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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 very bright, red-orange color.
Potassium dichromate is an oxidant (oxidizing agent). The reduction half-equation can be seen:
- Cr2O72âˆ’(aq) + 14H+ + 6eâˆ’â†’ 2Cr3+(aq) + 7H2O (E = +1.23 V)
In organic chemistry, potassium dichromate is a mild oxidizer compared with potassium permanganate. It is used to oxidizealcohols. 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 is possible. For example, menthone may be prepared by oxidation of menthol with acidified dichromate. Tertiary alcohols are not oxidized by potassium dichromate.
In an aqueous solution the color 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 color 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.
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:
- C2H5OH + [O] → CH3COOH
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.
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 or Photochromos 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.
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Answers:As the previous poster suggested, usually potassium manganate VII is used to test for unsaturation. If you use acidified dichromate it changes from orange to a green colour (Cr3+aq)
Answers:Have you studied redox reactions in general chemistry? If you recall, we assume two half-reactions, one oxidation and the other a reduction, with the reduction half-reaction on occasion needing a source of protons. The sulfuric acid supplies the protons for the half-reaction reduction of Chromium to Cr+++ : Cr2O7-- + 14 H+ + 6e- 2Cr+++ + 7H2O In the oxidation of, say, a primary alcohol to aldehyde, the electrons are supplied by the alcohol: 3 RCH2OH + Cr2O7-- + 8H+ 3RCHO + 2Cr+++ + 7H2O. I'm not entirely sure if this is balanced correctly or not, but at least that's what the sulfuric acid is doing.
Answers:Gently heat test substance with sodium dichromate acidified with dil H2SO4 Orange ====> green where the alcohol reduces the dichromate to Cr3+ aq which is green the alcohol is oxidised
Answers:A test for platinum would turn the solution red upon treatment with a solution of aqua regia. However, if he used a solution of nitric acid with potassium dichromate you should expect no color change. Here is a link that describes this a bit: http://www.shorinternational.com/TestGoldMakeSolution.htm