Some science behind the scenes

Potassium dichromate

 

Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health and 'must be handled and disposed of appropriately'. It is a crystalline ionic solid with a very bright, red-orange color. It is also known as potassium bichromate; bichromate of potash; dipotassium dichromate; dichromic acid, dipotassium salt; chromic acid, dipotassium salt; and lopezite.

Despite its toxicity it is used in various applications:

  • Cleaning products - potassium dichromate may be used to prepare "chromic acid", which can be used for cleaning glassware and etching materials.
  • Construction - It is used as an ingredient in cement in which it retards the setting of the mixture and improves its density and texture. This usage commonly causes contact dermatitis in construction workers.
  • Ethanol determination - The concentration of ethanol in a sample can be determined by back titration with acidified potassium dichromate. It was used in police breathalyzer tests. When alcohol vapor makes contact with the orange dichromate-coated crystals, the color changes from orange to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.
  • Leather - It is used to tan leather which is used for footwear.
  • Photography - Potassium dichromate has important uses in photography and in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.  A potassium dichromate solution in sulfuric acid, for example, can be used to produce a reversal negative (i.e., a positive transparency from a negative film).
  • Silver test - When dissolved in an approximately 35% nitric acid solution it is called Schwerter's solution and is used to test for the presence of various metals, notably for determination of silver purity.
  • Sulfur dioxide test - Potassium dichromate paper can be used to test for sulfur dioxide, as it turns distinctively from orange to green.
  • Wood treatment - Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produces deep, rich browns that cannot be achieved with modern color dyes.

Potassium dichromate is one of the most common causes of chromium dermatitis; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. Toxicological studies have further illustrated its highly toxic nature. With rabbits and rodents, concentrations as low as 14 mg/kg have shown a 50% fatality rate amongst test groups. Aquatic organisms are especially vulnerable if exposed.

As with other CrVI compounds, potassium dichromate is carcinogenic. The compound is also corrosive and exposure may produce severe eye damage or blindness. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.

 

Observations

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