Ammonium Alum: simple synthesis from OTC materials

Although originally referring to the potassium aluminium double sulphate (KAl(SO4)2.12H2O), once one of the most important sources of water soluble Al salts and precursor to the name ‘alumin(i)um’, alums are now considered a class of chemical compounds of the general structure MM’(SO4)2.12H2O where M is a monovalent cation (e.g. K+, Na+,… , NH4, even Tl+) and M’ a trivalent one (e.g. Al3+, Fe3+, Cr3+ and even Mn3+).

This post is a simple recipe for making about 0.5 mol (about 226 g) of NH4Al(SO4)2.12H2O, ammonium aluminium alum or ammonium alum for short, from OTC materials with high yield and a nice looking highly water soluble Al compound to show for one’s effort.

Chemicals needed:

13.5 g of fairly clean aluminium (I use opening tabs of Al pop cans).
55 ml of H2SO4 95 % (I use a toilet or drains unblocker).
Some 1 M H2SO4 - prepared from above.
33 g of (NH4)2SO4, ammonium sulphate. I use once-recrystallised garden grade, beige in colour.
Water - deionised, distilled or very soft.

Procedure:

Add aluminium to a roomy Pyrex container and add 100 ml of water to it. Add the 55 ml of concentrated sulphuric acid slowly while stirring. Reaction will start according to Al(s) +3/2 H2SO4(aq) === > ½ Al2(SO4)3 (aq) + 3/2 H2(g). Add another 100 ml of water and put the covered container on an electrical hot plate (avoid open flame: HYDROGEN!) Simmer lightly until nearly all of the metal is dissolved. Top up volume of solvent with 1 M H2SO4 if it drops below 200 ml. Dissolution will take 1 - 2 hours, depending conditions.

Below: aluminium reacting with H2SO4 in conditions described.



The last bits of metal can take a long time to dissolve and I do as follows. Add 33 g of ammonium sulphate dissolved in about 100 ml of hot water to the aluminium sulphate solution and keep hot. That should give you about 300 ml of total volume. Hot filter the ammonium alum solution quickly with a coffee filter (small amounts of alum may crystallise on the filter) and receive the filtrate in a sturdy but flexible container (HDPE or PP - it can be hard to remove the formed crystals from a glass container without damaging it).

Add some seed crystals; allow cooling to RT, then put on ice bath or in a refrigerator overnight. Separate crystals from mother liquor and determine yield. My slightly moist crystals amounted to about 96 % theoretical yield.

Recrystallise by adding 1 part of water to 2 parts of product, heat gently to about 100 C while stirring until all crystals have dissolved. Add some seed crystals; allow cooling to RT, then put on ice bath or in a refrigerator overnight. The recrystallised product tends to show larger, more regular, snow white crystals and yield of recrystallisation is nearly 100 %.

Below: just over 200 g of ammonium alum prepared with this procedure.

Quote: Originally posted by m1tanker78

Simple, clean, OTC. Thanks for sharing the synth. BTW, what color is your plumbing grade H2SO4? Mine is a very nasty dark brown so I often hesitate to use it. Tank

Mine is red. Mankind's tendency to put dyes and flagrances into just about anything never ceases to amaze me... Or vitamins in hair shampoo, what's that all about?

I've found though that for most purposes the red dye doesn't cause any problems and is probably distilled off during use.

Quote: Originally posted by blogfast25

... Or vitamins in hair shampoo, what's that all about?

Marketing and the plecebo effect?

Mav, I'll keep my eyes open for Rooto brand drain cleaner. I think mine's branded "clear shot" or something similar.

Tank

Quote: Originally posted by Mr. Wizard

@blogfast25 13.5 g of fairly clean aluminium (I use opening tabs of Al pop cans). What is the alloy they use for aluminum cans? I know the alloy for the tops is different from the main can. It usually has more magnesium in it. Other elements from memory include manganese, copper, and silicon Your results look very good, but I was wondering about Magnesium and other alloy components in the mix. Thanks for sharing.

Quote: Originally posted by blogfast25

Yes, marketing of course: 'here comes the science bit" (NOT!). Today the new symbol of the double helix is what you want to put on every jar of 'face paint for women' to flog your product...

"Well, if it has the DNA thingies on the bottle, it MUST be good! Aloe vera is sooooo yesterday." (satisfied customer testimony)

+++++++++++++++++++

Impurities from the alloy shouldn't be an issue (I can't speak for Mn). This synth is on the to-do list for the near future. It's a perfect excuse to break in (hopefully not break) my new boro kettle.

Tank

Quote: Originally posted by m1tanker78

Impurities from the alloy shouldn't be an issue (I can't speak for Mn). This synth is on the to-do list for the near future. It's a perfect excuse to break in (hopefully not break) my new boro kettle. Tank

Tut tut: no should need an excuse to break in a new boropot. But you do need one to break it!

Meanwhile I've been running a few more runs but with ferric ammonium alum (NH4Fe(SO4)2.12H2O). Really hard to get good yields. Saturday I lost a 1 mole size batch (turned to Fe2O3, basically), today by the looks of it I'm getting good Q but less than 50 % yield. Bugger!

[Edited on 31-7-2011 by blogfast25]