MadSci Network: Chemistry

Re: What is the product of water electrolysis when you use platinum electrodes?

Date: Sun Aug 27 01:25:28 2000
Posted By: Neil Saunders, Research fellow
Area of science: Chemistry
ID: 962594853.Ch

Hi Kevin,

Thanks for the question. The short answer is no, using platinum electrodes to electrolyse water, you get only hydrogen and oxygen. Now for the longer answer!

Electrolysis of water is what's called a redox reaction. This means that electrons are being moved from one molecule to another. To electrolyse water, you need 2 electrodes, the cathode (negative) and the anode (positive). You also need to dissolve ions in the water to improve its conductivity, sodium chloride is often used.

At the anode, electrons are removed from the water and it is oxidized, in this reaction:

2H2O -----> O2 + 4H+ + 4e-

At the cathode, electrons are added to water and it is reduced:

2H2O + 4e- -----> 2H2 + 4OH-

So at the anode we get oxygen gas and hydrogen ions, at the cathode hydrogen gas and hydroxide ions.

As you suggested, other products can be formed depending on what the electrodes are made from and how the electrolysis chamber is built. If the cathode is made from iron, it will react with the hydroxide ions to make brown, insoluble iron hydroxide:

Fe3+ + 3OH- -----> Fe(OH)3

Chlorine gas can be formed, but this isn't because the electrodes are made of carbon. It will only happen if sodium chloride is added to the water. In this case, the chloride ions in the water can oxidise to chlorine and combine with hydroxide ions to form hypochlorite:

Cl2 + 2OH- -----> Cl- + OCl- + H2O

This is in fact how bleach is made industrially.
Platinum electrodes are the preferred electrodes for this experiment, because platinum is chemically inert, so it won't react with the hydroxide ions produced. An interesting side-note is that certain types of platinum electrode can absorb the hydrogen produced. This is the basis of experiments involving "cold-fusion" a few years ago, in which it was thought that the stored hydrogen was fusing and releasing energy. This was unfortunately not true!

There's a good picture and explanation of water electrolysis at
Hope this helps,

Neil Saunders

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