Potassium Dichromate: Definition, Structure and Uses

Potassium Dichromate is a less potent oxidiser than potassium permanganate. It is a primary standard, and its solutions have long-term stability in acid and are resistant to light, most organic matter, and the chloride ion. It is always used in acid solutions. Potassium dichromate is the dipotassium salt of dichromic acid and is a potassium salt. It functions as an oxidizer, an allergen, and a sensitizer. Potassium bichromate crystals are orange-red. Water is soluble and denser than water.

One of the most extensively used chemical reagents in inorganic chemistry is potassium dichromate. It is largely used in laboratories and industries as a strong oxidising agent in a variety of chemical reactions. Potassium dichromate is a common chemical that is utilised in oxidation processes. It’s also one of the most significant chromium compounds.

This article will provide details about the definition, structure, and uses of Potassium Dichromate. Continue reading to know more.

What is Potassium Dichromate?

Potassium dichromate is one of the most important compounds of chromium, with the chemical formula \({{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}{\rm{.}}\) It is a crystalline ionic solid with a bright red-orange colour.

Potassium dichromate is a strong oxidising agent and is utilised in laboratories and industries for the purpose of oxidisation. It has an acute and long-term harmful influence on health, just like all hexavalent chromium compounds, and should be handled with caution.

Structure of Potassium Dichromate

It is an ionic molecule made up of two potassium ions \(\left( {{{\rm{K}}^ + }} \right)\) and the negatively charged dichromate ion \(\left( {{\rm{C}}{{\rm{r}}_{\rm{2}}}{\rm{O}}_{\rm{7}}^{{\rm{2 – }}}} \right){\rm{.}}\) The dichromate ion has two hexavalent chromium atoms (oxidation state \({\rm{ + 6}}\)) connected to three oxygen atoms and a bridging oxygen atom. Its molar mass is \({\rm{294}}{\rm{.185}}\,{\rm{g/mol}}{\rm{.}}\)

Examples on Potassium Dichromate Titrations

Preparation of Potassium Dichromate

Preparation of Potassium Dichromate

1. Conversion of Chromite Ore to Sodium Chromate

Soda ash is combined with finely powdered chromite ore. The mixture is subsequently roasted in the presence of extra air in a reverberatory furnace, resulting in the formation of a yellow substance containing sodium chromate.

2. Conversion of Chromate into Dichromate

The sodium chromate solution obtained in the above step is treated with concentrated sulphuric acid resulting in the formation of sodium dichromate. The product obtained contains some byproducts such as sodium sulphate \(\left( {{\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}.\,{{\rm{H}}_2}{\rm{O}}} \right)\) which is less soluble, and hence it crystallises out. The hot filtrate obtained is allowed to cool, resulting in the separation of sodium dichromate \(\left( {{\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7}.\,2\,{{\rm{H}}_2}{\rm{O}}} \right)\) crystals.

\(2\,{\rm{N}}{{\rm{a}}_2}{\rm{Cr}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}}\)

3. Conversion of Sodium Dichromate into Potassium Dichromate

Potassium dichromate can be made by adding potassium chloride to a sodium dichromate solution. A heated sodium dichromate solution is mixed with a specified amount of potassium chloride and allowed to cool until orange potassium dichromate crystals form.

\({\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_7} + 2\,{\rm{KCl}} \to {{\rm{K}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + 2\,{\rm{NaCl}}\)

Properties of Potassium Dichromate

The physical and chemical properties of Potassium Dichromate are given below:

Physical Properties of Potassium Dichromate

1. It is a red-orange crystalline odourless solid.
2. It is soluble in water.
3. The melting point of potassium dichromate is \({398^{\rm{o}}}{\rm{C}},\) and the boiling point is \({500^{\rm{o}}}{\rm{C}}{\rm{.}}\)
4. The density of potassium dichromate is \(2.676\,{\rm{g}}/{\rm{c}}{{\rm{m}}^3}.\)

Chemical Properties of Potassium Dichromate

1. Action of heat: Potassium dichromate decomposes when heated strongly and forms potassium chromate with the release of oxygen gas.

2. Action with alkalies: When potassium dichromate combines with alkalies like potassium hydroxide, it produces yellow potassium chromate, which is then acidified with sulphuric acid to produce potassium dichromate.

