Science Buddies: "Ask an Expert"

Greetings!

My son is doing a science experiment on electrolysis. He is in 7th grade. He used table salt and sodium sulphate as electrolytes so far. With both experiments, hydrogen was produced, but very little oxygen, not getting the 2:1 ratio he was hoping for. He tried looking up info on this experiment on various sites, but much of the information is quite complicated and written for older students. Seems like it might have to do with the electrolyte and another reaction is occuring. Any guidance you could offer or suggestions for junior high level explanations he could look up? Many thanks,

MM

Dear Mel's Mom,

Your son's choice of Sodium Sulphate for electrolytes is a good one. However, the table salt electrolyte will likely release chlorine gas instead of oxygen, which being very soluable in cold water will not appear to bubble much. Oxygen can also be disolved in cold water, so one may have to run the experiment for a while... or heat the water a bit, as hot water has lower oxygen soluability. Another hint is to start with distilled water, so as to be sure that no stray chemicals are in the experiment.

But I'm curious, what is he using for electrodes? The electrodes must be made of noble metals, preferably pure platinum... Metals to avoid include copper, the most common metal used in electrical wires. Also, tin and lead are often found on wires as they help with "pre-tinning" the wires for soldering. Lead is especially problamatic as it forms a non-soluable salt with sulphates. Iron, another common metal in cheap wire, also can form insolable salts. Some wires are made with aluminum, which is also chemically reactive in an electrochemical cell. So... platinum is really, really recommended.

Here is a good link for the process:

http://en.wikipedia.org/wiki/Electrolysis_of_water

While pure platinum is expensive, the amount needed for the experiment is small... only the exposed electrode need be platinum. Your son will not need much. A short bit of small diameter wire is all that is needed. Here is a link to a source:

http://www.fuelcellstore.com/cgi-bin/fu ... roduct=217

Please keep us informed of your son's progress and Good Luck to him.

Hi & thanks for the quick response!

I'm writing this for M because we are not near fax to send in for permission for him:

The electrodes are steel; he thinks that is good. He ran out of sodium sulfate to re-do; sent away for more. For now he tried baking soda. After a short time the results were okay -- about a 2:1 ratio; after several hours though the oxygen was as much as the hydrogen. He can't find a reason why that makes sense. Then he repeated with bottled water -- lots less hydrogen & oxygen, but about a 2:1 ratio. He is going to try baking powder, too. He says: thanks for the suggestions; some of the info on this is quite complicated, but the reaction is cool to watch; also got one pop out of the hydrogen with a splint when there was lots of hydrogen.


MM

Dear Mel's Mom,

Plain steel is an alloy of iron and carbon along with minor amounts of other metals. Stainless Steel is an allow of iron and chromium along with minor amounts of some other metals. Thus, the iron in the steel may react in the electrolysis experiment, especially with sulphate.

Platinum is still a better bet.

Baking Soda is Sodium Bicarbonate NaHCO3, which is a weak "base" (alkalai). Baking Power is a combination of anhydous Cream of Tartar KC4H5O6, which is a weak base, along with Sodium Bicarbonate. The combination, when mixed with water release CO2, carbon dioxide, gas to make cake and pancakes fluffy. Thus, baking power may result in impure gasses being collected. Best to stick with the baking soda.

As I had mentioned earlier, oxygen is soluable in cold water. The experiment will run a while, producing free oxygen which will then dissolve in the water until the water is saturated with oxygen. At that point, the oxygen will start to form bubbles on the electrode.

When your son is ready to start other types of electro-chemical experiments, he may want to try electroplating. My favorite when I was a kid was copper plating base metal objects in copper sulfate.