I'm trying to synthesize sodium aluminate from NaOH and old pop cans, but when the reaction goes it produces a dark gray solution, when it should theoretically be clear. Is this because of an impurity in the aluminum? Why do a lot of aluminum reactions like this produce dark, gray sludge?

 

Thanks for the info.

Edited February 21, 201114 yr by Runninfarmer

Do you remove the layer of aluminium oxde from the cans before you started? When exposed to air aluminium reacts almost instaneously to gerneate a very thin layer of aluminum oxide. Accoridng to Wiki, sodium aluminate is prepared by reacting aluminium hydroxide with sodium hydroxide:

 

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

 

Aluminium hydroxide is a white/pale grey solid.

The aluminium from which the cans are made contains other metals as an alloy. (One possibility is that it contains iron. It might be interesting to leave the solution next to a strong magnet overnight.)

Those metals don't dissolve in the NaOH and are left behind as a black solid.

 

The aluminium oxide coating on the surface of the metal will dissolve to form sodium aluminate anyway, so there's no need to remove it.

This is just as well as the oxide layer is thought to reform in a matter of picoseconds.

The same thing happens if you react fairly concentrated HCl with aluminum foil (product is [ce]AlCl_3[/ce]). The solution becomes opaque grey due to the alloyed [non-aluminum] metals in the foil. If you wait a while, the non-dissolved impurities will settle out to the bottom. I like John Cuthber's idea of sticking a strong magnet next to flask overnight though. It would be interesting to see if you pick up any iron chips.