# Potassium Alum Crystals

This is a question regarding the crystallization of $\ce{KAl(SO4)2 . 12 H2O}$. I have an idea of what to do (I'll explain later in the explanation), but I would like to hear other suggestions encase the following ideas do not work out.

I placed a specific amount of the salt into hot distilled water and began stirring, later setting it out to grow my seed crystals. I have been waiting for the past 3 days to receive any potential seed crystals, however, I have not received even one. I noticed that over the 4 days a significant amount of water has evaporated, and at the bottom of the beaker has accumulated a white powder I'm assuming is the $\ce{KAl(SO4)2 . 12 H2O}$. I fail to understand why it has not crystallized into a small singular crystal?

My idea is to scratch the bottom of the beaker (it is new) to provide a location for the crystal to begin it's growth. Another idea pertains to adding more solute ($\ce{KAl(SO4)2}$) into the solution. Honestly, I don't know why I would add more solute if I am seeing the powder fall out of the solution. This should mean it is already saturated. Finally, I am deciding about adding some of the undissolved salt from the packet, into the solution. Hopefully this would provide a nice "infant crystal" (I am aware that there is no such thing an "infant crystal", I simply want to refrain from referring to "seed crystal" since these are not my seed crystals.) for my ions (dissolved solute) to latch onto and form my seed crystals.

Any suggestions would be appreciated.

Just to mention, my final technique drives through a more mathematical approach. I found the molar solubility from the $\mathrm{p}K_\mathrm{sp}$ of the crystal ($\mathrm{p}K_\mathrm{sp}$ of salt from aqion.de) I did my calculations to see a total of 0.3 grams of $\ce{KAl(SO4)2}$ per $1~\mathrm{mL}$ of water to obtain a saturated solution. Aesthetically, this doesn't make any sense to me. I feel the website I used may not have provided accurate experiments to reveal the $\mathrm{p}K_\mathrm{sp}$. If you can link me to any other websites or locations I would be more than grateful.

• What's so aesthetically unpleasing about the solubility figure, whatever that might be? Mar 15, 2016 at 12:05
– MaxW
Mar 15, 2016 at 16:52
• @MaxW lmgtfy and similar references to Google are generally not all too welcome here … just sayin’.
– Jan
Jun 11, 2016 at 10:43
• There's a video on this page: thoughtco.com/is-alum-safe-to-eat-use-607367 Sep 6, 2020 at 23:49

Typically, you will want to make sure that your solution is saturated. Don’t add a specific, certain amount; rather, add the solid to hot distilled water until it actually won’t dissolve any more. Then quickly filtrate the hot solution through a hot funnel (don’t burn yourself though, please). Let this filtered solution cool down and you should obtain acceptable seeding crystals in your beaker.

By using this supersaturation technique, you can be sure that

1. you have a saturated solution so crystallisation can occur immediately and
2. since you cool down the solution rather quickly and since solubility of aluminium potassium sulphate increases with increasing temperature, it becomes very supersaturated very quickly.

Once you have a seeding crystal, e.g. by this method, you can use the hanging technique mentioned by Carlos with the remaining solution (which is still supersaturated). If you do so, you can use an almost infinitely thin string; a human hair typically works perfectly and is even less visible. Another option is to filtrate the solution again so that the bottom of the flask is clean and place your crystals there to grow. You will need to turn them around every now and again to allow for even growth.

And you can make your aluminium potassium sulphate crystals even nicer by cocrystallising it with chromium potassium sulphate. They crystallise in exactly the same structure but the chromium(III) ion is coloured (aluminium(III) is not).

I have done this experiment a few times in the past, and nice crystals have always been produced.

One thing you might consider doing is to drop a single tiny grain of alum (as small as a sand grain) into your saturated solution, so that the alum crystallizes around it. Don't add "some" alum crystals to the solution; add only a tiny one.

The nicest crystals are produced by means of this technique. The only problem here is that the thread becomes "part" of the crystal, and you may not like the final result, aesthetically speaking. And if the thread has a dark colour (like the one shown in the photo) you may like the final result even less. So if you decide to apply this technique be sure to use a white or even a transparent thread (for example, a thin nylon filament). The "seed" crystal doesn't need to be as big as the one in the photo -- a smaller one will be good enough.