Gram's iodine (Lugol's modification) recipe
The mordant in Gram staining that locks crystal violet into the cell wall: 1 g iodine + 2 g KI per 300 mL.
🧮 Volume calculator
| Component | Amount | Final concentration (1X) |
|---|---|---|
| Potassium iodide (KI) | 666.7 mg | 0.67% |
| Iodine (I₂) | 333.3 mg | 0.33% |
| Water (distilled) | to final volume |
Steps
- Dissolve KI in about 10 mL of water first.
- Add iodine and dissolve (it needs the KI), then bring to volume.
- Store in a brown bottle at room temperature.
Good to know
- Gram stain: crystal violet 1 min → iodine 1 min → alcohol (or acetone-alcohol) decolourise → safranin 30–60 s.
⚠ Safety Iodine stains skin and clothing and is irritating.
FAQ
The colour has faded.
Iodine degrades with light and time — remake it once the deep brown fades.
This is a widely used standard composition. Amounts depend on the hydrate form and purity of your reagents — check the formula weight (FW) on the bottle and follow your institution's safety rules and the manufacturers' instructions.