Residual chlorine test (orthotolidine method): When 1 ml of orthotolidine reagent is added to 100 ml of chlorinated water after the required contact period, what color does residual chlorine develop in the solution?

Introduction / Context:
Orthotolidine (OT) was historically used as a quick field test to indicate residual chlorine in treated water. While modern DPD methods are now preferred, understanding OT color reactions is useful for legacy references and exam contexts.

Given Data / Assumptions:

• 100 ml chlorinated sample taken after contact time.
• 1 ml of orthotolidine reagent added.
• Observation made promptly in good lighting.

Concept / Approach:
Residual chlorine oxidizes orthotolidine to produce a yellow color whose intensity is roughly proportional to chlorine concentration. The OT test cannot distinguish free and combined chlorine and has interferences, but the characteristic color response is well known.

Step-by-Step Solution:

Verification / Alternative check:
Use DPD colorimetric methods for accurate and differentiated measurement of free vs. total residual chlorine; compare results for cross-validation.

Why Other Options Are Wrong:
Green, blue, red, or violet are not the standard OT responses for residual chlorine under normal conditions.

Common Pitfalls:
Delays in reading; colored or turbid samples causing misinterpretation; using expired reagents.

Final Answer:
Yellowish