Potassium Dichromate

Potassium dichromate is one of the most widely used chemical reagents in inorganic chemistry. It primarily serves as a strong oxidizing agent in various chemical reactions in laboratories and for industries, it is an important chemical widely used wherever oxidation is involved.

Potassium dichromate is always handled with great precautions and care as it poses a health hazard to humans if exposed for a long time. It has a very beautiful bright red-orange colour appearance and is found in an ionic solid crystalline state. The reason that potassium dichromate is so widely used in analytical chemistry and industries is its non-deliquescent nature as compared to other industrially important salts like sodium dichromate.

The preparation of potassium dichromate is commonly done by using chromates and these chromates are formed by the reaction of chromite ore with sodium/potassium carbonate.

Chemical Formula and Molecular Structure

The chemical formula for potassium dichromate is K2Cr2O7 and the molar mass is calculated to be 294.185 g/mol. 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.

Physical Properties of Potassium Dichromate

  • Potassium dichromate has an appearance of red-orange crystals. Normally exist in a solid-state at room temperature.

Potassium dichromate solution

  • It is odourless.
  • It is non-combustible and also its nature is highly corrosive.
  • The density of Potassium dichromate in the solid-state is 2.676 g/cm3.
  • Its melting point is 398 °C (748 °F; 671 K) while its boiling point is 500 °C (932 °F; 773 K) . It decomposes on boiling.
  • The solubility of potassium dichromate in the water at different temperatures is 4.9 g/100 mL at 0 °C and 13 g/100 mL 20 °C and 102 g/100 mL at 100 °C which means it becomes highly soluble in water at higher temperatures. On the other hand, it is insoluble in alcohol, acetone.
  • Refractive index of potassium dichromate is 1.738.
  • Its Crystalline structure is triclinic and coordination geometry for the central atom Chromium is tetrahedral.

Chemical Properties of Potassium Dichromate

1. Action of Heat

K2Cr2O7 shows decomposition when heated to liberate potassium chromate and evolution of O2 gas is also observed.

The reaction for this can be depicted as follows:

4K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3 O2

2. With Alkalies

The orange-red colour of K2Cr2O7 solution turns into a yellow colour solution when potassium dichromate reacts with alkalies. This yellow colour is because of the formation of potassium chromate.

The reaction proceeds as follows:

K2Cr2O7 + 2KOH → 2K2CrO4 + H2O

Orange-red                   Yellow

Again the yellow colour of K2CrO4 converts to orange-red when acidified because of the formation of K2Cr2O7.

2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O

Orange-red

This colour change of orange to yellow and yellow to orange can be explained on the concept of equilibrium of the two ions. In the potassium dichromate solution, Cr2O72- ions (in orange colour) are in equilibrium with CrO42- ions (in yellow colour ).

Cr2O72- + H2O ⇋ 2CrO42- + 2H+

Orange-red               Yellow

3. Oxidising properties

Potassium dichromate is one of the most powerful oxidising agents. one mole of K2Cr2O7 liberates three moles of oxygen when reacted with dil.H2SO4.

The reaction can be depicted as follows:

K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2 (SO4)3 + 4H2O + 3(O)

Few more illustrations of oxidising properties of K2Cr2O7 are listed as follows:

  • It liberates I2 from KI. The reaction is listed as follows:

K2Cr2O7 + 7H2SO4 + 6KI → 4 K2SO4 + Cr2 (SO4)3 + 3 I2 + 7H2O

  • Ferrous salts are oxidised to ferric salt by Potassium dichromate.

The equation for this reaction is:

K2Cr2O7 + 7H2SO4 + 6 FeSO4 → K2SO4 + Cr2 (SO4)3 + 3 Fe2 (SO4)3+ 2H2O

  • Hydrogen sulphide is oxidised to sulphur by this compound.

The equation for this reaction is:

K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2 (SO4)3 + 7H2O + 3S

4. Chromyl chloride test

When Potassium dichromate and concentrated sulphuric acid are reacted with salt-containing chloride, reddish-brown vapours of chromyl chloride are evolved.

This reaction is also popularly known as “ The Chromyl chloride test”.

The equation for this reaction is:

K2Cr2O7 + 4KCl + 6H2SO4 → 2CrO2Cl2 + 6 KHSO4 + 3H2O

Chromyl Chloride

This above reaction is widely employed in the salt analysis of chloride ions.

