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Potassium dichromate, K 2 Cr 2 O 7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health.
The chromic acid is usually made by acidifying (with sulfuric acid) a solution of potassium dichromate. The old name for potassium dichromate is potassium bichromate and the cell is often called a bichromate cell. [1] This type of cell is now only of historical interest.
Sun printing may also refer to a photographic process using potassium dichromate which produces a negative plate for conventional lithographic printing. The process uses a film of gelatine spread on a flat and rigid surface. This is coated with a dilute solution of potassium dichromate and dried in low light conditions.
Chromate and dichromate salts of heavy metals, lanthanides and alkaline earth metals are only very slightly soluble in water and are thus used as pigments. The lead-containing pigment chrome yellow was used for a very long time before environmental regulations discouraged its use. [ 7 ]
Of these, potassium dichromate (K 2 Cr 2 O 7) is the most effective: it is relatively cheap, easy to purify, and can nearly completely oxidize almost all organic compounds. In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed.
Chromyl chloride can be prepared by the reaction of potassium chromate or potassium dichromate with hydrogen chloride in the presence of sulfuric acid, followed by distillation. [3] [4] K 2 Cr 2 O 7 + 6 HCl → 2 CrO 2 Cl 2 + 2 KCl + 3 H 2 O. The sulfuric acid serves as the dehydration agent.
However, increasing the concentration of dichromate also reduces the contrast which is very low at best. The right concentration of dichromate is always a compromise between speed and contrast. Using ammonium dichromate allows concentrations up to 15% of the active ingredient whereas potassium dichromate is limited to about 10%
Chrome alum. Chromium alum is produced from chromate salts or from ferrochromium alloys. Concentrated aqueous solutions of potassium dichromate can be reduced, usually with sulfur dioxide but also with alcohols or formaldehyde, in the presence of sulfuric acid at temperatures <40 °C.