industry) do to split water. To be fair, conventional photosynthesis also does not yield hydrogen but rather reduced cofactors like NADPH. $\endgroup$
    – Karsten
    Commented Feb 21, 2023 at 21:30
  • $\begingroup$ @Karsten I'd set mine up right next to the desalination plant, somewhere like in San Diego, CA. Plenty of sunshine. Pour water in, out comes pressurized H2! Scalable. Burn H2 for energy, back comes H2O for irrigation. $\endgroup$ Commented Feb 21, 2023 at 21:35
  • $\begingroup$ @RobertDiGiovanni Thank you for your response. But I'm not using any electrodes actually. I'll place my photocatalyst inside water and irradiate it with light of proper frequency. This should result in splitting of water. $\endgroup$ Commented Feb 25, 2023 at 8:04

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