Iodine Tincture
Formula: I₂ / KI in ethanol–water — Iodine tincture (2%)
Appearance: Deep amber-brown liquid; stains skin, fabric, and surfaces yellow-brown
Hazard: Irritant · Stains · Flammable (alcohol content)
Properties
Pharmacy-grade iodine tincture is typically 2% iodine (I₂) and 2.4% potassium iodide (KI) dissolved in approximately 47% ethanol with water. The KI is essential — it converts some I₂ to the more soluble triiodide ion (I₃⁻), which is what gives the solution its characteristic deep amber color and keeps the iodine in solution. Pure iodine is nearly insoluble in water alone. The alcohol carrier makes the solution dry quickly on skin and gives it mild antiseptic properties. Iodine is a moderately strong oxidizing agent, readily oxidizing iodide-free substrates while remaining stable in solution at dilute concentrations.
Starch test: a single drop of iodine tincture in a starch solution gives an immediate, intense dark blue-black color. This is one of the most sensitive colorimetric tests in chemistry — visible at iodine concentrations below 1 ppm. The color comes from iodine molecules threading inside the helical structure of amylose (the linear component of starch), forming a charge-transfer complex that absorbs red and orange light.
Historical Context
Iodine was discovered in 1811 by Bernard Courtois while processing seaweed ash for saltpeter. He noticed a violet vapor rising when he accidentally added too much sulfuric acid, which condensed into dark crystalline plates. The element was named for the Greek iodes (violet), referring to the color of iodine vapor. Gay-Lussac and Davy established it as a new element shortly after.
The use of iodine as an antiseptic dates from the 1830s, when doctors began applying it to wounds. Iodine tincture became widely used during the American Civil War and remained the standard battlefield antiseptic through the First World War, falling out of favor only with the development of less painful and less staining alternatives. The starch–iodine test was described almost simultaneously with iodine’s discovery and has been a standard analytical tool ever since.
Experiments
Starch indicator: Add a drop to any solution containing starch (bread, potato, paper) — immediate blue-black color confirms starch is present. A drop on a fresh-cut apple turns blue; on a ripe banana (where starch has converted to sugar) it stays amber. A simple test for starch conversion during ripening.
Iodine clock reaction: Combined with hydrogen peroxide and vitamin C, iodine tincture participates in a two-reaction cycle that produces a sudden color switch after a precise delay. The iodine is continuously produced and consumed until the vitamin C runs out, at which point it accumulates and triggers the starch indicator.
Tracking diffusion: Stir a drop into a gel (cornstarch slurry or agar) and watch the amber color diffuse over minutes to hours. The rate of diffusion follows Fick’s law and depends on gel concentration.
Experiments using this chemical:
- Iodine Clock — Drives the competing oxidation cycle; the source of I₂ and I⁻ for the clock reaction
Safety
Low hazard at pharmacy concentrations with normal precautions.
Skin and eyes: Mildly irritating; stains brown on contact. Wash off with water — stains fade within a day or two. If it gets in eyes, flush thoroughly with water.
Fabric and surfaces: Stains permanently or near-permanently. Work over a protected surface; wear an apron or old clothes.
Flammability: The ethanol content makes tincture moderately flammable. Keep away from open flames.
Iodine vapor: At room temperature, dilute tincture produces minimal vapor. Do not heat — concentrated iodine vapor is irritating to the respiratory tract.
Storage: Keep in a tightly sealed, amber glass bottle away from light and heat. Iodine slowly sublimes and the alcohol evaporates over time, changing the concentration.
Incompatible with: Strong reducing agents (rapid reduction to iodide); ammonia (forms explosive nitrogen triiodide — never mix iodine with ammonia solutions); aluminium foil (vigorous reaction).