Iron(III) Chloride
Properties
| State | Solid at room temperature |
| Color | Dark green-black (anhydrous), yellow-brown (hexahydrate) |
| Solubility | Very soluble in water (91.2 g/100 mL at 25 °C) |
| Melting Point | 306 °C |
| Boiling Point | 315 °C (sublimes) |
About Iron(III) Chloride
Anhydrous iron(III) chloride is one of the more visually striking inorganic reagents you'll meet — dark green-black flakes with a metallic purple sheen, sublimable around 315°C, and aggressive enough as a Lewis acid that it ranks alongside AlCl3 for Friedel-Crafts acylations and alkylations of activated arenes. The Fe(III) center is hard, electron-poor, and three-coordinate in the gas-phase dimer (Fe2Cl6, isostructural with Al2Cl6), which is what gives it Lewis acidity in the first place. In the lab it's standard for the FeCl3 phenol test: a drop of dilute aqueous FeCl3 added to an unknown produces violet-to-green color with phenol, salicylates, enols, and other compounds with an OH attached to an sp2 carbon — the color comes from a [Fe(OAr)6]3- chelate. Industrially, the bulk applications go to its aqueous form: water treatment (~2 million tonnes/year as a coagulant for municipal sewage and drinking water) and as a copper etchant in PCB manufacturing. Anhydrous FeCl3 is made by passing dry Cl2 over hot iron filings around 350°C, and once made it has to be stored in a sealed bottle because it deliquesces in seconds in lab air.
Where you'll encounter it
If you've ever run a Friedel-Crafts acylation in undergraduate organic lab, dosed a fish tank against algae, or watched a sewage plant's primary settling tank turn the orange-brown of ferric floc, you've encountered FeCl3. Sophomore organic students meet anhydrous FeCl3 as the dark green-purple flake catalyst for acylating anisole or toluene — drop in 10 mol% and a chilled stirred reaction kicks off in seconds. Aquarium hobbyists dose dilute FeCl3 to outcompete algae for phosphate and to provide trace iron for planted-tank crypts and Amazon swords. At the wastewater plant downstream, the same compound dosed at 30 mg/L into raw influent is what turns the primary settling tank that characteristic orange-brown as Fe(OH)3 flocs sweep down through the column carrying suspended solids, phosphate, and bacteria with them.
Common Uses
- Friedel-Crafts acylation and alkylation catalyst at 5-20 mol% loading
- Phenol qualitative test in organic chemistry teaching labs
- Coagulant for ~2 million tonnes/year of municipal water treatment globally
- Copper etchant for printed circuit boards via 2FeCl3 + Cu reaction
- Mordant in natural-dye textile work for browns and blacks
- Oxidative coupling agent in polypyrrole and polythiophene synthesis
- Iron source for nano-magnetite (Fe3O4) preparation
Safety Information
Corrosive to skin, eyes, and most metals — aqueous solutions reach pH 1-2. GHS H290, H302, H314, H318. Stains skin yellow-brown for several days due to keratin binding. Anhydrous form reacts vigorously with water releasing HCl gas and heat. Soluble iron salts carry a low airborne limit as Fe, so keep mist and dust down. Wear nitrile gloves, splash goggles, and a lab coat. Never store in steel containers — it will eat through.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.