The Amateur Chemist

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Synthesis of Alum


Alum, or aluminum potassium sulfate is the potassium double sulfate of aluminum. Alum forms clear/white regular octahedral crystals. Potassium alum is chemically compatable with other alum types such as chrome alum as well, so you can grow their crystals in tandum, layering them as many times as you want. Alum crystals are very easy to grow and form beautiful diamond like crystals. For this reason, many amateur crystal growers choose potassium alum as their first crystal. Potassium alum can usually be found in the spice section of grocery stores as it is used in the pickling process. The "Alum" I found at my local grocery store was not potassium alum though and did not behave or crystallize like an alum.  If you just want a small amount of potassium alum to experiment with than this is a good method. It is fairly easy and actually gives a pretty good yield too.


 

2Al + 2KOH + 6H2O -----> 2KAl(OH)4 + 3H2

2KAl(OH)4 + H2SO4 ----> 2Al(OH)3 + 2H2O + K2SO4

2Al(OH)3 + 3H2SO4 ------> Al2(SO4)3 + 6H2O

Al2(SO4)+ K2SO4 + 24H2O ----> 2KAl(SO4)2.12H2O

 


Ingredients

 

20 grams potassium hydroxide

5 grams aluminum (foil works well)

90 grams 98% sulfuric acid 

 

Procedure

 

First, make a 3M solution of potassium hydroxide by dissolving 20 grams of KOH into 125 ml of water. The solution will heat up substantially, so be prepared. Next, weigh out 5 grams of aluminum foil and tear it into small pieces. Slowly add several small pieces of aluminum foil to the KOH solution at a time and wait for them to finish reacting before adding any more. Resist the temptation to add them all at once because as the solution heats up, the reaction can become quite vigorous and could bubble over the sides of the beaker.  When the reaction has stopped and all of the aluminum foil has been reacted, then your solution will probably appear black. In reality, these are just really fine particulates floating around in the solution that would eventually settle out. Filter the solution while hot and you should be left with a clear, colorless solution. Allow it to cool to room temperature.

 

Now, we are going to add the sulfuric acid to the solution. When first adding it, a precipitate of white aluminum hydroxide will come out of solution. As you add more sulfuric acid though, the precipitate will redissolve and you'll be left with a colorless solution again. I list 90 grams of 98% sulfuric acid, but I would dilute this substantially, maybe by adding 50 or even 100 ml of water to it. This will make the next part easier.

 

When the solution is reasonably cool, add the sulfuric acid to the solution. Do this with care. If the acid is dilute enough, you can add it fairly quickly, but if it is more concentrated, you will want to take your time. The solution will heat up significantlyas you add the sulfuric acid. After adding the acid, if there are still some white flecks left, heat the solution up and stir until they are gone.

 

Allow the flask to cool a little, then place it into an ice bath for 5 minutes. If no crystals have started to form by then, scratch the inside of the flask with a stirring rod or add a seed crystal. Swirl the flask until you notice the start of crystal formation, then put it back into the ice bath for an additional 10 minutes. While the solution is cooling, pour 50-100 ml of a 50% alcohol/water mixture into a beaker and place it in the ice bath to cool as well. After crystals stop forming, set up for vacuum filtration, and filter the potassium alum crystals in the flask, making sure to get them all out. Rinse with 10ml portions of the alcohol/water mix, sucking dry in between additions.  Let them dry on the filter for 20 or so minutes, then put them in a petri dish or on another dry filter paper somewhere warm to dry.