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I've read in several places that when electrolyzing water into $\ce{O2}$ and $\ce{H2}$, that adding a salt or an acid will speed up the process. Why is this?

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Primarily due to

Pure water conducts an electric current very poorly and, for this reason, is difficult to electrolyze.

(Source: Water Electrolysis)

and that salt and acids are able to act as an electrolyte catalyst as most salt and acids dissolve (hence dissociate) in water, specifically, according to the Wikipedia page,

If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged $\ce{H+}$ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged $\ce{OH-}$ there. This allows the continued flow of electricity.

A diagram of the electrolysis of water with dissolved salt/acid ions is shown below:

enter image description here

Image source

Further information can be found in the article A Review of Water Electrolysis (Zoulias et al).

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  • $\begingroup$ So the salt only helps exchange the water around the electrodes? $\endgroup$ – Friend of Kim Feb 15 '14 at 9:31
  • $\begingroup$ the dissolved salt or acid increases the conductivity of the water, allowing a more continuous flow of electricity compared to that of pure water. More electric current = speedier electrolysis $\endgroup$ – user4076 Feb 15 '14 at 9:36
  • $\begingroup$ @FriendofKim is there anything else you would like to know? $\endgroup$ – user4076 Feb 15 '14 at 9:55
  • $\begingroup$ If I'm correct water is split up into oxygen and H+ on the anode releasing electrons in doing so. On the kathode electrons are supplied and splits water into H2 and OH-. H+ and OH- reacts and forms water. I don't see any flow of electrons, only two separate reactions. $\endgroup$ – Friend of Kim Feb 15 '14 at 10:19
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    $\begingroup$ Thank you! I'm reading the source you linked first. Then I'll ask if there is something that was unclear! $\endgroup$ – Friend of Kim Feb 15 '14 at 10:51
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I'd recommend using a strong base, e.g. NaOH. The reason is that if there's chloride in the solution (e.g NaCl as the easiest accessible salt or HCl as cheap strong acid), you'll produce chlorine gas instead of oxygen.

This is most suitable for industrial purposes, but not for home experiments ;-)

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    $\begingroup$ Yes, indeed. The best I have is $\ce{NaHSO4}$ (bath tub pH-lower). It says 95 % of the acid and 5 % inert ingredients. $\endgroup$ – Friend of Kim Feb 15 '14 at 21:30

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