Iodine
Properties
| State | Solid (lustrous dark purple-gray crystals with metallic sheen) |
| Color | Dark purple-gray (violet vapor) |
| Solubility | Slightly soluble in water (0.3 g/L at 20°C); soluble in KI solution and organic solvents |
| Melting Point | 113.7°C |
| Boiling Point | 184.3°C (sublimes readily at lower temperatures) |
About Iodine
Iodine is the violet-vapor halogen — dark, lustrous, almost metallic crystals at room temperature that sublime readily enough that a watch glass left in a beaker of I2 will be coated with purple needles by morning. Bernard Courtois isolated it in 1811 from the residual liquor of seaweed ash being processed for saltpeter, and Gay-Lussac named it the next year from Greek *ioeides* (violet-colored) for the sublimed vapor. As an element, I2 is everything its lighter halogen cousins are, just slower and softer: weaker oxidant than Cl2 or Br2, longer I-I bond (267 pm), polarizable enough to form charge-transfer complexes with everything from amines to amylose helices. The amylose complex is the famous starch-iodine test — triiodide and pentaiodide chains thread the inside of the helix and absorb in the orange to give the deep blue-black color, the textbook qualitative test for both starch and free iodine. In the body, iodine is the unique constituent of the thyroid hormones T4 and T3, where four (or three) iodine atoms decorate the diphenyl ether scaffold and tune the molecules' affinity for thyroid hormone receptors that regulate basal metabolic rate. Iodine deficiency in soil and water is the leading preventable cause of intellectual disability worldwide; iodized salt, introduced in Michigan in 1924 and now standard across most of the world, costs roughly $0.05 per person per year and prevents millions of cases of goiter, hypothyroidism, and cretinism. In the lab, I2 shows up in iodometric titrations, the Hofmann-Löffler-Freytag reaction, NIS-mediated electrophilic iodination, and as silver iodide in cloud seeding and silver-halide photography.
Where you'll encounter it
If you've ever titrated a vitamin C tablet with KI/I2 in lab, swabbed a wound with brown povidone-iodine before stitches, or cracked open a salt shaker labeled 'iodized' — that's the same element in three very different oxidation states doing very different jobs. A teaching-lab iodometric titration of a crushed vitamin C tablet against 0.005 M I2 in KI gives the endpoint as the first faint blue from a starch indicator, with one mole of I2 reacting per mole of ascorbate. An ER nurse swabs povidone-iodine onto a laceration before suturing because the slow PVP-bound release of free iodine kills broad-spectrum bacteria without the cytotoxicity of straight tincture. A nuclear-medicine tech preparing a thyroid scan administers a microcurie capsule of I-131 because the gland concentrates iodide selectively and burns the adjacent thyroid tissue from inside.
Common Uses
- Dietary iodide source (as KI in iodized salt, 15–80 ppm) for thyroid hormone synthesis
- Topical antiseptic — povidone-iodine (Betadine) and tincture of iodine
- Starch detection via the amylose-polyiodide blue-black charge-transfer test
- Iodometric titration of reductants (vitamin C, sulfite, thiosulfate) in analytical chemistry
- Electrophilic iodination of arenes and alkenes (often via N-iodosuccinimide or I2/oxidant)
- Therapeutic and diagnostic radioiodine (^131I, ^123I) for thyroid imaging and ablation
Safety Information
GHS: Acute Tox. Cat 4 (oral, dermal, inhalation), Eye Irrit. 2, Skin Irrit. 2, STOT-SE 3, Aquatic Acute 1. ACGIH TLV-C 0.1 ppm for I2 vapor (ceiling). Stains skin, clothing, and surfaces brown via covalent attachment to protein tyrosines and amylose. Vapor is a strong respiratory and ocular irritant. Acute oral toxicity LD50 ~14 mg/kg in rats. Excess iodide intake causes Wolff-Chaikoff effect (transient hypothyroidism); chronic excess causes Jod-Basedow hyperthyroidism — but recommended daily intake of 150 µg/day for adults is safe and necessary. Store in a tightly closed amber bottle away from reducing metals (Na, K, Al, Mg) and ammonia (forms shock-sensitive nitrogen triiodide).
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.