Question

Hypochlorous acid (HOCl) is produced during chlorination of drinking water.

a. Sketch the speciation diagram for HOCl as a function of pH, between pH 4 and pH 12. Clearly label the pH at any intersection points. You will need the Ka of the relevant acid-base reaction.

b. Considering that HOCl itself (as opposed to its conjugate base) is the more effective form of the disinfectant. What is the pH range that should be maintained for most effective disinfection?

Answer 1

The free activated chloride is a mixture of Cl2, HOCl, and OCl- where the distribution of the three components is mainly defined by the pH.

At pH 2, Cl2 is 50% and HOCl is 50%, where HOCl increases to 100% by pH 6; then decreases to 50% at pH 7.4, at which OCl- becomes 50%; and above pH 9, only OCl- is left. [pKa (Cl2/HOCl) = 2; pKa (HOCl/OCl-) = 7.4 .

2).Neutral pH (6.5 to 7.5) produces the maximum amount of hypochlorous acid. If the pH of the water is too low (pH < 6.0), chlorine will escape as a gas, decreasing effectiveness and increasing equipment corrosion. If chlorine is added to an alkaline water (pH > 8.5), the amount of hypochlorous acid formed will be greatly reduced, and the water will not be disinfected. Bleach (liquid chlorine) and calcium hypochlorite (solid chlorine) both increase the solution's pH. pH test kits are available from most chlorine equipment distributors. When the pH is too low, the water can be buffered with lime (CaO). If the pH is too high, it can be brought down with sulfuric acid (H2SO4). Both of these chemicals .