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Chromates and dichromates are salt
Salt

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and base . Salts are ionic compounds composed of cations and anions so that the product is electrically electric charge ....
s of chromic acid
Chromic acid

Chromic acid generally refers to a collection of chemical compound generated by the acidification of solutions containing chromate and dichromate ion or the dissolving of chromium trioxide in sulfuric acid....
 and dichromic acid, respectively. Chromate salts contain the chromate anion, Cr
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
O
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
42-, and have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O72-, and have an intense orange color.

chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
 centers in both of these anions in oxidation state
Oxidation state

In 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 Electrical charge that an atom would have if all bonds to atoms of different elements were 100% Ionic bond....
 +6.






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Chromates and dichromates are salt
Salt

A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and base . Salts are ionic compounds composed of cations and anions so that the product is electrically electric charge ....
s of chromic acid
Chromic acid

Chromic acid generally refers to a collection of chemical compound generated by the acidification of solutions containing chromate and dichromate ion or the dissolving of chromium trioxide in sulfuric acid....
 and dichromic acid, respectively. Chromate salts contain the chromate anion, Cr
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
O
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
42-, and have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O72-, and have an intense orange color.

Characteristics

The chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
 centers in both of these anions in oxidation state
Oxidation state

In 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 Electrical charge that an atom would have if all bonds to atoms of different elements were 100% Ionic bond....
 +6. The chromate and dichromate ions are fairly strong oxidizing agent
Oxidizing agent

An oxidizing agent can be defined as either:#a chemical compound that readily transfers oxygen atoms, or#a substance that gains electrons in a redox chemical reaction...
s. Chromate and dichromate anions exist in a chemical equilibrium
Chemical equilibrium

In a chemical process, chemical equilibrium is the state in which the Activity or concentrations of the reactants and products have no net change over time....
. 2 CrO42− + 2 H3O+ ? Cr2O72− + 3 H2O This equilibrium can be pushed toward dichromate by lowering the pH
PH

pH is a measure of the Acid or Base of a solution. It is defined as the cologarithm of the Activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations....
 (making the solution more acidic) or in the other direction towards chromate by raising the pH to basic
Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
. This is a classic example of Le Chatelier's principle
Le Châtelier's principle

In chemistry, Le Chatelier's Principle, also called the Le Chatelier-Braun principle, can be used to predict the effect of a change in conditions on a chemical equilibrium....
 at work. This equilibrium is also dependent on concentration of Chromium in solution.

Applications

Chromate and dichromate salts are widely used in chemical industry, most significantly in the isolation of chromium from its ores.

The sodium dichromate
Sodium dichromate

Sodium dichromate is the chemical compound with the formula sodiumChromiumOxygen. Usually, however, the salt is handled as its dihydrate Na2Cr2O7?2water of crystallization....
 (Na2Cr2O7) and potassium dichromate
Potassium dichromate

Potassium dichromate, K2Cr2O7, is a common inorganic compound chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications....
 (K2Cr2O7),]], as well as other derivatives are water soluble granular solids and are common reagent
Reagent

A reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s. Chromate and dichromate salts of heavy metals
Heavy metals

A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides....
, lanthanides and alkaline earth metals are only very slightly soluble in water and are thus used as pigments.

Chromates and dichromates are used in chrome plating
Chromate conversion coating

Chromate conversion coating is a type of conversion coating applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion....
 to protect metals for corrosion protection and to improve paint adhesion. When used as oxidizing agents or titrants in a redox
Redox

Redox describes all chemical reactions in which atoms have their oxidation number changed.This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane , or it can be a complex process such as the oxidation of sugar in the human body through a ser...
 chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
, chromates and dichromates convert turn into trivalent chromium, Cr3+, salts of which typically have a distinctively different blue-green color.

Safety

Chromium in the +6 (or VI) oxidation state is often referred to as hexavalent chromium
Hexavalent chromium

Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via conversion to sodium dichromate....
. Such compounds, especially when air-borne, are carcinogenic. All hexavalent chromium compounds are considered toxic. 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 2002/95/EC

Structures

The tetrahedral chromate ion, CrO42- The dichromate ion, Cr2O72-, consists of two corner-sharing tetrahedra


Natural occurrence

Chromate minerals are rarely found in the nature. The most commonly met is crocoite
Crocoite

File:Crocoite from the Dundas extended mine, Dundas, Tasmania, Australia.jpgCrocoite is a mineral consisting of lead chromate, PbCrO4, and crystallizing in the monoclinic system....
. Potassium-bearing chromates and related compounds are known from Atacama desert, but are very rare minerals.

