if you store water in 20 liter containers, you might drop by your local pharmacy and pick up a small 10ml syringe...I got mine free from Walgreens, and use it to add 2ml of Clorox bleach to the containers when I fill them with fresh water at the beginning of hurricane season
at 8 drops per gallon, 5 gallons equals 40 drops
20 drops per milliliter , so 2 ml