Dichromate Calculators

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Dichromate (Cr₂O₇²⁻) is a strong oxidizing agent, most commonly encountered as potassium dichromate (K₂Cr₂O₇), a bright orange crystalline solid. In acidic solution, dichromate is reduced from Cr(VI) to Cr(III) (green), accepting 6 electrons per Cr₂O₇²⁻ ion. It is used as a primary standard in redox titrimetry, as an oxidizing agent in organic synthesis, and as the basis of the chemical oxygen demand (COD) test for water pollution analysis. Dichromate compounds are highly toxic, carcinogenic, and must be handled with appropriate safety precautions and proper waste disposal.

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Dichromate Half-Reaction

In acidic solution: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

E° = +1.33 V (standard reduction potential). Dichromate is a strong oxidizer in acid — it oxidizes Fe²⁺ to Fe³⁺, reduces organic compounds, and reacts with alcohols (breathalyzer reaction: ethanol → acetic acid). In neutral or alkaline solution, its oxidizing power decreases significantly.

Potassium Dichromate as a Primary Standard

K₂Cr₂O₇ (MW = 294.18 g/mol) is an excellent primary standard for redox titrimetry: it is stable, non-hygroscopic, high purity, and has a high equivalent weight reducing dilution errors. Equivalent weight = MW / 6 = 49.03 g/equivalent (since 6 electrons are transferred per formula unit in acid). It is used to standardize ferrous ammonium sulfate (FAS) solutions for COD analysis.

COD Analysis by Dichromate Method

Chemical oxygen demand (COD) measures the oxygen equivalent of organic matter that can be oxidized in a water sample. In the dichromate COD method: a water sample is digested with excess K₂Cr₂O₇ in concentrated H₂SO₄ at 150°C for 2 hours. Unreacted dichromate is back-titrated with FAS or measured spectrophotometrically at 600 nm. COD is reported in mg O₂/L.

Safety and Disposal

Cr(VI) compounds including dichromate are classified as IARC Group 1 carcinogens (lung cancer, nasal cancer) and are highly toxic by ingestion and inhalation. They are also strong skin and eye irritants. All dichromate waste must be treated to reduce Cr(VI) to Cr(III) (e.g., using sodium bisulfite in acid) before disposal, as Cr(VI) is regulated under the Clean Water Act and RCRA hazardous waste regulations.

Glossary

Dichromate (Cr₂O₇²⁻)
A strong Cr(VI) oxidizing anion; in acid solution accepts 6 electrons per ion (Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O); used in COD analysis and redox titrimetry.
Chemical Oxygen Demand (COD)
The oxygen equivalent of organic matter oxidizable by dichromate in acidic conditions; reported in mg O₂/L; used to assess organic pollution in wastewater.
Primary Standard
A highly pure, stable, non-hygroscopic substance used to prepare or standardize solutions of known concentration; K₂Cr₂O₇ is a primary standard for redox titrimetry.

Frequently Asked Questions

Potassium dichromate (K₂Cr₂O₇) is used as a primary standard for redox titrations — its high purity, stability, and high equivalent weight make it ideal for standardizing reducing agent solutions. It is the basis of the COD (chemical oxygen demand) test for water quality. Historically, dichromate was used in chromate cleaning solutions for glassware, but this practice has been largely replaced due to Cr(VI) toxicity. It is also used in organic chemistry as an oxidant for primary alcohols to aldehydes or carboxylic acids.

In acidic solution: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. Each dichromate ion accepts 6 electrons, converting orange Cr(VI) to green Cr(III). The standard reduction potential E° = +1.33 V, making dichromate a strong oxidizing agent. The color change from orange to green is visible and is used analytically — the endpoint in some dichromate titrations is detected by the disappearance of orange color or by an indicator.

Chemical oxygen demand (COD) is measured by digesting a water sample with excess K₂Cr₂O₇ in concentrated H₂SO₄ at 150°C for 2 hours. Organic matter reduces Cr(VI) to Cr(III). The remaining unreacted dichromate is either back-titrated with ferrous ammonium sulfate (FAS) or measured spectrophotometrically at 600 nm. COD = mg O₂/L equivalent to the organic oxidation. Results are compared to regulatory discharge limits typically set in mg/L COD.

Chromium(VI) compounds including dichromate are classified as IARC Group 1 human carcinogens, causing lung and nasal cancer through inhalation. They are acutely toxic by ingestion, highly irritating to skin and eyes, and environmentally hazardous to aquatic organisms. Cr(VI) is regulated as a priority pollutant under the Clean Water Act and as a RCRA hazardous waste. Laboratory waste containing dichromate must be reduced to Cr(III) before disposal — typically with sodium bisulfite or ascorbic acid in acidic solution.