Chlorine
Properties
| State | Gas (greenish-yellow with strong, pungent, suffocating odor) |
| Color | Greenish-yellow |
| Solubility | Moderately soluble in water (7.3 g/L at 20°C, forming hypochlorous acid and HCl) |
| Melting Point | -101°C |
| Boiling Point | -34°C |
About Chlorine
Cl2 is a greenish-yellow diatomic gas, dense enough at 3.2 g/L to pool in low spots and trenches, and the dominant electrochemical product of the chlor-alkali process — about 75 million tonnes of it get made every year by electrolyzing brine in either membrane cells or older mercury and diaphragm cells. The half-reaction at the anode is 2 Cl- -> Cl2 + 2e- and the cathode simultaneously gives you NaOH and H2, so the entire commodity chemical industry's supply of caustic soda comes coupled to its supply of chlorine in a roughly 1.1 kg NaOH : 1 kg Cl2 ratio. That ratio is a real constraint on the industry: you can't make more caustic without making more chlorine, and the chlorine has to go somewhere. About 35% of it goes into PVC manufacture (via ethylene dichloride and vinyl chloride monomer), another 20% into other organic chlorinated chemicals like solvents and intermediates, around 15% into pulp bleaching (mostly as ClO2 these days, not Cl2 directly, since elemental-chlorine-free bleaching took over in the 1990s), and the rest into water treatment, inorganic chemicals, and pharmaceutical intermediates. The chemistry that makes chlorine useful is its position on the reduction-potential scale — Cl2/Cl- sits at +1.36 V vs SHE, oxidizing enough to kill most pathogens and to chlorinate aromatic rings, but not so oxidizing that you can't store it as a liquefied gas in steel cylinders.
Where you'll encounter it
If you've worked in a municipal water treatment plant, the green-tinted gas cylinders chained to the wall in the chlorination room are 1-ton chlorine cylinders feeding a vacuum-regulated chlorinator. The plant pulls Cl2 under vacuum into a contact basin where it hydrolyzes to HOCl and ClO-, the actual disinfectants. In a synthesis lab, you'll see chlorine gas occasionally for radical chlorination of alkanes (UV-initiated, in a quartz vessel) or for converting alcohols to alkyl chlorides via SOCl2 made in situ. The smell — sharp, oxidizing, distinct from HCl's sour acid smell — is detectable around 0.3 ppm, well below the 1 ppm OSHA TWA, which is the only reason chlorine leaks in industrial settings get caught before they injure people. Above 30 ppm a leak can put you on the floor in minutes.
Common Uses
- Disinfection of municipal drinking water and wastewater via on-site hypochlorous acid generation
- Feedstock for ethylene dichloride to vinyl chloride to PVC at chlor-alkali integrated complexes
- Production of chlorinated solvents including methylene chloride, chloroform, and 1,2-dichloroethane
- Anode product of the chlor-alkali electrolysis paired with cathodic NaOH and H2 generation
- Pulp and textile bleaching, increasingly displaced by ClO2 and peroxide alternatives
- Radical chlorination of alkanes and aromatics in industrial intermediates production
- Chlorination of titanium dioxide ore in the chloride process for pigment-grade TiO2
- Chemical feedstock for roughly 20% of all small-molecule pharmaceuticals containing C-Cl bonds
Safety Information
Chlorine is acutely toxic by inhalation — GHS H330, plus oxidizing gas H270 and skin/eye corrosive H315/H319. The OSHA PEL is 1 ppm as a ceiling, NIOSH REL is 0.5 ppm ceiling, and IDLH is 10 ppm. The German Army weaponized it at Ypres in April 1915 at concentrations estimated near 1000 ppm and killed several thousand Allied troops in the first attack. Modern industrial exposure controls rely on continuous gas detection (typically electrochemical sensors with 0.1 ppm resolution), self-contained breathing apparatus on standby, and water-spray scrubbers that hydrolyze leaks to HOCl/HCl before they spread. Liquid chlorine in cylinders is at about 7 bar at room temperature; a ruptured cylinder vents catastrophically, which is why chlorine is increasingly being replaced in water treatment by on-site sodium hypochlorite generation.
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.