<prompt> Residual chlorine of water should be:1 mgltr after 1 hr1 mgltr after 12 hr0.5 mgltr after 1 hr0.5 mgltr after 12 hr  <response>: Principles of chlorination: The chlorine demand of the water should be estimated. The chlorine demand of the water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact at a given temperature and pH of the water.The minimum recommended concentration of free chlorine is 0.5 mgliter for one hour. The free residual chlorine provides a margin of safety against subsequent microbial contamination which may occur during storage and distribution.Essentials of preventive and community dentistry Soben Peter 5th edition 