Potassium Chromate
Properties
| State | Solid at room temperature |
| Color | Bright lemon-yellow |
| Solubility | Soluble in water (629 g/L at 20 °C) |
| Melting Point | 968 °C |
| Boiling Point | 1000 °C (decomposes) |
About Potassium Chromate
Potassium chromate (K2CrO4, 194.19 g/mol) is the lemon-yellow Cr(VI) salt that almost every introductory chemistry student meets in two contexts: the chromate-dichromate equilibrium demo, and the Mohr titration. Add a drop of HCl to a yellow K2CrO4 solution and watch it shift to orange dichromate (2 CrO4^2- + 2 H+ <-> Cr2O7^2- + H2O); reverse it with NaOH and the yellow comes back. The demo is one of the cleanest classroom illustrations of Le Chatelier's principle because the color change is unmistakable and the equilibrium constant (about 10^14) means small acid additions move it dramatically. The Mohr method (Karl Mohr, 1856) uses K2CrO4 as the indicator for argentometric chloride titration: as AgNO3 is added to a chloride solution, AgCl (Ksp ~10^-10) precipitates first as a white curd; once chloride is exhausted, the next drop of Ag+ hits the chromate indicator to form the brick-red Ag2CrO4 (Ksp ~10^-12) and you stop the titration. The method is restricted to roughly pH 6.5-9 — too acidic and chromate protonates to dichromate, too basic and silver hydroxide precipitates. Outside the teaching lab, K2CrO4 has historically gone to textile dye mordanting, leather tanning, corrosion-inhibiting primers, and as an analytical oxidant. Hexavalent chromium's IARC Group 1 carcinogen designation has driven nearly every consumer-facing application toward Cr(III) replacements over the past two decades.
Where you'll encounter it
If you've taken general chemistry, you've watched a TA add HCl to a beaker of yellow K2CrO4 and seen it flash orange in real time — it's one of the stock demos for Le Chatelier's principle because the color change is so visual and the equilibrium responds within a second. If you've ever run a Mohr titration in an analytical chem lab, the indicator endpoint is unmissable: a clear pale-yellow solution, drops of silver nitrate going in, white AgCl building up at the bottom, and then the moment a single drop hits unconsumed CrO4^2- and the swirl of brick-red Ag2CrO4 appears that tells you you've titrated the last chloride. Industrially, hexavalent chromium is what *Erin Brockovich* was about — the Cr(VI) groundwater contamination in Hinkley, California that drove Pacific Gas and Electric to a $333 million settlement in 1996 came from chromate cooling-tower additives, not K2CrO4 specifically, but it is the same Cr(VI) chemistry and the same toxicology that has driven modern phase-out of chromate corrosion inhibitors in aerospace and automotive coatings.
Common Uses
- Mohr method indicator for argentometric chloride titration at pH 6.5-9 (endpoint = brick-red Ag2CrO4)
- Le Chatelier principle classroom demo via chromate-dichromate pH-driven equilibrium
- Mordant for chrome-dyeing wool with acid dyes (largely phased out under REACH)
- Corrosion-inhibiting pigment in primers and conversion coatings (legacy aerospace/automotive use)
- Analytical oxidant for primary-alcohol-to-aldehyde and primary-amine-to-nitroso reactions
- Selective oxidant in classical wood and textile bleaching processes (largely historical)
- Reference Cr(VI) species in environmental analytical chemistry (EPA Method 7196A)
- Gravimetric reagent for determining lead as PbCrO4
Safety Information
IARC Group 1 carcinogen (hexavalent chromium); confirmed human lung carcinogen via inhalation. OSHA PEL for total Cr(VI) compounds: 5 ug/m3 8-hour TWA (29 CFR 1910.1026), with action level 2.5 ug/m3 — among the lowest workplace limits set for any inorganic chemical. Causes severe skin sensitization (contact dermatitis), nasal septum ulceration, kidney and liver damage on chronic exposure. GHS H350 (carcinogen Cat 1A), H340 (mutagen Cat 1B), H361 (reproductive toxin Cat 2), H317 (skin sensitizer), H410 (very toxic to aquatic life). Handle in fume hood with nitrile gloves, full sleeve coverage, and dedicated chromate waste stream — 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.