In Ohotolidine Arsenite test errors caused by the presence of interfering substances are overcome for all except:

OHOTOLIDINE-ARSENITE (OTA) Modification of the OT test to determine the free and combined chlorine residuals separately. Errors caused by the presence of interfering substances such as nitrites, iron andmanganese will produce a yellow colour with Ohotolidine, are overcome by the OTA test. The test is carried out by adding 0.1 ml of the reagent to 1 ml of water. The yellow colour produced is matched against suitable standards or colour discs. It is essential to take the reading within 10 seconds after the addition of the reagent to estimate free chlorine in water. The colour that is produced after a lapse, say 15-20 minutes, is due to the action of both free and combined chlorine.