During iodine-based field water purification, which sequence of steps is correct?

• A. Dissolve 1 ampule in 1/2 full canteen cup
• B. Pour 1/2 full canteen cap into canteen
• C. Shake canteen
• D. Tighten cap and wait additional 30 min

Answer: (D)

Disinfection with iodine relies on enough contact time between the iodine and the water for pathogens to be inactivated. After you’ve added the iodine and mixed it so the solution is evenly distributed, the final step is to seal the canteen and let it sit for about 30 minutes. Tightening the cap helps prevent evaporation and keeps the iodine in contact with the water for the full duration, ensuring effective purification. The other options describe incomplete or improper steps (such as adding iodine in a separate cup, pouring cap material into the canteen, or just shaking without allowing time for the disinfectant to work). The essential idea is to secure the container and wait the 30-minute contact time for the purification to be complete.