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Chromate salts contain the chromate anion, . Dichromate salts contain the dichromate anion, . They are
oxyanionAn oxyanion, or oxoanion, is an ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyd ...
s of
chromium Chromium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science tha ...

chromium
in the 6+
oxidation state The oxidation state, or oxidation number, is the hypothetical charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * Charge (David Ford album), ''Charge ...
and are moderately strong
oxidizing agent 125px, Dangerous goods label for oxidizing agents An oxidizing agent, also known as an oxidant or oxidizer, is a substance that has the ability to oxidize (mild reducing agent) are added to powdered potassium permanganate (strong oxidizi ...

oxidizing agent
s. In an
aqueous An aqueous solution is a solution Image:SaltInWaterSolutionLiquid.jpg, Making a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chemistry, a solution ...
solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the water the solvent. In chemistry ...
, chromate and dichromate ions can be interconvertible.


Chemical properties

Potassium-chromate-sample.jpg,
potassium chromate Potassium chromate is the inorganic compound with the formula (K2 CrO4). This yellow solid is the potassium salt of the chromate anion. It is a common laboratory chemical, whereas sodium chromate Sodium chromate is the inorganic compound with ...

potassium chromate
Potassium-dichromate-sample.jpg,
potassium dichromate Potassium dichromate, , is a common inorganic compound, inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically ...

potassium dichromate
Chromates react with
hydrogen peroxide Hydrogen peroxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...

hydrogen peroxide
, giving products in which
peroxide Peroxides are a group of compounds with the structure R−O−O−R, where R = any element. The O−O group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

peroxide
, , replaces one or more oxygen atoms. In acid solution the unstable blue peroxo complex
Chromium(VI) oxide peroxide Chromium(VI) peroxide or chromium oxide peroxide is an unstable compound with the formula CrO5. This compound contains one oxo ligand and two peroxo ligands, making a total of five oxygen atoms per chromium atom. Preparation and properties Chromiu ...
, CrO(O2)2, is formed; it is an uncharged
covalent A covalent bond is a chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and take ...

covalent
molecule, which may be extracted into
ether Ethers are a class of organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reaction ...
. Addition of
pyridine Pyridine is a basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming language In computer science Computer science deals with the theoretical foundations of informati ...

pyridine
results in the formation of the more stable complex CrO(O2)2py.


Acid–base properties

In aqueous solution, chromate and dichromate anions exist in a
chemical equilibrium In a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes th ...
. :2  + 2 H+ + H2O The
predominance diagramA predominance diagram purports to show the conditions of concentration and pH where a chemical species A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up s ...

predominance diagram
shows that the position of the equilibrium depends on both
pH
pH
and the analytical concentration of chromium.pCr is equal to minus the decimal logarithm of the analytical concentration of chromium. Thus, when pCr = 2, the chromium concentration is 10−2 mol/L. The chromate ion is the predominant species in alkaline solutions, but dichromate can become the predominant ion in acidic solutions. Further condensation reactions can occur in strongly acidic solution with the formation of trichromates, , and tetrachromates, . All poly
oxyanionsAn oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a ...
of chromium(VI) have structures made up of tetrahedral CrO4 units sharing corners. The hydrogen chromate ion, HCrO4, is a
weak acid Acid strength is the tendency of an acid An acid is a molecule or ion capable of donating a proton (hydrogen ion H+) (a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid), or, alternatively, capable of forming a covalent bond wi ...

weak acid
: : + H+; p''K''a ≈ 5.9 It is also in equilibrium with the dichromate ion: :2  + H2O This equilibrium does not involve a change in hydrogen ion concentration, which would predict that the equilibrium is independent of pH. The red line on the predominance diagram is not quite horizontal due to the simultaneous equilibrium with the chromate ion. The hydrogen chromate ion may be protonated, with the formation of molecular
chromic acid The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid Sulfuric acid ( American spelling) or sulphuric acid ( Commonwealth spelling), also known as oil of vitriol, is a mineral acid composed of the elem ...
, H2CrO4, but the p''K''a for the equilibrium :H2CrO4 + H+ is not well characterized. Reported values vary between about −0.8 and 1.6. The dichromate ion is a somewhat weaker base than the chromate ion: : + H+, p''K'' = 1.18 The p''K'' value for this reaction shows that it can be ignored at pH > 4.


