2e. Other Common Analytical Methods
Orthotolidine Method
The orthotolidine (OT) method for chlorine was first reported by Ellms and Hauser (Ref. 2.14). The method has been modified several times to overcome stability problems and interferences related to monochloramine breakthrough in the free chlorine procedure.
The orthotolidine method was dropped from the 14th edition of Standard Methods after the results of two round-robin studies (Refs. 2.15, 2.16) were released. Both studies indicated the OT method gave poor accuracy and precision and a high overall error in comparison with the other chlorine methods.
Two aquatic toxicity studies (Refs. 2.17, 2.18) compared the DPD colorimetric, amperometric titration and orthotolidine methods for determining chlorine residuals. In both studies, the OT method gave lower values at all concentrations of total chlorine relative to the other two methods.
Because of relatively poor accuracy and precision and a lack of specificity, the orthotolidine method generally is not accepted in the United States and most developed countries. Usage of this method is mainly confined to low-cost pool testing applications.