How can I demonstrate the electrolysis of water safely?

I'm helping a 6th Grader with a science project where we're trying to separate hydrogen from oxygen using hydrolysis. While I'm a little rusty on the experiment, I recall using a Sulfuric Acid solution for the electolyte, I don't recall particulars and my trial and error techniques are not making much progress. I wondered if you might give me a reference for the experiment or answer the following questions: Is Sulfuric Acid key or will other acids (safer household sources such as Acetic Acid) work? I tried using vinegar with copper electrodes and all of the gas formed on the negative electrode. I want the gases to separate. Is there a way to modify the electrodes and electrolyte to have the right effect? What is the proper electrolyte concentration? Kris Beutel (kbeutel@gte.net)

Electrolyte is added so that the reaction will go at a nice rate using a low current. Acetic acid is a weak acid, and household vinegar is only 3% acetic acid. It doesn't work; the electrolysis is just too slow. Sulfuric acid works well. The trouble with a lot of other acids is that they undergo side reactions. For example, hydrochloric acid is easy to get (it's in brick-cleaning solutions) but you'll get chlorine gas (which is very poisonous!) from positive electrode. Try a 1.0 M solution of Na₂SO₄. The precise concentration doesn't matter. That's safer than sulfuric acid but still works well. You can buy sodium sulfate as 'Glauber's salt' in a drugstore.

You should use an inert metal or you'll drive side reactions that involve the metal. Platinum wire is a favorite electrode material for this reason. You might try nichrome wire; it's a lot cheaper than platinum. As you found out, copper won't do. You were seeing the hydrogen come off on the negative electrode (4 H₂O(l) + 4e- = 2 H₂(g) + 4 OH-(aq)) but instead of oxygen at the positive electrode (2 H₂O(l) = O₂(g) + 4 H⁺(aq) + 4e-), you were getting Cu(s) = Cu²⁺(aq) + 2e- at the other electrode. The latter reaction is easier, and when the positive electrode is made of copper, it occurs preferentially.

You should set up five six-volt lantern batteries in series to supply the current. Use anything less, and the reaction occurs very slowly.

You might try doing sulfuric acid with water in one setup, and sulfuric acid with household hydrogen peroxide in the other; that way you can show how the composition of water and H₂O₂ differ.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How can I demonstrate the electrolysis of water safely?