Potassium dichromate

Overview
Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidising agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is potentially harmful to health and must be handled and disposed of appropriately. It is a crystalline ionic solid with a vivid red-orange colour.

Uses in organic chemistry
Potassium dichromate is used to oxidise alcohols. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under reflux. Secondary alcohols are converted into ketones, with no further oxidation possible. Tertiary alcohols are not oxidized, due to lack of an additional hydrogen to be eliminated to form a carbonyl group.

In an aqueous solution the colour change exhibited can be use to test whether an aldehyde or ketone is present. When an aldehyde is present the chromium ions will be reduced from the +6 to the +3 oxidation state, changing colour from orange to green. This is because the aldehyde can be further oxidised to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidised further, and so the solution will remain orange.

Ethanol determination
Potassium dichromate (usually acidified with sulfuric acid), or any other dichromate for that matter, can be used to determine the amount of ethanol in a solution using back titration. First a known amount of dichromate is added, enough to react with all the ethanol and leave some spare in the solution. A blank solution with no ethanol is titrated using sodium thiosulfate with iodide ions (from e.g. potassium iodide) in the solution and a small amount of stach used as an indicator. The dichromate reacts with the iodide ions forming iodine, which will then react with thiosulfate ions forming iodide ions again. In this way, when all the dichromate has been used up, the thiosulfate will convert iodine to iodide ions and the blue black colour from the starch will disappear, giving a clear endpoint. From here, calculations using balanced formulae easily yield the amount of ethanol in the original solution.

Other Applications
K2Cr2O7 is used as an oxidizing agent in many chemical applications, and is often used for cleaning laboratory glassware of organic contaminants, usually in a solution with concentrated sulfuric acid. This solution must not be used to clean the glass tubes used in NMR spectroscopy, as residual contamination of the glass by the paramagnetic Chromium disrupts the NMR procedure.

Potassium dichromate also has important uses in photography and in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.

Chromium intensification uses potassium dichromate together with equal parts of concentrated hydrochloric acid diluted down to approximately 10% v/v to treat weak and thin negatives of black and white photograph roll. This solution reconverts the elemental silver particles in the film to silver chloride. After thorough washing and exposure to actinic light, the film can be redeveloped to its end-point yielding a stronger negative which is able to produce a more satisfactory print.

A potassium dichromate solution in sulfuric acid can be used to produce a reversal negative (i.e,. a positive transparency from a negative film). This is effected by developing a black and white film but allowing the development to proceed more or less to the end point. The development is then stopped by copious washing and the film then treated in the acid dichromate solution. This converts the silver metal to silver sulfate, a compound that is insensitive to light. After thorough washing and exposure to actinic light, the film is developed again allowing the previously unexposed silver halide to be reduced to silver metal.

The results obtained can be unpredictable, but sometimes excellent results are obtained producing images that would otherwise be unobtainable. This process can be coupled with solarisation so that the end product resembles a negative and is suitable for printing in the normal way.

CrVI compounds have the property of tanning animal proteins when exposed to strong light. This quality is used in photographic screen printing. In screen printing a fine screen of bolting silk of similar material that is required to be printed is then taped securely onto the surface of the screen and the whole thing exposed to strong light for a period - typically about half an hour in bright sunlight. When the design is removed, the gelatine on the screen is washed off with hot water. All the gelatine exposed to sun-light will have been hardened by the dichromate and will be retained on the screen leaving a precise mask of the required design which can be printed in the usual way.

Hazards
Potassium dichromate is one of the most common culprits in causing 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. As with other CrVI compounds, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection. Potassium dichromate is listed as one of the ingredients in the migraine over-the-counter homeopathic medication called HeadOn, though the product contains only one part per million of dichromate.