Knowledges on the mechanisms of disinfecting action by chlorine is still on the early stage of the investigation. The action varies with the classes of microorganisms, such as vegetative bacteria, spores, viruses, and amoebic cysts. In some reports, it is discussed that the effectiveness of N-chloramine to the disinfection is based on the amount of HOCl formed by dissociation, but in some experiments, it is proved that N-chloramine attacks microorganisms directly without dissociation. In order to know the mechanisms of the disinfecting action precisely, it is considered to be necessary to study the reactivity of active chlorine on various nitrogenous compounds, nucleic acids, amino acids, proteins, enzymes, and the like. According to the results obtained by present author's research on various N-chloramines, it is hard to draw a distinct line between free and combined chlorine. As the methods for fractional determination of available chlorine, DPD method, neutral ortho-tolidine method, and amperometric method are in practical use. The available chlorine of N-chloro-compound is differentiated into free or combined chlorine by these methods, but the end points of each fraction are not always adequately sharp, so that some confusions are caused for the evaluation of the data. The reactions between chlorine and nitrogenous compounds are reviewed here from a novel point of view.
