Potassium dichromate is a popular chemical for growing crystals because the crystals are easy to grow and are naturally vivid reddish orange. The crystals occur naturally in the rare mineral called lopezite. In fact, if you purchase lopezite for a mineral collection, it’s usually grown in a dish rather than collection from nature.
About Potassium Dichromate
The chemical formula of potassium dichromate is K2Cr2O7. It crystallizes in the triclinic system into orange or orange-red crystals. The crystals are soft, with a Mohs hardness of 2.5. Natural crystals may be granular or even spherical, but lab-grown crystals are prismatic. As with all salts, solubility greatly depends on temperature:
- 4.9 g/100 mL (0 °C)
- 13 g/100 mL (20 °C)
- 102 g/100 mL (100 °C)
Grow Potassium Dichromate Crystals
All you need is potassium dichromate and distilled water:
- Potassium dichromate
- Distilled water
You’ll also need a container for mixing the solution and growing the crystals. A shallow petri dish is ideal, but you can use an empty jar. Don’t use the container afterwards for food.
- Dissolve as much potassium dichromate as you can into warm to hot water. You can get a saturated solution by dissolving 100 grams of the chemical into freshly boiled water.
- Cover the solution with a paper towel or coffee filter to keep it clean. Let it sit undisturbed until you observe crystal growth. Cooling and evaporation produces a mass of crystals within a few hours to several days.
- Alternatively, pour a few drops of the solution into a shallow dish to produce a seed crystal. It takes from a few minutes to a couple of hours to get a seed crystal. You can tie this seed crystal with a nylon line and suspend it into the larger volume of solution. When you notice growth anywhere other than on the seed, decant the liquid into a clean container and move the seed crystal to it.
As with any crystals, the rate of cooling and evaporation control the rate of crystal growth and the form the crystals take. For large, perfect crystals, aim for slow cooling and evaporation. In other words, you’ll get the best crystals with slow cooling to room temperature (not refrigeration) and slow evaporation (not in a sunny window sill). You’ll get bright orange rectangle prisms. Large crystals are darker and look more red.
Where to Get Potassium Dichromate
Potassium dichromate is not one of those chemicals you can find at the grocery store or (usually) home supply store. However, it’s readily available online from Amazon, Walmart, and Carolina Biological. It’s also sold by photographer supply stores, ceramics stores, pyrotechnics supply stores, and all lab chemical suppliers. Potassium dichromate finds use as an analytical reagent, in leather tanning, as a precursor for potassium chrome alum, as a cement ingredient, for making chromic acid, for photography, and for staining wood.
The chemical is sold either as solid crystalline granules or as a liquid reagent. The solid runs about $15 per pound. The liquid works fine for growing crystals, but it takes longer because you need to let the excess water evaporate.
Safety Information
Wear gloves to avoid skin contact with the chemical or crystals. It’s a good idea to wear goggles to protect eyes from a splash. If you handle the crystals (not recommended, unless you seal them first), wash your hands. Touching potassium dichromate or its solution causes contact dermatitis.
Do not pour leftover solution down the drain. The best bet is to evaporate off the water and save the solid for later use. You can convert the hexavalent chromium into trivalent chromium by adding an aldehyde (e.g., formaldehyde or acetaldehyde). This turns the solution green.
This crystal-growing project is not suitable for children because they might touch or drink the liquid. Potassium dichromate is a carcinogen, so it should not be ingested.
References
- Anger, Gerd; et al. (2005) “Chromium Compounds” in Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a07_067
- Binnewies, M.; Milke, E. (2002). Thermochemical Data of Elements and Compounds (2nd ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-30524-7.
- Gaines, Richard V.; et al. (1997) Dana’s New Mineralogy. John Wiley & Sons.