Orthotoluidine Arsenite test determines –
Orthotolidine -Aresenite (OTA) Test

This is a modification of the OT test to determine the free and combined chlorine residual separately.
Further, the errors caused by the presence of interfering substances such as nittrine, iron and manganese all of which produce a yellow colour with orthotolidine, are overcome by the OTA test.

Also know

Quickest and simplest method for free residual chlorine → Diethyl parpaphenylene Diamine (DPD) indicator test.
Free residual chlorine is a measure of the portability of water.
Free residual chlorine
Combined chlorine
Free and combined chlorine separately
Free and combined chlorine together