The Amateur Chemist

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Synthesis of Chrome Alum

Chrome alum, or chromium (III) potassium sulfate is the double potassium sulfate of chromium. This compound is a beautiful dark violet, almost black, color that forms crystals in regular ocahedra. It is chemically compatable with regular potassium alum as well so you can grow these crystals in tandum, layering them as much as you want. 


Chrome alum has the formula KCr(SO4)2.12H2O, but is better represented as [K(H2O)6Cr(H2O)6](SO4)2. The hexaaquachromium (III) ion gives the compound it's characteristic violet color both in solution and as a solid. If the tempature ever rises above 60° however, the aqua ligands are replaced by sulfate ions and the solution turns green and becomes very difficult to crystalize.


Chrome alum can be prepared by reduction of a chromium (VI) salt (potassium dichromate) to a chromium (III) salt.


Cr2O72- + 14H+ + 6e-   ------------------------------>    2Cr3+ + 7H2O


The reducing agent is ethanol which is oxidised to ethanal (tempature below 60°). The characteristic smell of ethanal is evident at the end of the reaction.


CH3CH2OH ------------------------------> CH3CHO + 2H+ + 2e-

Balanceing electrons and cancelling gives...


Cr2O72- + 8H+ + 3CH3CH2OH -----------------------> 3CH3CHO + 2Cr3+ + 7H2O


Chrome alum is quite soluble in water so you should try to keep the volume of water used to a minimum. 



5.9 grams potassium dichromate

5 ml concentrated sulfuric acid

10 ml ethanol (or metylated spirits/denatured alcohol)



Dissolve 5.9 grams of potassium dichromate into 50 ml of hot water in a 250 ml beaker. Cool the orange solution (preferable until ice cold), and carefully add 5 ml of concentrated sulfuric acid. Slowly add 10 ml of ethanol drop by drop. Stir constantly with a thermometer and do not let the tempature rise above 60°.  The color of the solution will change from orange to violet - it might even appear black.  Initially, the tempature rises very rapidly with only a small amount added, but as you add more the tempature rise will slow down. 


When you are done adding the ethanol, cover the solution with a watch glass and cool in an ice bath or a freezer/fridge. Let it sit at least overnight, preferably several days to allow for crystallization to complete. You may need to add a seed crystal to induce crystallization, but I've never had that problem. Decant the solution - it should be easy as all of the crystals are compact on the bottom. Filter the crystals and wash with a SMALL amount of ice cold water. Transfer the crystals to a dry filter paper or a watch glass to fully dry.

Here are some pictures of my crystals... You can see that they look almost black. Some of that is the camera and light, but they are quite dark violet crystals.

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