Iodine from Sodium Iodide
Difficulty: Easy | Time: 10 minutes | Visual Impact: High
Historical Context
Iodine was discovered in 1811 by Bernard Courtois while working in his family’s saltpeter factory in Paris. He accidentally added too much sulfuric acid to seaweed ash and observed a striking violet vapor that condensed into dark metallic crystals — the first isolation of iodine.
The element takes its name from the Greek ioeides (violet-colored), reflecting the color of its vapor. Within a decade, chemists had established that iodine forms a deep blue-black complex with starch — one of the most sensitive and visually dramatic color tests in chemistry, still used today in biochemistry, food science, and medical diagnostics.
This experiment demonstrates that same chemistry: a mild oxidant converts colorless iodide (I⁻) to brown elemental iodine (I₂), and the starch test then confirms what the eye can already see.
Materials
- Sodium iodide (NaI) — 0.5 g dissolved in 20 mL water
- Hydrochloric acid (10%) — 1.5 mL
- Hydrogen peroxide (3%, household) — 2 mL
- Cornstarch solution (optional) — 1 tsp in 50 mL hot water, cooled
- Beaker or glass — 50 mL
- Gloves, safety glasses
Procedure
- Dissolve 0.5 g sodium iodide in 20 mL water in the beaker — colorless solution
- Add 1.5 mL dilute hydrochloric acid — still colorless
- Add 2 mL hydrogen peroxide — solution turns orange-brown within seconds as iodine forms
- Optional starch test: add a few drops of the starch solution — turns deep blue-black, confirming iodine
Extension: add a pinch of ascorbic acid (vitamin C) to the brown solution — it vanishes instantly, showing how vitamin C acts as a reducing agent. The color returns if you add more hydrogen peroxide.
The Science
\[\ce{2 NaI + H2O2 + 2 HCl -> I2 + 2 NaCl + 2 H2O}\]
Net ionic form:
\[\ce{2 I^- + H2O2 + 2 H+ -> I2 + 2 H2O}\]
Hydrogen peroxide is a mild oxidizing agent that converts colorless iodide ions (I⁻) into elemental iodine (I₂), which dissolves in water to give the orange-brown color. The acid supplies H⁺ ions consumed in the reaction — without them the reaction is slow and incomplete.
This is a clean example of halide oxidation: iodide is oxidized (loses electrons) while hydrogen peroxide is reduced (gains electrons) to water. Iodine is the easiest halide to oxidize because iodide has the highest energy electrons of the halide series; chloride and bromide require stronger oxidants.
The starch–iodine complex forms when I₂ slips into the helical coils of amylose (the linear component of starch), shifting its absorption into the visible range and producing the characteristic blue-black color. The effect disappears on heating (the helix unwinds) and reappears on cooling — a thermochromic indicator.
Safety
Work in a well-ventilated area. Wear gloves — iodine stains skin and clothing brown and is difficult to remove. Do not mix concentrated hydrogen peroxide or concentrated acid.