Potassium dichromate, K
2Cr
2O
7, is a common
inorganicInorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
chemical reagent, most commonly used as an
oxidizing agentAn oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
in various laboratory and industrial applications. As with all
hexavalent chromiumHexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
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. It is also known as
potassium bichromate;
bichromate of potash;
dipotassium dichromate;
dichromic acid, dipotassium salt;
chromic acidThe term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
, dipotassium salt; and
lopeziteLopezite is a rare red chromate mineral with chemical formula: K2Cr2O7. It crystallizes in the triclinic crystal system.It occurs as rare vug fillings in nitrate ores in association with tarapacáite , dietzeite and ulexite in the Chilean Atacama and is reported from the Bushveld igneous complex of...
.
Chemistry
Potassium dichromate is an oxidant (oxidizing agent). The reduction half-equation can be seen:
- Cr2O72−(aq) + 14H+ + 6e− → 2Cr3+(aq) + 7H2O (E = +1.23 V)
In organic chemistry, potassium dichromate is a mild oxidizer compared with
potassium permanganatePotassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...
. It is used to
oxidizeOrganic reductions or organic oxidations or organic redox reactions are redox reactions that take place with organic compounds. In organic chemistry oxidations and reductions are different from ordinary redox reactions because many reactions carry the name but do not actually involve electron...
alcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s. It converts primary alcohols into aldehydes, or into carboxylic acids if heated under
refluxReflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations...
. In contrast, with permanganate, carboxylic acids are the sole products. Secondary alcohols are converted into ketones — no further oxidation is possible. For example,
menthoneMenthone is a naturally occurring organic compound with a molecular formula C10H18O. l-Menthone , shown at right, is the most abundant in nature of the four possible stereoisomers. Menthone is a monoterpene and a ketone...
may be prepared by oxidation of
mentholMenthol is an organic compound made synthetically or obtained from peppermint or other mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is -menthol, which is assigned...
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 stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
, 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.
Cleaning
Like other chromium(VI) compounds (
chromium trioxideChromium trioxide is the inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis...
,
sodium dichromateSodium dichromate is the chemical compound with the formula Na2Cr2O7. Usually, however, the salt is handled as its dihydrate Na2Cr2O7·2H2O. Virtually all chromium ore is processed via conversion to sodium dichromate. In this way, many millions of kilograms of sodium dichromate are produced...
), potassium dichromate may be used to prepare "
chromic acidThe term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...
", which can be used for cleaning glassware and etching materials.
Construction
It is used as an ingredient in
cementIn the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
in which it retards the setting of the mixture and improves its density and texture. This usage commonly causes
contact dermatitisContact dermatitis is a term for a skin reaction resulting from exposure to allergens or irritants . Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight....
in construction workers.
Ethanol determination
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 acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
:
- C2H5OH + [O] → CH3COOH
The excess dichromate is determined by titration against
sodium thiosulfateSodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
. 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
reactionA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
is in old police
breathalyzerA breathalyzer or breathalyser is a device for estimating blood alcohol content from a breath sample...
tests. When alcohol vapor makes contact with the yellow dichromate-coated crystals, the
colorColor or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
changes from yellow to green. The degree of the color change is directly related to the level of alcohol in the suspect's breath.
Photography
Potassium dichromate has important uses in
photographyPhotography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
and in photographic screen printing, where it is used as an oxidizing agent together with a strong mineral acid.
Gum bichromateGum bichromate is a 19th century photographic printing process based on the light sensitivity of dichromates. It is capable of rendering painterly images from photographic negatives. Gum printing is traditionally a multi-layered printing process, but satisfactory results may be obtained from a...
printing was one of the very first stable photographic printing processes, dating back to about 1850. A solution of
gum arabic220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...
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 acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric 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 chlorideSilver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
. 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 acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
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
silverSilver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
metal to
silver sulfateSilver sulfate is an ionic compound of silver used in silver plating and as a non-staining substitute to silver nitrate. This sulfate is stable under ordinary conditions of use and storage, though it darkens upon exposure to air or light...
