Breakpoint chlorination – residual behaviour: At the breakpoint in a chlorination curve for treated water, which statement best describes the observed residual?

Introduction / Context:
Breakpoint chlorination is the process of adding chlorine until the demand from reducing agents, ammonia, and chloramine formation is satisfied, after which a free chlorine residual appears and increases with further dosage.

Given Data / Assumptions:

• Typical post-filtration chlorination of potable water.
• Presence of ammonia/organics initially consumes added chlorine.

Concept / Approach:
As dosage rises, chlorine first reacts with reducing substances and forms chloramines; residuals may remain low or non-free. Near the breakpoint, chloramines are destroyed and nitrogen is released. Beyond this point, additional chlorine produces a measurable free-chlorine residual that “reappears” and then increases with dose.

Step-by-Step Solution:
Add chlorine incrementally; observe residual pattern.Approaching breakpoint: combined residual drops as chloramines are oxidized.Past breakpoint: free residual reappears and increases with dose.

Verification / Alternative check:
Chlorine residual curves consistently show the characteristic dip at breakpoint followed by a rising free-chlorine residual region, confirming the “reappearance.”

Why Other Options Are Wrong:

• “Residual chloride is zero/maximum”: chloride ion is not the measured disinfectant residual; the focus is free/combined chlorine.
• “Chlorine is used to oxidise” is true in general but does not describe the breakpoint residual behaviour.

Common Pitfalls:

• Confusing chloride (Cl−) with chlorine residual (Cl2 as free/combined forms).

Final Answer:
residual chlorine reappears