2 drops per liter of 4-6% chlorine is for use in "clear" water. Double that amount if murky, cloudy, or colored water or if the water is extremly cold.
http://water.epa.gov/drink/emerprep/...sinfection.cfm
BTW, chlorine and chlorine dioxide are two completely different chemicals.
http://www.frontierpharm.com/faqs.php
Cheers!