In chemistry, an oxidizing agent is a type of substance which has the ability to oxidize other substances. You may wonder what oxidation means. Oxidation is a phenomenon where a substance can accept electrons from other substances. Some of the most common oxidizing agents are halogens, hydrogen peroxide, oxygen, ozone, chromium hexavalent compounds etc.
A chemical compound that has chromium element in its +6oxidation state is known as Hexavalent chromium (Chromium +6, Cr (VI), chromium (VI)). They are considered to be highly toxic as they are genotoxic carcinogens.
What is Potassium Dichromate?
A hexavalent chromium compound which acts as a common inorganic chemical reagent is called as potassium dichromate. The chemical formula of potassium dichromate is K2Cr2O7. It is used in many industrial applications and laboratories as a conventional oxidizing agent.
Being hexavalent, potassium dichromate is highly toxic in the nature and harmful to skin and body. Potassium dichromate is crystalline ionic solid with very bright red-orange color.
Potassium dichromate is more popular in laboratory experiments as it is not deliquescent (a tendency to absorb air moisture and dissolve in it) as compared to most industry-relevant sodium dichromate salts.
A reaction of potassium chloride with sodium dichromate gives potassium dichromate.
Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
Alternatively, it can be obtained from potassium chromate through roasting chromite ore with potassium hydroxide.
It ionizes in water:
K2Cr2O7 → 2K+ + Cr2O72−
Cr2O72− + H2O ⇌ 2CrO42− + 2H+
This reaction when performed in the laboratory gives orange red crystals of potassium dichromate. It is soluble in water. It does not have any distinctive odor. It may, however, severely irritate the eyes and respiratory tract. Avoid contact of potassium dichromate with organic materials. It is noncombustible but most commonly used in pyrotechnic displays along with tungsten and iron.
The Structure of Potassium Dichromate (K2Cr2O7))
The chemical formula of potassium dichromate is K2Cr2O7 with 294.185 g/mol molar mass. It is an ionic compound with two potassium ions (K+) and the negatively charged dichromate ion (Cr2O7-), in which two hexavalent chromium atoms (with oxidation state +6) are each attached to three oxygen atoms as well as a bridging oxygen atom.
Figure 1: Properties of Potassium Dichromate (K2Cr2O7)
Properties of Potassium Dichromate (K2Cr2O7)
It forms orange red crystals which melt at 396oC.
It is easily soluble in hot water but moderately soluble in cold water.
Action of Heat: On heating, K2Cr2O7 decomposes to give potassium chromate with evolution of O2 gas.
4K2Cr2O7 -> 4K2CrO4 + 2Cr2O3 + 3O2
Reaction with Alkalis: When an orange-red solution of K2Cr2O7 reacts with an alkali, a yellow solution is obtained because of the potassium chromate formation.
K2Cr2O7 + 2KOH ->2K2CrO4 + H2O
On acidifying the yellow color of K2CrO4 again changes to orange red due to reformation of K2Cr2O7.
2K2CrO4 + H2SO4 -> K2Cr2O7 + K2SO4 + H2O
The inter conversion can be explained on the basis of the fact that in K2Cr2O7 solution, orange red Cr2O72- ions are in equilibrium with yellow CrO42- ions.
Cr2O72- + H2O -> 2CrO42- + 2H+
Orange red Yellow
Oxidization: K2Cr2O7 is known as a powerful oxidizing agent. In presence of dil.H2SO4, one mole of this compound gives three atoms of oxygen as shown below.
K2Cr2O7 + 4H2SO4 -> K2SO4 + Cr2 (SO4)3 + 4H2O + 3(O)-
Some other examples of oxidizing property of K2Cr2O7 are given below:
It liberates I2 from KI.
K2Cr2O7 + 7 H2SO4 + 6KI -> 4 K2SO4+ Cr2(SO4)3 + 3I2 + 7H2O
It oxidizes ferrous sulphate to ferric sulphate.
K2Cr2O7 + 7H2SO4 + 6FeSO4 -> K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3+ 2H2O
It oxidizes H2S to sulphur.
K2Cr2O7 + 4H2SO4 + 3H2S -> K2SO4 + Cr2(SO4)3 + 7H2O + 3S
Chromyl Chloride Test: When salt containing chloride is treated with K2Cr2O7 and con. H2SO4 chromyl chloride (reddish brown) vapors are produced.
K2Cr2O7+ 4KCl + 6 H2SO4 -> 2CrO2Cl2 + 6KHSO4 + 3H2O
The reaction is used to detect chloride ions in a qualitative analysis.
Uses of Potassium Dichromate (K2Cr2O7)
Potassium dichromate has widespread industrial use. Some of its uses are listed below:
K2Cr2O7 is an oxidizing agent for variety of reactions in laboratories and industry.
It is used in leather industry for chrome tanning by acting as a precursor for potassium chrome alum.
It is used in volumetric analysis.
It is used in dyeing and calico printing.
In photography, K2Cr2O7 is used with strong mineral acid as an oxidizing agent to harder the gelatin film.
It is used to prepare chromic acid, which is used to clean glassware (like other chromium (VI) compounds, sodium dichromate, chromium trioxide). However, due to the toxicity of hexavalent chromium, this practice is now discouraged and discontinued now.
K2Cr2O7 finds use in construction industry as an ingredient in cement.
It is a non-hygroscopic reagent and used in “wet tests” in analytical chemistry.
It is known as Schwerter’s solution when dissolved in 35 percent nitric acid, and used in test to detect the presence of different metals in determining silver purity.
It is used as wood darkening in the wood tannins that produced deep brown color on wood. It is effective treatment for mahogany.
Effects on Health
Potassium dichromate can cause chronic diseases like chromium dermatitis in hands and forearms.
The toxicity of potassium dichromate can cause harm to animals such as rabbits, rodents etc.
It is toxic to aquatic organisms too, and poses a big threat to the environment.
Potassium dichromate is corrosive in nature and its exposure may develop several eye problems or, even blindness.
It may cause heritable genetic damage, impaired fertility, and is dangerous to unborn children.