Chemical Properties and Role as a Primary Standard

Potassium dichromate (K₂Cr₂O₇) is a bright orange-red crystalline solid with a molecular weight of 294.18 g/mol. In acidic aqueous solution, the dichromate ion (Cr₂O₇²⁻) acts as a strong oxidizing agent, accepting six electrons per formula unit and being reduced to chromium(III) (Cr³⁺) according to the half-reaction:

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Because potassium dichromate can be obtained in high purity, is thermally stable, non-hygroscopic, and does not oxidize atmospheric components under normal storage conditions, it qualifies as a primary standard. Solutions prepared by direct weighing and dissolution in water do not require external standardization against another reference material, making preparation straightforward and reproducible. Its equivalent weight is M/6 = 49.03 g/eq, which forms the basis for expressing concentration in the normality convention still used in some pharmacopoeial and industrial methods.

Preparation and Standardization of the Titrant

To prepare a 0.1 mol/L potassium dichromate titrant (expressed as c(1/6 K₂Cr₂O₇) = 0.1 mol/L), dry reagent-grade or primary-standard-grade K₂Cr₂O₇ at 120 °C for at least two hours to remove adsorbed moisture. Allow it to cool in a desiccator, then accurately weigh the required mass. For one litre of 0.1 mol/L solution (equivalent weight basis), dissolve approximately 4.903 g in a small volume of purified water, transfer quantitatively to a 1 L volumetric flask, and dilute to the mark with purified water. Mix thoroughly.

Because K₂Cr₂O₇ fulfils primary-standard criteria, no further standardization is strictly necessary when prepared by direct weighing. However, good laboratory practice recommends verifying the prepared solution against a certified reference material or by coulometric methods if the highest metrological traceability is required. Store the titrant in a tightly sealed amber glass bottle away from light and reducing agents. The solution is stable for at least six months under these conditions.

Key Applications in Analytical Chemistry

Iron determination (Bichromate titration of iron): The most classical application is the titrimetric determination of total iron in ores, alloys, and environmental samples. Iron(II) in acidic solution is titrated directly with K₂Cr₂O₇ using diphenylamine sulfonate or ferroin as a redox indicator. The endpoint is marked by a sharp color change from green (Cr³⁺) to violet, providing excellent precision.

Chemical Oxygen Demand (COD) determination: In water and wastewater analysis, a known excess of K₂Cr₂O₇ is added to the sample under reflux in concentrated sulfuric acid. Residual dichromate is back-titrated with ferrous ammonium sulfate (FAS), and the COD value is calculated from the amount of dichromate consumed. This method is described in standard methods including those published by APHA and ISO.

Organic and pharmaceutical assays: K₂Cr₂O₇ titrant is employed in the assay of certain organic compounds and in quality control testing of pharmaceutical raw materials where an oxidizable functional group is present. It is also used as a reference oxidant in calibrating electrochemical instruments.

Safety, Handling, and Waste Disposal

Hazard classification: Potassium dichromate is classified as a Category 1A carcinogen (hexavalent chromium compounds) by IARC and regulatory bodies in multiple jurisdictions. It is also toxic if ingested or inhaled, corrosive to skin and eyes, and an environmental hazard to aquatic organisms. The substance is subject to SVHC (Substance of Very High Concern) listing under REACH regulation in the European Union.

Personal protective equipment (PPE): Always work in a fume hood when preparing or using the titrant. Wear chemical-resistant gloves (nitrile, minimum 0.15 mm thickness), safety goggles or a face shield, and a laboratory coat. Avoid generating dust when handling the solid form.

Spill management: Contain spills immediately using inert absorbent material. Do not use combustible materials. Collect waste into labeled, sealed containers compliant with local hazardous waste regulations.

Waste disposal: Chromium-containing waste solutions must NOT be poured down the drain. Treat with a reducing agent (e.g., sodium bisulfite) under acidic conditions to convert Cr(VI) to Cr(III), then precipitate as Cr(OH)₃ at neutral pH before disposal through a licensed hazardous waste contractor. Consult your institution's Environmental Health and Safety office for site-specific protocols.