I have seen a great many number of diagrams which show the electrolysis of brine in a diaphragm cell, but show twice the volume of hydrogen produced. For example:

Brine Electrolysis Diagram 1

On the other hand, other diagrams show that the same volume of hydrogen and chlorine is produced. For example:

Brine Electrolysis Diagram 2

So who's right and why?

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    $\begingroup$ These are just qualitative schemes; they don't specify volumes at all. At least, I don't see any indication of it whatsoever. $\endgroup$ Aug 1, 2016 at 13:31
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    $\begingroup$ No they did not pay attention to the volumes (otherwise they would have shown them equal, as the equation requires). You are inferring meaning which is simply not there. $\endgroup$ Aug 1, 2016 at 14:16
  • $\begingroup$ @IvanNeretin I appreciate your response. Are you suggesting that there is evidence of systematic error in the creation of these diagrams relating to the electrolysis of brine? $\endgroup$
    – Turbo
    Aug 1, 2016 at 14:17
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    $\begingroup$ I would not even call that an error. They never said their volumes are 2:1 or whatever; you inferred that from some random details which (I think) simply aren't supposed to mean anything. $\endgroup$ Aug 1, 2016 at 14:24
  • $\begingroup$ @IvanNeretin I think I've worked it out - see my answer below $\endgroup$
    – Turbo
    Aug 1, 2016 at 14:32

2 Answers 2


It appears, as the others have said that the same number of moles of gas of chlorine and hydrogen is produced.

Then why are there different levels of solution?

Since the solution is taken out in the cathode chamber, the level of solution present is lower than the anode chamber where sodium chloride solution is being added in.

Electrolysis of Brine Diagram


I'm not sure how the first diagram shows a $2:1$ volume ratio. But I am sure about the balanced chemical equations:

$$ \begin{align} \ce{2 Cl^- &→ Cl2 + 2 e-} \\ \ce{2 H2O + 2 e- &→ H2 + 2 OH-} \\ \hline \ce{2 H2O + 2 Cl- &→ H2 + Cl2 + 2 OH-} \end{align} $$

Thus equal amounts of both gases are produced, and at the same temperature and pressure that implies equal volumes by the ideal gas law.


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