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Potassium Dichromate - Molecular Formula, Uses, Health Effect, Chemical and Physical Properties

 

When you entered your chemistry lab, you may have seen beakers with an orange liquid solution. It looks so beautiful, teachers ask you to do titrations using this orange solution. 

You may have seen an orange strain on the apron which is used by researchers while performing experiments. Do you know what is the name of that compound? 

Or what is the major element involved in this one?

Or how versatile is this compound?
We will study more about this coloured compound and see how useful is this in chemistry.

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Table of content:

  • Occurrence 
  • Potassium Dichromate
  • Molecular Formula and Structure of Potassium Dichromate
  • Preparation of Potassium Dichromate
  • Physical properties of Potassium Dichromate
  • Chemical properties of Potassium Dichromate
  • Use of Potassium Dichromate
  • Effects of Potassium Dichromate on health
  • Practice Problems
  • Frequently asked questions-FAQ

Occurrence: 

Potassium dichromate is found in nature as lopezite, an uncommon mineral. It is claimed to have been discovered as vug fills in the nitrate deposits of Chile's Atacama desert and South Africa's Bushveld igneous complex.

Potassium Dichromate

  • Potassium dichromate is the compound of potassium. It is one of the most popular chemical reagents used in the field of inorganic chemistry. 
  • It is a very strong oxidizing agent. Hence, it is widely used in laboratories and industries whenever there is an involvement of oxidation.
  • Potassium dichromate is produced using chromates which are thus obtained by chromite ore reacting with either sodium or potassium carbonate

Molecular Formula and structure of Potassium Dichromate

  • The molecular formula of Potassium Dichromate is K2Cr2O7.while its molecular mass is found out to be 294.185 gmol-1
  • Potassium dichromate is an ionic compound that has two potassium ions (K+) and a negatively charged dichromate ion (Cr2O72-). 
  • In dichromate ion, two chromium ions are bonded to three oxygen atoms each in a tetrahedral manner in which one oxygen atom acts as a bridging element. Both the chromium atoms have an oxidation state of +6.

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Preparation of Potassium Dichromate

  • Dichromate is often made from chromate, which is made by fusing chromite ore (FeCr2O4) with sodium or potassium carbonate in the presence of oxygen. The following is the reaction with sodium carbonate:
    4FeCr2O4(s)+8Na2CO3(s)+7O2(g)8Na2CrO4(s)+2Fe2O3(s)+8CO2(g)
  • The orange sodium dichromate,Na2CrO7.2H2O, can be crystallised from a yellow sodium chromate solution that has been filtered and acidified with sulphuric acid.
    2Na2CrO4(s)+2H+(aq)Na2Cr2O7(s)+2Na+(aq)+H2O (l)
    Potassium dichromate is less soluble than sodium dichromate. As a result, Potassium dichromate is made by treating sodium dichromate solution with potassium chloride.
    Na2Cr2O7(s)+2KCl (s)K2Cr2O7(s)+2NaCl (s)

Physical Properties of Potassium Dichromate

  • Potassium dichromate exists as a solid at room temperature and has an orange-red crystal.
  • Potassium dichromate is odourless in nature.
  • Potassium dichromate is highly corrosive and non-combustible in nature.
  • The melting point of potassium dichromate is 398°C, and its boiling point is 500°C.

Chemical properties of Potassium Dichromate

1. The action of heat on potassium dichromate:

The action of heat on potassium dichromate: Upon heating potassium dichromate, chromate is obtained along with oxygen gas. The reaction is as follows: 

4K2Cr2O7(s) →4K2CrO4(s) + 2Cr2O3(s) + 3O2(g) ↑

2. The action of potassium dichromate on alkalis: 

On reacting with alkalis, potassium dichromate loses its orange-red colour and turns yellow. The reaction is as follows:
K2Cr2O7(s)+2KOH(aq)2K2CrO4(s) (yellow)+H2O(l)

