Dichrometry: Principle, Reactions, Procedure and Applications
Introduction
Dichrometry is a type of volumetric redox titration in which potassium dichromate (K₂Cr₂O₇) is used as an oxidizing agent for the quantitative estimation of reducing substances.
The titration is generally performed in an acidic medium. During the reaction, dichromate ions are reduced to chromium(III) ions, while the reducing substance undergoes oxidation.
Dichrometry is commonly used for estimating ferrous salts and other suitable reducing substances in chemical and pharmaceutical analysis.
1. Principle of Dichrometry
The principle of dichrometry is based on the oxidation of a reducing substance by potassium dichromate in an acidic medium.
In this reaction, dichromate ions accept six electrons and are reduced to chromium(III) ions.
Reduction half-reaction:
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Example: Oxidation of ferrous ions
Ferrous ions lose one electron and are converted into ferric ions.
Fe²⁺ → Fe³⁺ + e⁻
The overall ionic reaction is:
Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺
Thus, one mole of potassium dichromate reacts with six moles of ferrous ions in an acidic medium.
[IMG 1 — Horizontal educational infographic illustrating dichrometry: orange dichromate ions accept six electrons and become green chromium(III) ions, while six ferrous ions are oxidized to ferric ions.]
2. Reagents Used in Dichrometry
1. Potassium Dichromate (K₂Cr₂O₇)
It acts as the oxidizing agent and is used as the standard titrant. It is available in a highly pure form and can be used as a primary standard.
2. Dilute Sulphuric Acid (H₂SO₄)
It provides the acidic medium required for the reduction of dichromate ions to chromium(III) ions.
3. Phosphoric Acid (H₃PO₄)
It may be added in certain iron estimations to complex ferric ions and improve endpoint detection.
4. Redox Indicator
Potassium dichromate is not a self-indicator, so a suitable indicator is generally required.
Common indicators include:
Diphenylamine.
Sodium diphenylamine sulphonate.
N-Phenylanthranilic acid.
5. Distilled Water
Used for preparing and diluting solutions.
3. Titration Procedure
The general procedure for dichrometric estimation of ferrous ions is:
Accurately weigh or measure the prescribed quantity of the sample.
Dissolve the sample in distilled water or a suitable solvent.
Add the required quantity of dilute sulphuric acid to provide an acidic medium.
Add phosphoric acid if required by the analytical procedure.
Add a few drops of a suitable redox indicator.
Fill the burette with standard potassium dichromate solution.
Titrate the sample gradually with continuous stirring.
Observe the indicator colour change near the endpoint.
Record the volume of potassium dichromate consumed.
Calculate the amount of reducing substance using the balanced chemical reaction.
[IMG 2 — Horizontal laboratory infographic showing dichrometry setup: burette containing standard potassium dichromate solution, conical flask containing an acidic ferrous salt sample and redox indicator, followed by endpoint colour change.]
4. Endpoint Detection
Since potassium dichromate is not a self-indicator, a suitable redox indicator is used to identify the endpoint.
Diphenylamine Indicator:
Diphenylamine is commonly used in dichrometric titrations. It undergoes oxidation near the endpoint and produces a colour change.
Observation:
Before the endpoint: The reducing substance consumes the added dichromate.
At the endpoint: A slight excess of dichromate oxidizes the indicator, producing a violet or blue-violet colour.
Important Note: Phosphoric acid may be used in certain iron estimations to reduce interference from the intense colour of ferric ions and produce a sharper endpoint.
5. Applications of Dichrometry
Dichrometry is used for the quantitative estimation of different reducing substances.
1. Estimation of Ferrous Salts
Potassium dichromate oxidizes ferrous ions (Fe²⁺) into ferric ions (Fe³⁺). This reaction is widely used for estimating suitable ferrous salts.
2. Determination of Iron
It is used to determine iron content in suitable ores, alloys, chemical samples and pharmaceutical materials.
3. Estimation of Reducing Agents
Dichrometry can be used to analyse substances that undergo quantitative oxidation by potassium dichromate.
4. Pharmaceutical Analysis
It is useful in the assay of suitable pharmaceutical substances containing reducing components.
5. Chemical Quality Control
It supports quantitative testing of chemical raw materials and other suitable samples.
[IMG 3 — Horizontal educational infographic showing applications of dichrometry: ferrous salt estimation, iron determination, analysis of reducing agents and pharmaceutical quality control.]
6. Advantages of Dichrometry
Primary Standard: Potassium dichromate is available in high purity and can be used to prepare a standard solution directly.
Stable Solution: Its standard solutions are relatively stable during storage.
Definite Reaction: It undergoes a well-defined six-electron reduction in acidic medium.
Accurate Estimation: It provides reliable quantitative results when suitable conditions are maintained.
Wide Applications: Useful for estimating ferrous salts and other reducing substances.
7. Limitations and Precautions
Maintain the prescribed acidic medium using sulphuric acid.
Do not use hydrochloric acid unless specifically permitted by the analytical method, as chloride ions may undergo oxidation under certain conditions.
Use a suitable redox indicator.
Add potassium dichromate carefully near the endpoint.
Ensure complete dissolution of the sample.
Avoid interference from other oxidizing or reducing substances.
Potassium dichromate contains hexavalent chromium, which is toxic and carcinogenic. Handle it with appropriate protective equipment and dispose of waste according to laboratory safety procedures.
8. Difference Between Dichrometry and Permanganometry
Dichrometry | Permanganometry |
|---|---|
Uses potassium dichromate (K₂Cr₂O₇). | Uses potassium permanganate (KMnO₄). |
Dichromate is reduced to Cr³⁺. | Permanganate is generally reduced to Mn²⁺ in acidic medium. |
Not a self-indicator. | Acts as a self-indicator. |
Requires a suitable redox indicator in ordinary visual titrations. | Usually does not require an additional indicator. |
Potassium dichromate can be used as a primary standard. | Potassium permanganate is generally standardized before use. |
Commonly used for ferrous salt estimation. | Used for ferrous salts, oxalic acid and other reducing substances. |
Key Takeaways
Dichrometry is a redox titration method using potassium dichromate as an oxidizing agent.
It is generally performed in an acidic medium.
Dichromate ions are reduced to chromium(III) ions.
One mole of dichromate reacts with six moles of ferrous ions.
Diphenylamine and sodium diphenylamine sulphonate are commonly used indicators.
Potassium dichromate can be used as a primary standard.
Dichrometry is useful for estimating ferrous salts, iron and other suitable reducing substances.
Potassium dichromate requires careful handling because of its toxicity.