What is the best ratio of salt to water when using a salt solution in the electrolysis of water?

I am using the electrolysis of water to provide middle school students with a phenomena to explore chemical and physical change. The set up is producing much more hydrogen gas than oxygen gas and I am wondering if this is caused by the salt water solution I am using? I was looking to get a 2;1 ratio of hydrogen to oxygen.




$\ce{NaCl}$ is not a good choice for showing the electrolysis process to high school students. The electrolysis of $\ce{NaCl}$ starts by producing $\ce{Cl2}$ (and a little $\ce{O2}$) at the anode, plus $\ce{NaOH + H2}$ at the cathode. But $\ce{Cl2}$ is a bit soluble in water, so its gaseous volume is not consistent with the stoichiometric proportions. Furthermore, after some time the dissolved $\ce{Cl2}$ reacts with the dissolved $\ce{NaOH}$ according to : $$\ce{Cl2 + 2 NaOH -> NaClO + NaCl + H2O}$$ As a consequence the solution contains now a new substance, sodium hypochlorite $\ce{NaClO}$ or its ions $\ce{Na^+ + ClO^-}$. The gaseous volume observed at the anode is much less than $1/2$ times the volume of $\ce{H2}$ produced at the cathode.


So if you want to show that the gaseous volumes produced at the cathode is twice what is produced at the anode, it is better to use a solution of sodium sulfate $\ce{Na2SO4}$, which produces exactly the gaseous volumes in a proportion $2 : 1$. The secondary reactions observed with $\ce{NaCl}$ do not occur with sodium sulfate.


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