Test: Chlorination - Civil Engineering (CE) MCQ

When chlorine is added to water, the chlorine acts as molecular chlorine only when its pH is less than 5.

The chlorine existing in water as hypochlorous acid, hypochlorite ions, molecular chlorine is termed as free available chlorine.

HOCl <——> H⁺ + HOCl^–
Where, HOCl is hypochlorous acid and HOCl^– is hypochlorite ions.
At pH<10, only HOCl^– is produced.

When the pH is between 5 and 10, the chlorine in the water acts as hypochlorous acid and hypochlorite ions. As pH increases, the concentration of hypochlorous acid decreases while of hypochlorite ions increases.

Chlorine has the oxidizing power which oxidizes organic and inorganic impurities present in water and the amount of chlorine, which gets consumed before disinfection is called as chlorine demand.

The hypochlorous acid is 80 times more effective as hypochorite ions, so the pH of water to be treated should be less than 7 to prevent the ionization of it.

The water is satisfactorily disinfected if the free residual chlorine is 0.2 ppm at the contact period of 10 minutes.

When chlorine demand is fulfilled, then chlorine is available s free residual chlorine, which contains hypochlorous acid and hypochlorite ions.

Bleaching powder contains 33.3% of available chlorine. It loses its strength during storage and therefore, it is used during emergency conditions.

The chemical formula of bleaching powder is Ca (OCl)₂. It is called as chlorinated lime.