What is the effective difference between potassium alum and
ammonium alum when used as a pickle? Are there other "alum"s??
G’day; potassium alum is potassium aluminium sulphate, KAl(SO4)2 +
water of crystallization, and ammonium alum is ammonium aluminium
sulphate which has the same formula except that the ammonium ion (NH3)
is substituted for the potassium (K) ion. Yes, there are a large
number of alums; chrome alum, magnesium, manganese, ferric (iron) and
etc. All of them generally have the same crystal shape which looks
much like a cube standing on a corner. The reason why potassium alum
(commonly just referred to as ‘alum’) is used as a pickle is because
the molecule continuously breaks up and rejoins when dissolved in
water (dissociation) and ‘ionises’. The sulphate or acid part of the
molecule tends to seize upon any base it finds, and this includes
copper oxide firestain from sterling and low carat golds. The
solution eventually becomes pale blue due to copper sulphate forming.
I don’t know for certain, but I think that ammonium alum should
behave similarly. There is no simple test you can do to determine
the presence of potassium, other than taking a little on a little loop
of very clean stainless or nichrome wire and holding it in the edge
of a hot flame, when a pale violet (hard to see) colour will be
introduced to the flame to indicate the presence of potassium. If
ammonium alum solution is heated with an excess of strong alkali like
caustic soda (lye) a very strong smell of ammonia (don’t sniff too
hard or it will ‘blow’ your head off! - achoo!!) will be produced.
You may have, I suggest, some plain potassium sulphate which doesn’t
ionise as easily as alum, so isn’t much good for use as a pickle.
Add a solution of your ‘salt’ to washing soda or baking soda
solution, and if it is any use as a pickle it will fizz strongly.
Remember the old ‘strike the knob’ fire extinguishers? They contained
a solution of bicarbonate plus either soap or a detergent, and
striking the knob broke a thin walled glass tube of alum solution. The
reaction produced large amounts of carbon dioxide gas which threw the
resulting foam over the fire and put it out - one hoped. Here endeth
the (chemistry) lesson. Cheers, – John Burgess; @John_Burgess2
of Mapua Nelson NZ