ave369 - 11-9-2017 at 12:49
Today I've ran an experiment and tried to make some aluminium-potassium alum. I did the following:
* Dissolved a piece of aluminium foil in KOH, resulting in potassium hydroxo aluminate;
* Acidified this solution with H2SO4. At first, a precipitate of Al(OH)3 fell out, but dissolved on adding more H2SO4;
* Filtered the resulting solution and measured its pH; it turned out to be about 0.5.
* Regulated the pH by adding potassium carbonate until it was about 2;
* Boiled the solution down to produce crystals.
While I boiled down a portion of the solution, some tiny crystals of alum appeared in the main solution as well. But boiling down resulted in a mass
of crystals. I filtered those out and washed them first with ethanol (to get rid of excess H2SO4) then with water (to wash out salts such as K2SO4,
more readily soluble than the alum).
Did I do anything wrong? What possible contaminants could be in my alum?
nezza - 12-9-2017 at 05:59
In the old days when chemistry was taught at school this was an occasional exam question on how to make alum. Dissolving aluminium in KOH gives
potassium aluminate containing Potassium and Aluminium in the correct ratio to form alum. The aluminate is acidified with H2SO4 when aluminium
hydroxide should precipitate and then re-dissolve and the solution is filtered and evaporated to give crystals of Alum. The potassium carbonate should
not be necessary and as you only used Aluminium and KOH the alum should be pretty pure.
MrHomeScientist - 13-9-2017 at 10:23
I did the same process in one of my videos, but as nezza said once you add the sulfuric acid and everything redissolves, you're done. All you need for
alum is K<sup>+</sup>, Al<sup>3+</sup>, and SO<sub>4</sub><sup>2-</sup>; pH shouldn't make a
difference as far as I know. The benefit of using KOH first to dissolve the aluminum is that most of the contaminants in your Al foil (or soda cans in
my experiment) aren't soluble and can be filtered off immediately, yielding pretty pure alum. If you're concerned about purity, you can always do a
few recrystallizations of course.