Skip to main content

Break point chlorination; pH and temperature effects

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.

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.

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.

 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 (Fe2+^{2+}, Mn2+^{2+}, H2_2S, 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 (NH2_2Cl, NHCl2_2). The combined residual rises.
  3. Zone 3 (a to break point b): more chlorine oxidises the chloramines to nitrogen gas, N2_2O 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:

Cl2+H2O⇌HOCl+H++Cl−Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^-

The hypochlorous acid is a weak acid and dissociates:

HOCl⇌H++OCl−,Ki=[H+][OCl−][HOCl]=2.7×10−8 mol/l (at 20 °C)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 11+Ki/[H+]\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):

pH677.589
% HOCl97.478.753.927.03.6
% OCl−^-2.621.346.173.096.4

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: KiK_i increases with temperature (pKa_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.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Water Supply Engineering chapter 6 questions →