Potassium Dichromate
Properties
| State | Solid at room temperature |
| Color | Bright orange-red crystalline solid |
| Solubility | Soluble in water (125 g/L at 20 °C) |
| Melting Point | 398 °C |
| Boiling Point | 500 °C (decomposes) |
About Potassium Dichromate
Potassium dichromate (K2Cr2O7, 294.185 g/mol) is the bright orange-red Cr(VI) salt whose two chromium atoms sit in corner-sharing tetrahedra bridged by a single oxygen — the dichromate ion (Cr2O7)^2-. Three properties have made it a workhorse of classical analytical chemistry: it can be obtained in primary-standard purity by recrystallization from water, it is non-hygroscopic so it can be weighed accurately straight from the bottle, and its half-reaction (Cr2O7^2- + 14 H+ + 6 e- -> 2 Cr^3+ + 7 H2O, E0 = 1.33 V in 1 M acid) gives a strong, well-defined oxidation in acidic solution with a sharp orange-to-green color change at the endpoint. The dichromatometric titration of Fe(II) in HCl/H2SO4 — used to assay iron ore, steel, and ferrous pharmaceutical raw materials — is the classical iron determination, and ASTM E322 is essentially this reaction. Outside titrimetry, the chromic-acid cleaning solution that generations of glassware washers used to remove the worst organic films from chromatography columns and reaction flasks was K2Cr2O7 dissolved in concentrated H2SO4. Photographic dichromate processes (carbon printing, gum bichromate, dichromated gelatin holography) all exploit the photoreduction of Cr(VI) to Cr(III) hardening a colloid layer. In Roadside Breathalyzer Test 1.0 (1954-1980s), an acidified K2Cr2O7-on-silica-gel cartridge oxidized exhaled ethanol to acetaldehyde and turned green in proportion to BAC. Hexavalent chromium's IARC Group 1 carcinogen designation has pushed nearly every modern application — including the breathalyzer — toward fuel-cell or IR-absorption replacements.
Where you'll encounter it
If you trained in analytical chemistry before about 1995, you weighed K2Cr2O7 to four decimal places, dissolved it in volumetric water to make a primary standard solution, and titrated it against unknown Fe(II) in HCl using diphenylamine sulfonate as the redox indicator — watch for the moment the green Cr(III) product just barely shifts to violet at the endpoint. If you worked in any organic lab through the 1980s, the brown 4 L bottle of chromic acid solution next to the sink (K2Cr2O7 dissolved in concentrated H2SO4) was what you reached for when a column had crud baked onto it that simple base bath would not touch — modern labs replaced it with NoChromix or Piranha because Cr(VI) waste disposal got expensive and the cancer hazard got hard to ignore. If you got pulled over for a roadside alcohol test in 1975 and the officer used a glass tube with orange crystals at one end, you were watching the dichromate breathalyzer in action — the orange-to-green band length was matched to a printed BAC scale on the side of the tube.
Common Uses
- Primary standard for redox titrations including iron determination by ASTM E322
- Chromic acid cleaning solution (K2Cr2O7 in concentrated H2SO4) for laboratory glassware (largely phased out)
- Dichromated gelatin and gum bichromate processes for carbon printing and holographic optics
- Roadside breathalyzer (1954-1980s) via ethanol oxidation to acetaldehyde and Cr(VI) reduction to Cr(III)
- Tanning agent in chrome leather production via reduction to basic chromium(III) sulfate in the bath
- Wood preservative formulation component in CCA-treated lumber (banned for residential use in U.S. since 2003)
- Pyrotechnic oxidizer in some specialty compositions
- Mordant in textile dyeing and printing (largely replaced by safer alternatives)
Safety Information
IARC Group 1 carcinogen (hexavalent chromium); confirmed human lung carcinogen by inhalation. OSHA PEL for total Cr(VI) compounds: 5 ug/m3 8-hour TWA, action level 2.5 ug/m3 (29 CFR 1910.1026) — one of the lowest workplace limits for any inorganic chemical. Strong oxidizer that can ignite organics on contact, particularly when mixed with concentrated H2SO4 (chromic acid). Severe skin sensitizer; nasal septum perforation and chrome ulcers are classic occupational signs. GHS H350 (carcinogen Cat 1A), H340 (mutagen Cat 1B), H360FD (reproductive toxin Cat 1B), H317/H334 (skin and respiratory sensitization), H272 (oxidizer), H410 (very toxic to aquatic life). Handle in fume hood with full PPE; dedicated Cr(VI) waste stream required, no aqueous disposal.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.