A number of reagents have been proposed for determination of free chlorine or chloramine in water. Phelps suggested the use of 0tolidine in acetic acid (1) and Ellms and Hauser devised a method for estimation of chlorine based upon the use of o-tolidine in hydrochloric acid (2). The starch iodide test has been used for some time by certain workers. More recent suggestions are the use of dimethylp-phenylenediamine hydrochloride as a reagent for chlorine (3) and methyl orange for the estimation of chlorine in the absence of chloramine.The chief interfering substances in the accurate determination of chlorine by o-tolidine have been found to be manganese, nitrites and iron (1,5,6,7,8). At an earlier date Boruff and Buswell showed that there was no interference from nitrites and iron under the conditions necessary for making the o-tolidine test for free chlorine (9). But with the present general use of chloramine instead of chlorine longer reaction time must be allowed for the development of full color. Where this long reaction time is used, nitrites and iron, as well as manganese, interfere in the test. Obviously, the o-tolidine reagent is not entirely satisfactory and attempts have been made to adapt the reagent to the purpose by making corrections or eliminating the cause of interference either mechanically or chemically (4).In the o-tolidine test, each of the following would exert an effect: concentration of chlorine, oxidation potential of the chlorine or chloramine, pH and HCl concentration, concentrations of manganese and iron and the degree of oxidation of each, concentration of nitrite, concentration of the reagent and reaction time. With these things in mind work has been carried on in an attempt to find a reagent which would eliminate part of the difficulties experienced with 0tolidine. This investigation has been divided into three parts, involving (I) the application of the principle of oxidation-reduction potentials to the problem (II) nitrite interference in the o-tolidine test and (III) the nature of o-tolidine reactions. 1645

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