3. Action with acids:
a. When potassium dichromate reacts with cold and concentrated sulphuric acid, it gives red crystals of chromium trioxide, also known as chromic anhydride.
\({{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + 2{\mkern 1mu} {{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to \mathop {2{\mkern 1mu} {\rm{Cr}}{{\rm{O}}_{\rm{3}}}}\limits_{{\rm{Chromic}}{\kern 1pt} {\rm{an}}\,{\rm{hydride}}}  + 2{\mkern 1mu} {\rm{KHS}}{{\rm{O}}_{\rm{4}}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)
b. When the mixture of potassium dichromate and sulphuric acid is heated, oxygen gas is evolved.
\({\rm{2}}\,{{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + 8\,{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to 2\,{{\rm{K}}_2}{\rm{S}}{{\rm{O}}_{\rm{4}}} + 2\,{\rm{C}}{{\rm{r}}_{\rm{2}}}{\left( {{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)_{\rm{3}}}{\rm{ + 8}}\,{{\rm{H}}_{\rm{2}}}{\rm{O + 3}}\,{{\rm{O}}_2}\)
c. Potassium dichromate reacts with hydrochloric acid forming chromic chloride with the evolution of chlorine.
\({{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}} + 14\,{\rm{HCl}} \to 2\,{\rm{CrC}}{{\rm{l}}_{\rm{3}}} + 2\,{\rm{KCl}} + 7\,{{\rm{H}}_{\rm{2}}}{\rm{O}} + 3\,{\rm{C}}{{\rm{l}}_{\rm{2}}}\)
4. Oxidising Character: Potassium dichromate acts as a powerful oxidising agent in an acidic medium. When it is treated with dilute sulphuric acid, nascent oxygen is liberated as per the following equation.
\({{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}} + 4\,{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to {{\rm{K}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} + {\rm{C}}{{\rm{r}}_{\rm{2}}}{\left( {{\rm{S}}{{\rm{O}}_{\rm{4}}}} \right)_{\rm{3}}} + 4\,{{\rm{H}}_{\rm{2}}}{\rm{O}} + 3\,{\rm{O}}\)
The reaction actually involves the reduction of \({\rm{C}}{{\rm{r}}_{\rm{2}}}{\rm{O}}_{\rm{7}}^{{\rm{2 – }}}\) ions to \({\rm{C}}{{\rm{r}}^{3 + }}\) Ions.
Some of the oxidising properties of potassium dichromate \(\left( {{{\rm{K}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_{\rm{7}}}} \right)\) are:

a. It liberates iodine with potassium iodide.

b. It oxidises ferrous salts to ferric salts.

5. Chromyl Chloride Test: Orange-red vapours of volatile oily chromyl chloride are produced when solid potassium dichromate is heated with concentrated sulphuric acid and a soluble metal chloride such as sodium chloride, potassium chloride, etc. When chromyl chloride vapours are passed through water, a yellow solution of chromic acid is formed. Treating chromic acid solution with lead acetate and acetic acid gives a yellow precipitate of lead chromate. This test is known as the chromyl chloride test.

The chemical reactions involved are as follows:

6. Precipitation of insoluble chromates: When potassium dichromate is treated with the aqueous solution of soluble salts of lead, barium, etc., it forms its insoluble chromates.

For Example:

Structure of Chromate \(\left( {{\rm{CrO}}_4^{2 – }} \right)\) and Dichromate Ions \(\left( {{\rm{CrO}}_7^{2 – }} \right)\)

The structure of the chromate ion is tetrahedral, with four oxygen atoms arranged tetrahedrally around the central chromium atom. The dichromate ion is made up of two tetrahedra that share one corner and have a \({\rm{Cr}} – {\rm{O}} – {\rm{Cr}}\) bond angle of \({126^{\rm{o}}}.\) As a result, it has a \({\rm{Cr}} – {\rm{O}} – {\rm{Cr}}\) linkage.

Acidified Potassium Dichromate

Acidified potassium dichromate oxidises various types of alcohol. When acidified potassium dichromate is added to alcohol, the solution turns orange at first and then changes to green as the alcohol oxidises. Only alcohols change colour from orange to green when exposed to acidified potassium dichromate solution, which can be used as an alcohol test.

\({\rm{C}}{{\rm{r}}_{\rm{2}}}{\rm{O}}_7^{2 – } + 14\,{{\rm{H}}^ + } + 6\,{{\rm{e}}^ – } \to 2\,{\rm{C}}{{\rm{r}}^{3 + }} + 7\,{{\rm{H}}_{\rm{2}}}{\rm{O}}\)

Uses of Potassium Dichromate

The uses of potassium dichromate are as follows:

