Remember that colder water tempuratures will slow down the reaction so it may take longer to purify (i.e., mountain stream, melted snow, etc.) Also, the concentration of sodium hypochlorite may differ from brand to brand. Check the label. The chart I have lists the following dosage per gallon:<br><br>1% 40 drops<br>4 - 6% 8 drops<br>7 - 10% 4 drops<br><br>But I also must caution that I never used chlorine bleach personally.