A study of analysis errors caused by nitrite and free available chlorine during iodometric titration of total residual chlorine in wastewater

Nitrite and free available chlorine (FAC) are reported to cause errors in total residual chlorine (TRC) measurements when following the Standard Methods back-titration procedure with iodate titrant. Nitrite caused elevated TRC results when using either phenylarsine oxide or thiosulfate, which are the alternative reducing reagents specified in the procedure. FAC caused no error when phenylarsine oxide was used; however, TRC results were low when using thiosulfate. These errors are explained based on studies of the analysis chemistry, and method modifications are suggested for eliminating those errors. Thus, when using phenylarsine oxide, errors from nitrite were prevented by sulfamic acid, which was used to destroy nitrite. However, sulfamic acid could not be used with thiosulfate because the acidity of sulfamic acid induced a reaction between nitrite and thiosulfate. Errors from FAC are traced to an upset in the analysis reaction stoichiometry. With FAC present in samples, thiosulfate was not exclusively oxidized to tetrathionate. Instead, thiosulfate was partially converted to sulfate due to the strong oxidizing power of FAC. However, errors from FAC were eliminated by changing the order of reagent addition used in the analysis.