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| Potassium dichromate | |
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| IUPAC name | Potassium dichromate(VI) |
| Other names | Potassium bichromate Bichromate of potash |
| Identifiers | |
| CAS number | [7778-50-9] |
| EINECS number | |
| RTECS number | HX7680000 |
| Properties | |
| Molecular formula | K2Cr2O7 |
| Molar mass | 294.19 g/mol |
| Appearance | Red-orange crystalline solid |
| Density | 2.676 g/cm3, solid |
| Melting point |
398°C |
| Boiling point |
500°C decomp. |
| Solubility in water | 4.9 g/100 ml (0°C) |
| Structure | |
| Crystal structure | Triclinic (α-form,<241.6 °C |
| Coordination geometry |
Tetrahedral (for Cr) |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
-2033 kJ/mol |
| Standard molar entropy S |
291,2 J.K−1.mol−1 |
| Hazards | |
| Main hazards | Highly toxic Carc. Cat. 1 Muta. Cat. 2 Repr. Cat. 2 Oxidant Dangerous for the environment |
| R-phrases | R45, R46, R60, R61, R8, R21, R25, R26, R34, R42/43, R48/23, R50/53 |
| S-phrases | S53, S45, S60, S61 |
| Flash point | Non-flammable |
| Related compounds | |
| Other anions | Potassium chromate Potassium molybdate Potassium tungstate |
| Other cations | Ammonium dichromate Sodium dichromate |
| Related compounds | Potassium permanganate |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Potassium dichromate, K2Cr2O7, 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 potentially harmful to health and must be handled and disposed of appropriately. It is a crystalline ionic solid with a bright, red-orange colour.
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Potassium dichromate is an oxidizer. The oxidation half-equation can be seen:
In organic chemistry, potassium dichromate is a mild oxidizer compared with potassium permanganate. It is used to oxidize alcohols. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under reflux. In contrast, with permanganate, carboxylic acids are the sole products. Secondary alcohols are converted into ketones — no further oxidation possible. For example, menthone may be prepared by oxidation of menthol with acidified dichromate.[1] Tertiary alcohols are not oxidized by potassium dichromate.
In an aqueous solution the colour change exhibited can be used 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 oxidized to the corresponding carboxylic acid. A ketone will show no such change because it cannot be oxidized further, and so the solution will remain orange.
Like other chromium(VI) compounds (chromium trioxide, sodium dichromate), potassium dichromate may be used to prepare "chromic acid", which can be used for cleaning glassware and etching materials.
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.[2]
The concentration of ethanol in a sample can be determined by back titration with acidified potassium dichromate. Reacting the sample with an excess of potassium dichromate, all ethanol is oxidized to acetic acid:
The excess dichromate is determined by titration against sodium thiosulfate. Subtracting the amount of excess dichromate from the initial amount, gives the amount of ethanol present. Accuracy can be improved by calibrating the dichromate solution against a blank.
One major application for this reaction is in old police breathalyzer tests. When alcohol vapor makes contact with the yellow dichromate-coated crystals, the color changes from yellow to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.
Potassium dichromate is used in the homeopathic preparation of Kali (or Kalium) bichromicum, although little or none remains in the final dilution.[3] One notable homeopathic product that contains it as the active ingredient is HeadOn,[4] in which the potassium dichromate is diluted to one part per million.
It is used to tan leather which is used for footwear.[5]
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.
Gum bichromate printing was one of the very first stable photographic printing processes, dating back to about 1850. A solution of gum arabic and potassium dichromate, once applied to paper and dried, will harden when exposed to ultraviolet light.
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.
Potassium dichromate is used to finish certain types of wood. It brings out the color and grain to a very deep, rich appearance. This is particularly effective on mahogany.[6]
Potassium dichromate occurs naturally as the rare mineral lopezite. It has only been reported as vug fillings in the nitrate deposits of the Atacama desert of Chile and in the Bushveld igneous complex of South Africa.[7]
Potassium dichromate is one of the most common causes of chromium dermatitis[8]; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. It is also toxic, with doses of approximately 100 mg/kg being fatal in rabbits and rodents.[9]
As with other CrVI compounds, potassium dichromate is carcinogenic and should be handled with gloves and appropriate health and safety protection. The compound is also corrosive and exposure may produce severe eye damage or blindness.[10]
Potassium dichromate(VI) paper can be used to test for sulfur dioxide, as it turns distinctively from orange to green.