Sodium Chromate
Properties
| State | Solid (crystalline) |
| Color | Bright yellow |
| Solubility | Highly soluble in water (873 g/L at 30 °C) |
| Melting Point | 792 °C |
| Boiling Point | Decomposes above melting point |
About Sodium Chromate
Sodium chromate is the bright yellow Cr(VI) compound that sits at the entry point of the entire chromium chemicals industry. Formula Na2CrO4, molar mass 161.974 g/mol, and an aqueous solution that is unmistakably canary yellow from the tetrahedral chromate anion CrO4^2-. The first step of every commercial chromium process starts here: roast finely ground chromite ore (FeCr2O4) with sodium carbonate at 1,100 °C in air, and the trivalent chromium oxidizes to hexavalent and leaches out as Na2CrO4. Acidify and you shift the chromate-dichromate equilibrium 2 CrO4^2- + 2 H+ ⇌ Cr2O7^2- + H2O all the way to orange Na2Cr2O7, which is the workhorse intermediate for chromium tanning agents, chrome-plating baths, pigments, and (until recent restrictions) chromated copper arsenate wood preservative. The yellow-to-orange color shift across the equivalence point is one of the most visually arresting equilibrium demonstrations in inorganic chemistry. Sodium chromate also serves as the indicator in the Mohr titration for chloride in water analysis: silver nitrate titrant precipitates AgCl first (white), and only when chloride is exhausted does AgCrO4 (brick red) appear, signaling the endpoint. Modern production is shrinking — REACH Annex XIV restricted Cr(VI) compounds heavily in Europe in 2017, and the U.S. EPA classifies Cr(VI) as a known human carcinogen via inhalation.
Where you'll encounter it
If you've ever sat through the Erin Brockovich movie, the contamination at Hinkley, California was hexavalent chromium leaching from a Pacific Gas & Electric cooling tower where Na2CrO4 had been added as a corrosion inhibitor for the carbon-steel piping. That single case drove a $333 million settlement and a sea change in how Cr(VI) corrosion inhibitors are used. Aerospace engineers who maintain pre-2000 aluminum airframes still encounter chromate conversion coatings (alodine) on rivets and structural panels — a thin Cr(VI) film passivates aluminum against pitting corrosion in salt-spray service better than any trivalent replacement, which is why the FAA has been slow to ban it for safety-critical applications. In an analytical lab, the Mohr titration is still taught in every quantitative analysis course because the indicator chemistry is so visually clean: the white silver chloride precipitate suddenly turns brick red as the first drop of excess silver hits chromate. Wood-preservative-treated lumber sold before 2003 is loaded with Cr(VI) as part of the CCA formulation, and burning that lumber in a fireplace concentrates carcinogenic chromate in the ash.
Common Uses
- Roast-leach intermediate in chromium-from-chromite extraction metallurgy
- Indicator for the Mohr argentometric titration of chloride in water
- Chromate conversion coating bath for corrosion-resistant aluminum aerospace parts
- Closed-loop cooling-tower corrosion inhibitor (now heavily restricted)
- Sodium dichromate precursor for industrial oxidation chemistry
- Demonstration of chromate-dichromate pH-dependent equilibrium in inorganic teaching
- Wool and leather mordant in legacy textile dyeing operations
- Ceramic and glass yellow pigment precursor in lead chromate synthesis
Safety Information
Acutely toxic and a Category 1A human carcinogen. GHS: H272 (oxidizer), H301 (toxic if swallowed), H312 (harmful in skin contact), H314 (severe skin and eye burns), H317 (skin sensitizer), H334 (respiratory sensitizer), H340 (genetic defects), H350 (causes cancer by inhalation), H360 (reproductive toxicant), H372 (chronic organ damage), H410 (very toxic to aquatic life). OSHA PEL is 5 µg/m³ Cr(VI) as an 8-hour TWA — among the lowest exposure limits in the OSHA chemical list, dropped from 52 µg/m³ in 2006 after epidemiology showed lung cancer risk at the older limit. Skin contact causes ulcerating chrome holes that take weeks to heal; chronic inhalation produces nasal septum perforation. EU REACH Annex XIV restricts use without specific authorization. Spills require chemical reduction to Cr(III) with sodium bisulfite plus acid before disposal as F006 hazardous waste. Use a fume hood, double nitrile gloves, P100 respirator, and dedicated coveralls.
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.