MadSci Network: Chemistry
Query:

Re: How can I 'electrolysize' water so I don't get and oxide on my annode?

Date: Fri Apr 24 06:39:21 1998
Posted By: Ewen McLaughlin, Lecturer, Chemistry, Swansea College
Area of science: Chemistry
ID: 893300205.Ch
Message:

Electrolysis of water produces hydrogen and oxygen. Water splits into H+ and OH- ions: H2O ® H+ + OH- At the cathode, electrons are donated to the hydrogen ions and hydrogen gas is produced: 2 H+ + 2 e- ® H2 At the anode, electrons are taken from the hydroxide ions and oxygen is produced: 4 OH- ® 4 e- + 2 H2O + O2 Now, because water is a poor conductor of electricity (very few ions are actually present) an ionic compound is usually dissolved in the water to show the electrolysis of water. We use 1 mol dm-3 sulphuric acid. If the class is an advanced one, we will explain this extra step, but because the only purpose of the additional chemical is to hasten the electrolysis of water, the presence of the additional chemical is often not mentioned. You said that you are using sodium hydrogen carbonate (‘sodium bicarbonate’) for this purpose. At the anode, where H+ ions are left after the OH- is discharged, the acidic conditions are going to cause carbon dioxide production: H+ + HCO3- ® H2O + CO2 This complicates things because the anode gas is a mixture of oxygen and carbon dioxide, so if you wanted to show things like the 2:1 ratio of hydrogen to oxygen or test the anode gas to show that it is oxygen, then you would be in trouble. Also note that the remaining solution is sodium hydroxide, which is caustic and dangerous. Sulphuric acid, which we use, does not interfere with the electrolysis. Sodium sulphate is a good alternative for the same reason and is less dangerous but more expensive and difficult to obtain. The electrodes we use are either platinum, which does not react with anything in the system, or carbon, which reacts some of the oxygen produced but still allows most of the gas to escape. Carbon is much cheaper. You did not say what electrodes you used, and it usually does not matter much what the cathode is made of. However, if the anode is made of a metal such as copper, then instead of oxygen forming at the anode, the copper reacts to form copper ions, which show up as a blue solution; Cu ® Cu2+ + 2 e- Now, you did not say what your electrodes were made of but most metals will react if placed at the anode so your weird oxide could have been any of a number of things, maybe not even an oxide, though this is unlikely. Either use an unreactive metal (expensive) or carbon, which will react but not in an interfering way and can cheaply be replaced if it becomes to corroded I hope that this helps! Happy electrolysing!  


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