Potassium dichromate

Potassium dichromate, K₂Cr₂O₇ is used in oxidation reactions. As a powerful oxidizing agent, it is the preferred compound for cleaning laboratory glassware of any possible organics.

Potassium dichromate also has important uses in photography and in photographic screen printing. In both uses it is used an oxidising 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% vol to vol to treat weak and thin negatives of black and white film. This solution reconverts the elemental silver particles in the film to silver chloride. After through 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 with sulphuric 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 sulphate , a compound that is insensitive to light. After through 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.

Cr⁶ ions 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 has inked squeezed through it onto paper or cloth underneath. If a mask or design is placed on the screen so that ink cannot pass, the design is transferred onto the material underneath. Many artistic prints, T-shirts etc are printed in this manner. To produce a photographic quality print, the screen is first coated with gelatine and then a solution of potassium dichromate is brushed on evenly in dull light. A full size translucent image 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.

Potassium dichromate is one of the most common culprits in causing chromium dermatitis. More significantly chromium is highly likely to induce sensistisation leading to chronic and dificult to treat derrmatitis , especially of the hand and fore-arms. All Cr⁶ products should be handled with gloves and appropriate health and safety protection.

Dichromate can also drive the oxidation of organic compounds as in Alcohol --> Aldehyde --> Ketone.