What Is Dichromate? Key Facts, Formula, and Its Importance in Chemistry
According to experts, dichromate can be defined as an anion with a chemical formula of Cr2O2-7. This dichromate formula provides this compound with the power to act as a strong oxidizing agent in organic chemistry. It is also used as a primary standard solution in volumetric analysis.
It is important to note that both the chromate and dichromate ions are interconvertible in aqueous solutions. Further, the most common dichromate compound is known as potassium dichromate.
Potassium dichromate is an orange crystalline solid that readily decomposes to give chromic oxide and potassium chromate. The formula of dichromate is Cr2O2-7. The density of the dichromate structure is 2.68 gram per centimetre cube.
The value of molecular weight over molar mass is 294.185 g/mol. The dichromate formula provides this compound with a boiling point of 500 C and a melting point of 398 C.
Important Structures
When it comes to the topic of dichromate, there are some important structures that students should be familiar with. We have prepared a list of those structures, and that list is mentioned below.
Dichromate Structure
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Structure of Dichromate Ion
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Structure of Chromate and Dichromate Ion
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Properties of Dichromate
Now, you must understand the formula of dichromate and the structure of chromate and dichromate ion. The next step is to learn about the various properties of dichromate.
Since there are so many dichromate properties, we have divided all the properties into the divisions of physical and chemical properties. This will allow readers to understand this topic in a better manner.
Let’s first begin with physical properties.
Dichromate is odourless and has an appearance of red-orange crystalline solid. The structure of dichromate ion has a valency of two and ph level four. In the oxidation state, it holds the value of +6. Dichromate is soluble in cold water and somewhat soluble in hot water.
Let’s move on to the topic of chemical properties of the dichromate ion formula.
It is important for readers to note that the acidified solution of dichromate forms a deep blue colour with peroxide. This is done because of the formation of [CrO(O2)2]. The reaction can be represented by:
Cr2O72- + 4H2O2 + 2H+ ----> 2CrO5 + 5H2O
Dichromate also reacts with hydrogen sulphide. It also oxidizes it to sulphur. It also oxidizes sulphites to sulphates, chlorides to chlorine, nitrites to nitrates, thiosulphates to sulphates, and sulphates and sulphur to stannic salts. This reaction can be represented by:
Cr2O72- + 3H2S + 8H+ -----> 2Cr3+ + 3S + 7H2O
Uses of Dichromate
There are many important uses of dichromate. We have prepared a list of those uses. And that list is mentioned below.
The dichromate ion formula is used in photography to harden the gelatin film
It is also used in chrome tanning in the leather industry
Dichromate is also used in dyeing as Cr(OH)3 acts as a moderent
It is used in the volumetric estimation of ferrous salts, sulphites, and iodine
This compound can also be used in the preparation of other chromium compounds like chrome alum, chrome red, and chrome yellow
Fun Facts about Dichromate
Did you know that approximately 136,000 tonnes or 150,000 tons of hexavalent chromium or sodium dichromate was produced in 1985? Chromates and dichromates are also used in chrome plating. Chrome plating helps in protecting metals from corrosion. It also improves the adhesion of paint.
The dichromate and chromate salts of heavy metals, alkaline earth metals, and lanthanides are also slightly soluble in water. Because of this reason, these salts are used as pigments. Also, the lead-containing pigment chrome yellow was used for various purposes before. Eventually, the use of chrome yellow was stopped due to environmental regulations.
When dichromates are used as oxidizing agents of titrants in redox reactions, both chromates and dichromates convert into trivalent chromium or Cr3+ salts. These salts have a distinctive blue-green colour.
On top of all this, did you know about the natural occurrence and production of dichromate? If not, then continue reading to find out.
The primary chromium ore constitutes the mixed metal oxide chrome or FeCr2O4. It is found as a brittle metallic black crystal or granule. For the production, chromite ore is heated with a mixture of sodium carbonate and calcium carbonate. This is done in the presence of air. The chromium is also oxidized to a hexavalent form. Simultaneously, iron forms iron (III) oxide or Fe2O3. This entire reaction can be represented by:
4 FeCr2O4 + 8Na2CO3 + 7O2 ----> 8 Na2CrO4 + 2 Fe2O3 + 8CO2
Further leaching of the material obtained through this reaction is done at higher temperatures. This dissolves the chromates and leaves behind a residue of insoluble iron oxide. Usually, the chromate solution is still further processed to form a chromium metal. However, a chromate salt can also be obtained directly from this liquor.
It is important for readers to note that chromate containing minerals is very rare. Crocoite or PbCrO4 occurs as very long red crystals. And this can be commonly found in chromate minerals. In the Atacama desert, rare potassium chromate minerals and related compounds can also be found. Amongst those rare compounds is lopezite, which is the only known dichromate mineral.