, 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
solarisationSolarisation is a phenomenon in photography in which the image recorded on a negative or on a photographic print is wholly or partially reversed in tone. Dark areas appear light or light areas appear dark...
so that the end product resembles a negative and is suitable for printing in the normal way.
Cr
VI compounds have the property of
tanningTanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...
animal
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s when exposed to strong light. This quality is used in photographic
screen-printingScreen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...
.
In
screen-printingScreen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...
a fine screen of bolting silk or similar material is stretched taut onto a frame similar to the way canvas is prepared before painting. A
colloidA colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...
sensitized with a dichromate is applied evenly to the taut screen. Once the dichromate mixture is dry, a full-size photographic negative is attached securely onto the surface of the screen, and the whole assembly exposed to strong light - typically about half an hour in bright sunlight - hardening the exposed colloid. When the negative is removed, the unexposed mixture on the screen can be washed off with warm water, leaving the hardened mixture intact, acting as a precise mask of the desired pattern, which can then be printed with the usual
screen-printingScreen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...
process.
Silver test
When dissolved in an approximately 35%
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
solution it is called Schwerter's solution and is used to test for the presence of various metals, notably for determination of silver purity. Pure silver will turn the solution bright red,
sterling silverSterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925....
will turn it dark red, low grade coin silver (0.800 fine) will turn brown (largely due to the presence of copper which turns the solution brown) and even green for 0.500 silver.
Sulfur dioxide test
Potassium dichromate paper can be used to test for
sulfur dioxideSulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, as it turns distinctively from orange to green. This is typical of all redox reactions where hexavalent chromium is reduced to the less harmful trivalent chromium. Therefore, it is not a conclusive test for sulfur dioxide.
Wood treatment
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
mahoganyThe name mahogany is used when referring to numerous varieties of dark-colored hardwood. It is a native American word originally used for the wood of the species Swietenia mahagoni, known as West Indian or Cuban mahogany....
.
Natural occurrence
Potassium dichromate occurs naturally as the rare
mineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
lopeziteLopezite is a rare red chromate mineral with chemical formula: K2Cr2O7. It crystallizes in the triclinic crystal system.It occurs as rare vug fillings in nitrate ores in association with tarapacáite , dietzeite and ulexite in the Chilean Atacama and is reported from the Bushveld igneous complex of...
. It has only been reported as
vugVugs are small to medium-sized cavities inside rock that may be formed through a variety of processes. Most commonly cracks and fissures opened by tectonic activity are partially filled by quartz, calcite, and other secondary minerals. Open spaces within ancient collapse breccias are another...
fillings in the
nitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
deposits of the
Atacama desertThe Atacama Desert is a plateau in South America, covering a strip of land on the Pacific coast, west of the Andes mountains. It is, according to NASA, National Geographic and many other publications, the driest desert in the world...
of
ChileChile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
and in the
Bushveld igneous complexThe Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
of
South AfricaThe Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
.
Safety
Potassium dichromate is one of the most common causes of chromium
dermatitis-Etymology:Dermatitis derives from Greek derma "skin" + -itis "inflammation" and genetic disorder.-Terminology:There are several different types of dermatitis. The different kinds usually have in common an allergic reaction to specific allergens. The term may describe eczema, which is also called...
; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. Toxicological studies have further illustrated its highly toxic nature. With rabbits and rodents, concentrations as low as 14 mg/kg have shown a 50% fatality rate amongst test groups. Aquatic organisms are especially vulnerable if exposed, and hence responsible disposal according local environmental regulations is advised.
As with other Cr
VI compounds, potassium dichromate is
carcinogenA carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
ic and should be handled with gloves and appropriate health and safety protection. The compound is also
corrosiveA corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...
and exposure may produce severe eye damage or blindness. Human exposure further encompasses impaired fertility, heritable genetic damage and harm to unborn children.
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