This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Orthotoluidine Arsenite test determines – 
Select and explain. 
A. Free residual chlorine, B. Combined chlorine, C. Free and combined chlorine separately, D. Free and combined chlorine together
C: Free and combined chlorine separately
Answer: C: Free and combined chlorine separately 
Explanation: 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.