Water supplies in many towns are subjected to bacterial purification effected by the action of free chlorine, introduced in the form of alkaline hypochlorites, chloride of lime, or Javel water. Quite apart from any injurious effects on the system of free chlorine in drinking water, the odor and taste of water containing a greater quantity of free chlorine than o.o 5 rag. per litre are objectionable. Hence the chemical control of waters so treated is important. Unfortunately the usual methods for the determination of free chlorine in water are inefficient and inoperative when the quantity of chlorine per litre falls below o.o 5 mg. The method here proposed, capable of determining much smaller proportions of chlorine, is based on the partial freezing and concentration of the water to be examined. In freezing, the water separates as pure ice, leaving the chlorine concentrated in the remaining small quantity of uncongealed water. Under these conditions, using IO litres of water, it is easily possible to estimate with starch-iodide mixture reagent 0.0o05 rag. per litre.
This qualitative method may be made quantitative by precise determination of the original quantity of water and that uncongealed, and subjecting the latter to colorimetrie comparisons with standard titrated solutions.