FAQs on Dichromate: Structure, Properties, and Applications
1. What is the detailed structure of the dichromate ion (Cr₂O₇²⁻) as per the NCERT syllabus?
The dichromate ion, Cr₂O₇²⁻, has a structure where two tetrahedral CrO₄ units are joined by sharing one oxygen atom at a corner. Key structural features include:
Central Atoms: Two Chromium (Cr) atoms, each in a +6 oxidation state.
Bridging Oxygen: A single oxygen atom forms a Cr-O-Cr bridge, connecting the two chromium atoms. The bond angle for this bridge is approximately 126°.
Terminal Oxygens: There are six terminal oxygen atoms in total, with three bonded to each chromium atom. The Cr-O terminal bonds are shorter than the Cr-O bridging bonds.
Overall Geometry: The ion consists of two tetrahedra sharing a common vertex, resulting in a non-linear, symmetrical structure with an overall charge of -2.
2. How is potassium dichromate (K₂Cr₂O₇) commercially prepared from chromite ore?
The commercial preparation of potassium dichromate from chromite ore (FeCr₂O₄) involves three main steps:
Step 1: Conversion to Sodium Chromate: The chromite ore is fused with sodium carbonate (Na₂CO₃) in the presence of excess air. This oxidises chromium to sodium chromate (Na₂CrO₄).
Step 2: Conversion to Sodium Dichromate: The resulting sodium chromate solution is filtered and then acidified with concentrated sulphuric acid. This converts the yellow sodium chromate into orange sodium dichromate (Na₂Cr₂O₇).
Step 3: Conversion to Potassium Dichromate: A hot, concentrated solution of sodium dichromate is treated with potassium chloride (KCl). Since potassium dichromate is less soluble than sodium dichromate, it crystallises out upon cooling.
3. What are the most common industrial and laboratory applications of potassium dichromate?
Potassium dichromate is widely used due to its strong oxidising properties. Its main applications include:
Volumetric Analysis: It is used as a primary standard in redox titrations to estimate the concentration of reducing agents like Fe²⁺ ions and iodides.
Organic Chemistry: It serves as a powerful oxidising agent for the preparation of aldehydes, ketones, and carboxylic acids from alcohols.
Leather Tanning: It is used in the chrome tanning process, which makes leather more resistant to water and microbial action.
Cleaning Glassware: A mixture of potassium dichromate and concentrated sulphuric acid, known as chromic acid, is used to clean laboratory glassware.
4. Why is potassium dichromate a powerful oxidising agent, particularly in an acidic medium?
Potassium dichromate acts as a powerful oxidising agent because the chromium atom is in its highest oxidation state of +6. In this state, chromium has a strong tendency to accept electrons and get reduced to the more stable +3 oxidation state. This tendency is significantly enhanced in an acidic medium, where H⁺ ions are required for the reaction. The half-reaction in an acidic solution is: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. The high standard electrode potential (E° = +1.33 V) for this reaction confirms its strong oxidising nature.
5. How does the equilibrium between chromate (CrO₄²⁻) and dichromate (Cr₂O₇²⁻) ions depend on pH?
Chromate and dichromate ions exist in an equilibrium that is highly sensitive to the pH of the solution.
In alkaline or basic solutions (high pH), the equilibrium shifts towards the formation of the yellow chromate ion (CrO₄²⁻).
In acidic solutions (low pH), chromate ions combine to form the orange dichromate ion (Cr₂O₇²⁻).
This interconversion is a reversible reaction represented by the equation: 2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O. This explains why potassium dichromate solutions turn yellow upon adding a base and why chromate solutions turn orange upon adding an acid.
6. What is the dichromate test, and what chemical change causes the colour to shift when exposed to sulfur dioxide?
The dichromate test is a qualitative analysis method used to detect the presence of sulfur dioxide (SO₂) gas. It involves exposing a filter paper moistened with an acidified potassium dichromate solution to the gas. If SO₂ is present, the paper's colour changes from orange to green. This colour change is due to a redox reaction where the orange dichromate ion (Cr₂O₇²⁻), with chromium in the +6 oxidation state, is reduced by SO₂ to the green chromium(III) ion (Cr³⁺). Simultaneously, sulfur dioxide is oxidised to the sulfate ion (SO₄²⁻).
7. How does the oxidising strength of potassium dichromate (K₂Cr₂O₇) compare to that of potassium permanganate (KMnO₄)?
While both are strong oxidising agents, potassium permanganate (KMnO₄) is generally a stronger oxidising agent than potassium dichromate (K₂Cr₂O₇) in an acidic medium. This is evident from their standard electrode potentials (E° for MnO₄⁻/Mn²⁺ is +1.51 V, while E° for Cr₂O₇²⁻/Cr³⁺ is +1.33 V). A key difference is their use in titrations: K₂Cr₂O₇ is a stable, non-hygroscopic solid that can be used as a primary standard. In contrast, KMnO₄ is a secondary standard because it is difficult to obtain in a pure state and can decompose in the presence of light or organic matter.