1. Cleaning: The chromic acid prepared from potassium dichromate is used for cleaning glassware and etching materials. But this practice has been discontinued mainly due to safety concerns about hexavalent chromium.
2. Construction: It is a component of cement that slows the setting of the mixture and increases the density and texture of the finished product.
3. It is used in the estimation of \({\rm{F}}{{\rm{e}}^{2 + }}\) and \({{\rm{I}}^ – }\) in volumetric analysis.
4. Analytical Reagent: Potassium dichromate is a common reagent in traditional “wet tests” in analytical chemistry because it is non-hygroscopic.
5. It is used in chrome tanning in the leather industry.
6. A black titration with acidified potassium dichromate is used to determine the concentration of ethanol in a sample. The ethanol in a sample is oxidised to acetic acid when it reacts with an excess of acidified potassium dichromate.
   (a) \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{OH}} + 2\left[ {\rm{O}} \right] \to {\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{COOH}} + {{\rm{H}}_{\rm{2}}}{\rm{O}}\)
7. It is used in the preparation of chrome alum and several other industrially important chromium compounds.
8. The mixture of potassium dichromate with approximately \(35\% \) nitric acid solution is known as Schwerter’s Solution. This solution is used to test the purity of the silver metal.
9. Potassium dichromate is used to stain certain types of wood by darkening the tannins in the wood. It produces deep, rich brown that are impossible to achieve with modern colour dyes.

Health Hazards of Potassium Dichromate

Potassium dichromate is a carcinogenic and highly poisonous hexavalent chromium chemical. It is also extremely corrosive and can cause severe irritation and burning on the skin and eyes. In some cases, it can also cause blindness. It is also known to have an effect on reproductive health and to be mutagenic.

Summary

Potassium dichromate is a less potent oxidiser than potassium permanganate and non-hygroscopic. We understood the various properties of potassium dichromate and its utilisation in the industrial field.

Frequently Asked Questions

We have provided some frequently asked questions on potassium dichromate here:

Q.1. Is potassium permanganate harmful to humans?
Ans: The upper gastrointestinal system may be damaged if potassium permanganate is consumed. It can also produce systemic side effects include adult respiratory distress syndrome, coagulopathy, hepatic-renal failure, pancreatitis, and even death in extreme situations.

Q.2. Is potassium dichromate toxic?
Ans: Yes, It has an acute and long-term harmful influence on health, just like all hexavalent chromium compounds; hence it is always handled with caution.

Q.3. Why do you acidify potassium dichromate?
Ans: Potassium dichromate is an oxidising agent used in the organic industry. For dichromate to act as an oxidising agent, it has to be reduced. An acidic medium is required to provide hydrogen ions/ protons for making stable products. Hence, potassium dichromate is acidified.

Q.4. How do you get potassium dichromate?
Ans: Preparation of Potassium dichromate involves the following steps;
1. Conversion of chromite ore to sodium chromate: Soda ash is combined with finely powdered chromite ore. The mixture is subsequently roasted in the presence of extra air in a reverberatory furnace, resulting in the formation of a yellow substance containing sodium chromate.

2. Conversion of chromate into dichromate: The sodium chromate solution obtained in the above step is treated with concentrated sulphuric acid resulting in the formation of sodium dichromate. The product obtained contains some byproducts such as sodium sulphate \(\left( {{\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}.\,{{\rm{H}}_2}{\rm{O}}} \right)\) which is less soluble, and hence it crystallises out. The hot filtrate obtained is allowed to cool, resulting in the separation of sodium dichromate \(\left( {{\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7}.\,2\,{{\rm{H}}_2}{\rm{O}}} \right)\) crystals. \(2\,{\rm{N}}{{\rm{a}}_2}{\rm{Cr}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4} + {{\rm{H}}_2}{\rm{O}}\)

3. Conversion of sodium dichromate into potassium dichromate: Potassium dichromate can be made by adding potassium chloride to a sodium dichromate solution. A heated sodium dichromate solution is mixed with a specified amount of potassium chloride and allowed to cool until orange potassium dichromate crystals form.
\({\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{C}}{{\rm{r}}_{\rm{2}}}{{\rm{O}}_7} + 2\,{\rm{KCl}} \to {{\rm{K}}_2}{\rm{C}}{{\rm{r}}_2}{{\rm{O}}_7} + 2\,{\rm{NaCl}}\)

Q.6. Is potassium dichromate a normal salt?
Ans: A dipotassium salt of dichromic acid is known as Potassium salt. Potassium dichromate is a common inorganic chemical reagent that is most commonly used as an oxidising agent in various laboratory and industrial applications.

Q.5. What is potassium dichromate used for?
Ans: The following are the uses of potassium dichromate:
1. It is used as an oxidising agent.
2. It is used in the estimation of \({\rm{F}}{{\rm{e}}^{2 + }}\) and \({{\rm{I}}^ – }\) in volumetric analysis
3. It is a component of cement that slows the setting of the mixture and increases the density and texture of the finished product.
4. It is used in chrome tanning in the leather industry.
5. It is used in the preparation of chrome alum and several other industrially important chromium compounds.

Preparation & Properties of Potassium Dichromate

We hope this detailed article on Potassium Dichromate is helpful to you. If you have any queries on this page, ping us through the comment box below and we will get back to you as soon as possible.