3. Oxidizing properties of potassium dichromate: 

When reacted with dilute sulphuric acid, one mole of potassium dichromate produces three moles of nascent oxygen gas is released. The reaction is as follows:
K2Cr2O7(s)+4H2SO4(aq)4K2SO4(s)+Cr2(SO4)3(s)+4H2O(l)+3[O] (g)↑

When potassium dichromate is reacted with KI, iodine gas is liberated. The reaction is as follows:K2Cr2O7(s)+4H2SO4(aq)4K2SO4(s)+Cr2(SO4)3(s)+7H2O(l)+3[O] (g)↑

Uses of potassium dichromate

The following are some of the most common industrial applications for potassium dichromate.

  • Potassium dichromate is primarily utilised in the manufacture of potassium chrome alum and in the factory of the leather industry.
  • Potassium dichromate can be used as a starting material for the production of chromic acid, which is used as an etchant for glass and for cleaning glassware. Because of its harmful nature, its use is restricted nowadays.
  • It's also utilised in the construction industry as a component of cement, increasing the hardening time and improving the density and colour of concrete.
  • Long-term exposure to potassium dichromate causes skin problems in construction workers.
  • It is utilised as an oxidising agent in the photography industry.

Effect of potassium dichromate on health

  • Potassium dichromate can cause ongoing infections like chromium dermatitis in the hands and lower arms.
  • The harmfulness of potassium dichromate can hurt creatures like bunnies, rodents, and so forth.
  • It is harmful to amphibian creatures as well and represents a major danger to the climate.
  • Potassium dichromate is destructive, and its openness might foster a few eye issues or even visual deficiency.
  • It might cause heritable hereditary harm, and weakened ripeness and is hazardous to unborn youngsters.

Practice problems

Q1.  The coefficient of the proton in the reduction reaction of dichromate in an acidic medium is ____ 

a. 7
b. 10
c.  12
d. 14

Answer: D
In an acidic environment, the following is a representation of its reduction reaction of dichromate ion:
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Q2. The reddish-brown gas evolved in the chromyl chloride test is ______

a. CrO2Cl2
b. CrO3
c. CrOCl4
d. CrSO4

Answer: A
The chromyl chloride test is a qualitative analytical test for Cl- ion conformation.Potassium dichromate reacts with concentrated sulphuric acid to give reddish-brown vapours of chromyl chloride. This reaction is also called Chromyl Chloride Test. The reaction is as follows:

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Q3. How do we use Potassium dichromate in drink and drive tests?
The colour of the orange dichromate-coated crystals changes from Cr(VI) orange to Cr(III)green when alcohol vapour comes into contact with them. The amount of alcohol in the suspect's breath is precisely proportional to the colour shift.

Q4. The oxidation number of chromium (Cr) in chromium peroxide is _______

a.  +2
b. +6
c. +4
d. +5 

Answer: B
Chromium peroxide can be prepared by treating it with H2O2 in an acidic medium. The reaction is
explained as follows:

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The oxidation state of chromium in Chromium peroxide is +6

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Frequently asked questions-FAQ

Q1. Is potassium dichromate safe to eat?
Answer: Consumption of even a small amount of potassium dichromate can be lethal. A dose of 40 g or more has been known to cause rapid mortality and severe adverse effects in humans.

Q2. Is potassium dichromate corrosive in nature?
Answer: The substance is highly corrosive in nature, and prolonged exposure can result in serious eye damage or blindness.

Q3. Why is K2Cr2O7 referred to as the primary standard solution?
Answer: Because it is exceedingly pure, stable, and does not fluctuate its concentration with environmental conditions, potassium dichromate solution is used as a primary standard solution

Q4. Why is K2Cr2O7 ineffective as a self-indicator?
Answer: Because potassium dichromate can only work as an oxidising agent in an acidic media, it cannot be employed as a self-indicator because its reduction product blocks the visual detection at the end of the reaction.

Related topics

Iron

Potassium Permanganate

f block elements

Water

Hydrogen

Transition Metals

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