Oxidation–reduction properties

The chromate and dichromate ions are fairly strong
oxidizing agent 125px, Dangerous goods label for oxidizing agents An oxidizing agent, also known as an oxidant or oxidizer, is a substance that has the ability to oxidize (mild reducing agent) are added to powdered potassium permanganate (strong oxidizi ...

oxidizing agent
s. Commonly three electrons are added to a chromium atom, reducing it to oxidation state +3. In acid solution the aquated Cr3+ ion is produced. : + 14 H+ + 6 e → 2 Cr3+ + 7 H2O ''ε''0 = 1.33 V In alkaline solution chromium(III) hydroxide is produced. The
redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species A chemical species is a chemical substance A chemical substance is a form of matter In clas ...
shows that chromates are weaker oxidizing agent in alkaline solution than in acid solution.. : + 4 + 3 e → + 5 ''ε''0 = −0.13 V


Applications

Approximately of
hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is ...
, mainly sodium dichromate, were produced in 1985. Chromates and dichromates are used in
chrome plating Chrome plating (less commonly chromium plating) is a technique of electroplating Electroplating is a general name for processes that produce a metal coating on a solid substrate through the redox, reduction of cations of that metal by means o ...
to protect metals from corrosion and to improve paint adhesion. Chromate and dichromate salts of
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as m ...
,
lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elem ...
s and
alkaline earth metal The alkaline earth metals are six chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a n ...
s are only very slightly soluble in water and are thus used as pigments. The lead-containing pigment
chrome yellow __NOTOC__ Chrome yellow is lead(II) chromate (PbCrO4). It occurs naturally as the mineral crocoite but the mineral ore was never used as a pigment for paint. After the French chemist Louis Vauquelin discovered the new element chromium in 1797 l ...
was used for a very long time before environmental regulations discouraged its use. When used as oxidizing agents or titrants in a
redox Redox (reduction–oxidation, pronunciation: or ) is a type of chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter ...

redox
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
, chromates and dichromates convert into trivalent chromium, Cr3+, salts of which typically have a distinctively different blue-green color.


Natural occurrence and production

The primary chromium ore is the mixed metal oxide
chromite Chromite is a crystalline mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of ...

chromite
, FeCr2O4, found as brittle metallic black crystals or granules. Chromite ore is heated with a mixture of
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
and
sodium carbonate Sodium carbonate, ·10, (also known as Natrium Carbonate, washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield modera ...

sodium carbonate
in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms iron(III) oxide, Fe2O3: :4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2 Subsequent leaching of this material at higher temperatures dissolves the chromates, leaving a residue of insoluble iron oxide. Normally the chromate solution is further processed to make chromium metal, but a chromate salt may be obtained directly from the liquor. Chromate containing minerals are rare.
Crocoite Crocoite is a mineral consisting of lead chromate, PbCr O4, and crystallizing in the monoclinic crystal system. It is identical in composition with the artificial product chrome yellow used as a paint pigment. Description Crocoite is commonl ...

Crocoite
, PbCrO4, which can occur as spectacular long red crystals, is the most commonly found chromate mineral. Rare potassium chromate minerals and related compounds are found in the . Among them is lópezite – the only known dichromate mineral.


Toxicity

All
hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is ...
compounds are
toxic Toxicity is the degree to which a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects t ...

toxic
(due to their oxidizing power) and
carcinogenic A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis, the formation of cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other part ...
(
IARC Group 1Substances, mixtures, and exposure circumstances in this list have been classified as Group 1 by the International Agency for Research on Cancer (IARC): The agent (mixture) is carcinogenic to humans. The exposure circumstance entails exposures that a ...
), especially if airborne and inhaled where they cause
lung cancer Lung cancer, also known as lung carcinoma, since about 98–99% of all lung cancers are carcinomas, is a malignant lung tumor characterized by uncontrolled cell growth in tissue (biology), tissues of the lung. Lung carcinomas derive from trans ...

lung cancer
. Also positive associations have been observed between exposure to
chromium (VI) Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is ...
compounds and
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of cells Cell most often refers to: * Cell (biolo ...

cancer
of the
nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside the mouth. Behind the nose are the olfactory mucosa and the Paranasal sinus, sinuses. Be ...

nose
and
nasal sinuses Paranasal sinuses are a group of four paired skeletal pneumaticity, air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the human eye, eyes; the frontal sinuses are above the eyes; the Ethmoid sinus, ethmoidal ...
. The use of chromate compounds in manufactured goods is restricted in the EU (and by market commonality the rest of the world) by EU Parliament directive on the Restriction of Hazardous Substances (RoHS) Directive (2002/95/EC).


See also

*
Chromate conversion coating 240px, Zinc chromate conversion coating on small steel parts. Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel Steel is an alloy of iron with typically a few tenths of a percent of carb ...


Notes


References


External links


National Pollutant Inventory - Chromium(VI) and compounds fact sheetDemonstration of chromate-dichromate equilibrium
{{Chromates and dichromates Oxidizing agents Transition metal oxyanions