Potassium dichromate

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Potassium dichromate
IUPAC name Potassium dichromate(VI)
Other names Potassium bichromate
Identifiers
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RTECS number HX7680000
Properties
K2Cr2O7
Molar mass 294.19 g/mol
Appearance Red-orange crystalline solid
Density 2.676 g/cm3, solid
Melting point
Boiling point
Structure
Crystal structure Triclinic (α-form,<241.6 °C
Coordination
geometry
Tetrahedral (for Cr)
Thermochemistry
Std enthalpy of
formation
ΔfHo298
-2033 kJ/mol
Standard molar
entropy
So298
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 {{{value}}}
Related compounds
Other anions {{{value}}}
Other cations {{{value}}}
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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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.

References


External links

Cost Effectiveness of Potassium dichromate

| group5 = Clinical Trials Involving Potassium dichromate | list5 = Ongoing Trials on Potassium dichromate at Clinical Trials.govTrial results on Potassium dichromateClinical Trials on Potassium dichromate at Google


| group6 = Guidelines / Policies / Government Resources (FDA/CDC) Regarding Potassium dichromate | list6 = US National Guidelines Clearinghouse on Potassium dichromateNICE Guidance on Potassium dichromateNHS PRODIGY GuidanceFDA on Potassium dichromateCDC on Potassium dichromate


| group7 = Textbook Information on Potassium dichromate | list7 = Books and Textbook Information on Potassium dichromate


| group8 = Pharmacology Resources on Potassium dichromate | list8 = AND (Dose)}} Dosing of Potassium dichromateAND (drug interactions)}} Drug interactions with Potassium dichromateAND (side effects)}} Side effects of Potassium dichromateAND (Allergy)}} Allergic reactions to Potassium dichromateAND (overdose)}} Overdose information on Potassium dichromateAND (carcinogenicity)}} Carcinogenicity information on Potassium dichromateAND (pregnancy)}} Potassium dichromate in pregnancyAND (pharmacokinetics)}} Pharmacokinetics of Potassium dichromate


| group9 = Genetics, Pharmacogenomics, and Proteinomics of Potassium dichromate | list9 = AND (pharmacogenomics)}} Genetics of Potassium dichromateAND (pharmacogenomics)}} Pharmacogenomics of Potassium dichromateAND (proteomics)}} Proteomics of Potassium dichromate


| group10 = Newstories on Potassium dichromate | list10 = Potassium dichromate in the newsBe alerted to news on Potassium dichromateNews trends on Potassium dichromate


| group11 = Commentary on Potassium dichromate | list11 = Blogs on Potassium dichromate

| group12 = Patient Resources on Potassium dichromate | list12 = Patient resources on Potassium dichromateDiscussion groups on Potassium dichromatePatient Handouts on Potassium dichromateDirections to Hospitals Treating Potassium dichromateRisk calculators and risk factors for Potassium dichromate


| group13 = Healthcare Provider Resources on Potassium dichromate | list13 = Symptoms of Potassium dichromateCauses & Risk Factors for Potassium dichromateDiagnostic studies for Potassium dichromateTreatment of Potassium dichromate

| group14 = Continuing Medical Education (CME) Programs on Potassium dichromate | list14 = CME Programs on Potassium dichromate

| group15 = International Resources on Potassium dichromate | list15 = Potassium dichromate en EspanolPotassium dichromate en Francais

| group16 = Business Resources on Potassium dichromate | list16 = Potassium dichromate in the MarketplacePatents on Potassium dichromate

| group17 = Informatics Resources on Potassium dichromate | list17 = List of terms related to Potassium dichromate


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