Short Notes on Residual Chlorine

Residual Chlorine:

Residual Chlorine is a low level of chlorine remaining in water after its initial application. This is an indicator that Chlorine Demand is satisfied. It constitutes an important safeguard against the risk of subsequent microbial contamination after treatment The provision of IRWM 2000 are as under:

The Health Inspectors should check the presence of residual chlorine daily at various distribution points e.g. platforms, refreshment rooms, waiting halls, hospitals, schools and in the Railway colonies (preferably from farthest taps in the distribution systems), randomly and record of the same should be kept in a register. Suitable remedial measures should be taken in case of deficiency. Health Inspectors should also test the bleaching powder used once in 3-4 months for Chlorine content (must contain at least 25% of chlorine). The residual free chlorine available one-hour after chlorination should be 0.5 mg/L (Which is 0.5 PPM).

Detection of residual chlorine : Detection of residual chlorine can be done by a simple apparatus known as chloroscope using orthotolidine test (OT) and orthotolidine arsenite test (OTA). The OT is used to determine the total residual chlorine concentration and the OTA is used to determine the free and combined residual chlorine separately. When orthotolidine reagent is added to water sample containing chlorine, a greenish yellow colour develops, the intensity of which is proportional to the amount of residual chlorine available in the water sample.

The OT test is not as accurate as the presence of nitrates; iron and manganese also produce a yellow colour with orthotolidine. The test using OTA is more reliable.