Potassium dichromate (K2Cr2O7 ) Uses

Potassium dichromate is formed using potassium. It is used as an agent for oxidising elements. It has a bright orange colour and is inorganic and toxic. It is used in laboratories as well as industries on a large scale. It can be hazardous to the health of humans and, therefore, must be handled with great care and precaution. 

The production of potassium dichromate involves the reaction of chromates with either sodium or potassium carbonate. It is used to clean glassware and as an etching material, in photographic screen printing, and more. 

Structure of Potassium dichromate.

It is a hexavalent chromium compound. Its structural formula is written as K2Cr2O7. It has two potassium ions and a negatively charged dichromate ion. This structure of chromate makes it ionic. 

Availability and Preparation

Potassium dichromate rarely occurs naturally in the form of a mineral called lopezite. It is made in industries by reacting potassium chloride (KCl) with sodium dichromate (Na2Cr2O7). Potassium chromate (K2CrO4) reacts with acids to give the salts of dichromate.

The reaction for the preparation of potassium dichromate is as follows.

Na2Cr2O7 + 2 KCl → K2Cr2O7 + 2 NaCl

Physical Properties

1. It is in a solid form at room temperature and has bright-orange crystal colour. 

2. It doesn’t have any smell and has a disagreeable bitter taste.

3. It has an irritating effect on the eyes and is toxic.

4. It tends to get rust easily and is non-combustible. 

5. The melting point of potassium dichromate is 398°C, while its boiling point is 500°C. Decomposition of potassium dichromate takes place when it is boiled. 

6. It is easily soluble in water at high temperatures. Solubility varies with the temperatures. 

7. It does not dissolve in alcohol as well as acetones. 

8. The Refractive Index is 1.738. Chromium ion of potassium dichromate has a tetrahedral structure in coordinate geometry.

9. Its crystalline structure is triclinic. 

Potassium Dichromate – Chemical Properties

• Heat action: When K2Cr2O7 is heated, it decomposes to give potassium chromate with the release of O2 gas. 

       4 K2Cr2O7 → 4K2CrO4 + 2Cr2O3 + 3O2

• Reaction with alkalis: The reaction of K2Cr2O7 solution with an alkali reactant tends to change its colour from orange-red to yellow solution. This happens due to the formation of potassium chromate.  

                                                                         K2Cr2O7 + 2KOH   →  2 K2CrO4 + H2O

                                                                        Orange-red                         Yellow

• If the above-formed yellow colour K2CrO4 is acidified, it again changes to orange-red. This happens due to the formation of K2Cr2O7.

2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O

• This process of formation of potassium chromate and reformation of potassium dichromate happens due to the presence of orange-red Cr2O72- ions in equilibrium with yellow CrO42- ions in the solution of K2Cr2O7. 

                                                                Cr2O72- + H2O → 2CrO42- + 2H+

                                                                              Orange-red            Yellow

Oxidation reaction: One mole of potassium dichromate, when reacted with dil.H2SO4, gives three atoms of oxygen, as shown below:

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

Uses of Potassium Dichromate

1. Potassium dichromate is used as an oxidising agent in research facilities and industry.

2. It is used to clean glassware and as an etching material.

3. It is used for chrome tanning in the calfskin industry.

4. It is used in laboratories for volumetric examinations. 

5. It is used in the cowhide tanning industry and to prepare potassium chrome alum. 

6. It is used in photographic screen printing. It is used along with the solid mineral acid as the oxidising agent to harden the gelatin film. 

7. It is the ingredient used in cement and is thus useful in the construction industry.  

8. Potassium dichromate is dissolved in the 35 per cent solution of nitric acid to make a solution called Schwerter’s solution. This solution is used to check the purity of silver by testing the presence of different metals in the silver. 

9. Potassium dichromate is used to darken the woods to produce a deep brown colour and is mainly used as a wood tanner for mahogany wood. 

10. It is non-hygroscopic and therefore used for wet tests in analytical chemistry. 

Conclusion

Thus, we can conclude that potassium dichromate is a hexavalent chromium compound with a structural formula of K2Cr2O7. It shows the reconversion reaction when reacted with alkali and again being acidified due to orange-red Cr2O72- ions in equilibrium with yellow CrO42- ions in the solution of K2Cr2O7. It is a strong oxidising agent and has a wide range of applications in industries and laboratories. It is used to clean glassware and as an etching material, to clean glassware as an etching material, in photographic screen printing, and much more, as we have seen above.