--- title: "Break point chlorination; pH and temperature effects" subject: "Water Supply Engineering" chapter: 6 key: "semester_5/water_supply_engineering:bpc-ph-temperature-hocl" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpc-ph-temperature-hocl site: BCT Engineering --- # Break point chlorination; pH and temperature effects Water Supply Engineering · Chapter 6: Water Treatment - Asked in: 2074 Chaitra, 2+6 marks; 2080 Baisakh, 2+6 marks (asked 2 times) - Video: [Breakpoint chlorination/Disinfection method in drinking water treatment](https://www.youtube.com/watch?v=LqHrxoGHbNk) — Revathi Purushothaman - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/bpc-ph-temperature-hocl - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6 ## Question Define break point chlorination. Describe in detail how pH and temperature affect the relative distribution of hypochlorous (HOCl) and hypochlorite ions (OCl$^-$) in the unit process of chlorination. ## Short answer Break-point chlorination is the dose that oxidizes all chlorine‑demanding substances, leaving free residual. pH controls HOCl/OCl⁻ equilibrium: at 20°C HOCl is 97% at pH 6, 78% at pH 7, 27% at pH 8. Higher temperature slightly lowers HOCl fraction (at pH 7, 86% at 0°C vs 77% at 25°C) but speeds disinfection. ## Answer #### Break-point chlorination **Break-point chlorination** is the addition of chlorine to water in a quantity big enough to oxidise all the chlorine-demanding substances (organic matter, ammonia, etc.) so that a **free residual chlorine** is left. The dose at which the combined residual falls to its minimum and then starts to rise as free chlorine is the **break point**. ```text Residual chlorine | / | a / free | /\ / residual | / \ / | / \ / | / combined \ / |______/ residual \/ +------+-------+--------b-------------- Zone 1 Zone 2 Zone 3 Zone 4 Chlorine dose added -> ``` a = peak of chloramines, b = break point 1. **Zone 1 (initial dose)**: the chlorine first reacts with reducing compounds (Fe$^{2+}$, Mn$^{2+}$, H$_2$S, organic matter) and is used up. There is no residual. 2. **Zone 2 (up to peak a)**: after the demand is met, chlorine reacts with ammonia and amines to form **chloramines** (NH$_2$Cl, NHCl$_2$). The combined residual rises. 3. **Zone 3 (a to break point b)**: more chlorine oxidises the chloramines to nitrogen gas, N$_2$O and nitrate; so the residual **falls**. Tastes and odours are worst here. 4. **Zone 4 (beyond b)**: all ammonia and organic matter have been oxidised; the added chlorine remains as **free chlorine** (HOCl and OCl$^-$) and the residual increases in proportion to the dose. #### Effect of pH and temperature on HOCl and OCl$^-$ When chlorine is added to water it hydrolyses: $$ Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^- $$ The hypochlorous acid is a weak acid and dissociates: $$ HOCl \rightleftharpoons H^+ + OCl^-, \qquad K_i=\frac{[H^+][OCl^-]}{[HOCl]}=2.7\times10^{-8}\ \text{mol/l (at 20 °C)} $$ The fraction as HOCl is $\dfrac{1}{1+K_i/[H^+]}$. HOCl (neutral, small molecule) is about 80 times more effective as a germicide than OCl$^-$ (charged, cannot enter the cell wall easily). **Effect of pH** (computed at 20 °C): | pH | 6 | 7 | 7.5 | 8 | 9 | |---|---|---|---|---|---| | % HOCl | 97.4 | 78.7 | 53.9 | 27.0 | 3.6 | | % OCl$^-$ | 2.6 | 21.3 | 46.1 | 73.0 | 96.4 | Below pH 6 nearly all is HOCl; at pH 7.5 the two forms are about equal; above pH 8 mainly OCl$^-$. So lower pH (6 to 7) gives better disinfection; higher pH needs a larger dose or contact time. **Effect of temperature**: $K_i$ increases with temperature (pK$_a$ about 7.8 at 0 °C and 7.54 at 25 °C), so at pH 7 the HOCl share is about 86 % at 0 °C but about 77 % at 25 °C. A higher temperature therefore reduces the HOCl fraction slightly, but it also speeds up the kill reaction, which more than makes up for it; in cold water the contact time must be increased. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the break point in chlorination? A. The dose where the residual is maximum B. The dose where combined residual falls to a minimum, after which added chlorine remains as free chlorine C. The dose that gives zero chlorine demand D. The dose at which pH becomes 7 Answer: B. Beyond the break point, ammonia and organics are oxidised and the residual rises as free chlorine. (Key point: Break point) 2. In the break-point curve between the peak and the break point, the residual falls because: A. Chlorine is lost to the air B. Chloramines are oxidised to nitrogen gas and other products C. More ammonia is added D. Free chlorine is destroyed by light Answer: B. Further chlorine oxidises the chloramines, so the combined residual drops; taste and odour are worst here. (Key point: Zone 3) 3. As pH rises from 6 to 9, the HOCl fraction: A. Rises from about 3 % to 97 % B. Stays at 50 % C. Falls from about 97 % to about 4 %, with OCl- becoming dominant D. Is unaffected Answer: C. HOCl dissociates to OCl- as pH increases, so disinfection is weaker at high pH. (Key point: Effect of pH) 4. Why is HOCl a better germicide than OCl-? A. It is charged B. It is a larger molecule C. It is less reactive D. It is neutral and small, so it can enter the cell wall easily Answer: D. HOCl is about 80 times more effective since the charged OCl- penetrates cells poorly. 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