Preparation

Preparation of potassium dichromate from chrome Iron Ore

A step by step process for the preparation of K2Cr2O7 from its ore is explained as follows:

i) Conversion of chrome iron ore to sodium chromate

The ore is first converted to a powdered form and then blended with Sodium carbonate Na2CO3 and CaO (quick lime), then it is heated inside a reverberatory furnace in presence of sufficient amounts of air. The reaction occurring in this step can be written as:

4FeO.Cr2O3 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2

ii) Conversion of Na2CrO4 to Na2Cr2O7

From the first step, we get Sodium chromate solution. This Sodium chromate solution is purified properly by the process of filtration so as to remove iron oxide and other impurities and then it is further reacted with concentrated H2SO4, which results in the conversion of sodium chromate into sodium dichromate.

2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

iii) Conversion of sodium dichromate into potassium dichromate

In the final step, the sodium dichromate obtained from the previous step is concentrated and heated and then this hot concentrated solution of sodium dichromate is reacted with potassium chloride KCl, which results in the formation of potassium dichromate.

The reaction for this step can be written as follows:

Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl

The Potassium dichromate is sparingly soluble in water as compared to sodium chloride NaCl.

Due to the potassium dichromate separates out in the form of crystals on cooling.

Potassium Dichromate Uses

The important uses of Potassium dichromate in industries are  as follows:

  1. The potassium dichromate is mainly used in the production of potassium chrome alum, and in the leather tanning industry.
  2. Potassium dichromate can be used as a raw material for the manufacturing of chromic acid which is used as an etchant for glass materials and also for the purpose of cleaning glassware. Nowadays its use is very limited because of its hazardous nature.
  3. It is also used in the construction industry as an ingredient in cement in which it increases the hardening time of the concrete mixture and improves its density and colour. Long term exposure of potassium dichromate to construction workers causes skin diseases.
  4. In the photography industry, it is used as an oxidising agent and in photography screen printing.
  5. It is also used as a hardening agent for organic solvents like gelatin, dye, carbon black and gum arabic. These hardened solvents find their uses in the creation of metal printing plates used in photomechanical printing processes.
  6. The solution of potassium dichromate solution with dilute H2SO4 is used for the development of negatives in photography.
  7. It is also used in screen printing photography because it has a property of tanning leathers and skin on the exposure of light.
  8. Potassium dichromate acts as an excellent chemical reagent in analytical chemistry because of its non- hygroscopic properties. One of the major uses is in the determination of the concentration of ethanol.
  9. One important application for potassium dichromate is in old police breathalyzer tests for the determination of drink and drive cases. When alcohol vapour makes contact with the orange dichromate-coated crystals, the colour changes from Cr(VI) orange to Cr(III) green. The degree of the colour change is directly related to the level of alcohol in the suspect’s breath.
  10. Schwerter’s solution ( solution of 35% nitric acid and potassium dichromate) is used for the analysis of various metals and their purity. This is more common for silver so it is called the silver test. It also gives different colours for different purity grades of silver.
  11. Potassium dichromate paper is widely used for the analytical test of sulfur dioxide, as it gives a cloud change from orange to green.
  12. Many times it is also used for the staining, polishing and tanning of wood which gives its a glossy finish with a rich brown texture.

Health Hazards

  1. Potassium dichromate is highly carcinogenic. Prolonged exposure to this compound can cause severe damage to respiratory organs and even can in some cases.
  2. Even a small amount of consumption of this salt can prove fatal. It has been found that a dose of 40 grams or more leads to immediate death and heavy side effects in the human body.
  3. As it is highly corrosive in nature, it burns the inner lining of the stomach and intestines if consumed.
  4. Symptoms of illness such as excess urination and vomiting and loose motion are generally seen after its light exposure or consumption.
  5. The Potassium ion present in this compound has been known for the cause of heart diseases such as low blood pressure, heartbeat malfunction, heart attack, choking of blood veins.
  6. Skin also gets burnt by its corrosive nature.
  7. The compound is also corrosive and exposure may produce severe eye damage or blindness. Prolonged exposure of the human body to this compound leads to reproductive disease, loss of fertility in males, genetic disorders and harm to unborn children.

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