Examples

  • Ammonium dichromate
    Ammonium dichromate

    Ammonium dichromate is the inorganic compound with the Chemical formula 2Cr2O7. In this compound, and in all chromates and dichromates, chromium is in a +6 oxidation state, commonly known as hexavalent chromium....
      (NH4)2Cr2O7
  • Barium chromate
    Barium chromate

    Barium chromate is a fine light yellow powder with the formula BaCrO4.It is very Solubility in water, but is soluble in acids:It is used in some pyrotechnic compositions, especially delay compositions, as an oxidizer and/or burn rate modifier....
      BaCrO4
  • Cadmium chromate CdCrO4
  • Caesium chromate
    Caesium chromate

    Caesium chromate is used to produce caesium vapour by reaction with silicon or boron, which is used to in the final stages of creating a vacuum tubes; the caesium vapour reacts with the remaining gases, including nitrogen and oxygen....
      Cs2CrO4
  • Calcium chromate
    Calcium chromate

    Calcium chromate is a bright yellow solid. It normally occurs as the dihydrate....
      CaCr2O7
  • Cobalt(II) chromate CoCrO4
  • Cobalt(II) dichromate CoCr2O7
  • Cobalt(III) chromate Co2(CrO4)3
  • Cobalt(III) dichromate Co2(Cr2O7)3
  • Copper(I) chromate Cu2CrO4
  • Copper(I) dichromate Cu2Cr2O7
  • Copper(II) chromate CuCrO4
  • Copper(II) dichromate CuCr2O7
  • Iron(III) chromate
    Iron(III) chromate

    Iron chromate chemical compound with the chemical formula Fe23.References...
     Fe2(CrO4)3
  • Iron(III) dichromate Fe2(Cr2O7)3
  • Lead(II) chromate
    Lead(II) chromate

    Lead chromate is a chemical compound, a chromate of lead. It has a vivid yellow color and is practically insoluble in water, and as a result, is used in paints under the name "chrome yellow"....
     PbCrO4
  • Lead(II) dichromate PbCr2O7
  • Lead(IV) chromate Pb(CrO4)2
  • Lead(IV) dichromate Pb(Cr2O7)2
  • Nickel(II) chromate NiCrO4
  • Nickel(II) dichromate NiCr2O7
  • Nickel(III) chromate Ni2(CrO4)3
  • Nickel(III) dichromate Ni2(Cr2O7)3
  • Potassium chromate
    Potassium chromate

    Potassium chromate is a yellow chemical indicator used for identifying concentrations of chloride ions in a salt solution with silver nitrate ....
      K2CrO4
  • Potassium trioxochlorochromate
    Potassium trioxochlorochromate

    Potassium trioxo?chloro?chromate, Potassium chlorochromate, Peligot's salt, or P?ligot's salt is a chemical substance named after Eug?ne-Melchior P?ligot....
      KCrO3Cl
  • Potassium dichromate
    Potassium dichromate

    Potassium dichromate, K2Cr2O7, is a common inorganic compound chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications....
      K2Cr2O7
  • Pyridinium chlorochromate
    Pyridinium chlorochromate

    Pyridinium chlorochromate is a reddish orange solid reagent used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. Pyridinium chlorochromate, or PCC, will not fully oxidize the alcohol to the carboxylic acid as does the Jones reagent....
  • Silver chromate
    Silver chromate

    Silver chromate is a brown-red monoclinic crystal and is a chemical precursor to modern photography. It can be formed by combining silver nitrate and potassium chromate ....
      Ag2CrO4
  • Silver dichromate Ag2Cr2O7
  • Sodium chromate
    Sodium chromate

    Sodium chromate is a yellow solid chemical compound used as a corrosion inhibitor in the petroleum industry, a dyeing auxiliary in the textile industry, as a wood preservative....
      Na2CrO4
  • Strontium chromate SrCrO4
  • Tin(II) chromate SnCrO4
  • Tin(II) dichromate SnCr2O7
  • Tin(IV) chromate Sn(CrO4)2
  • Tin(IV) dichromate Sn(Cr2O7)2
  • Zinc chromate
    Zinc chromate

    Zinc chromate, zincchromiumoxygen, is a chemical compound containing the chromate anion. It is used industrially in chromate conversion coatings....
      ZnCrO4
  • Zinc dichromateZnCr2O7


See also

  • Chromate conversion coating
    Chromate conversion coating

    Chromate conversion coating is a type of conversion coating applied to passivate aluminium, zinc, cadmium, copper, silver, magnesium, tin and their alloys to slow corrosion....


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