Lesson 1 – Free Chlorine for Water Disinfection (2:05)

Watch the video to learn about how free chlorine is added to water for disinfection purposes.> Chlorine gas (Cl2) forms hypochlorous acid (HOCl) and hydrochloric acid (HCl) when added to water (H2O):

Cl2(g) + H2O(l) ↔ HOCl(aq) + HCl (aq)

> Liquid bleach (NaOCl) forms hypochlorous acid and sodium hydroxide (NaOH) when added to water:

NaOCl(aq) + H2O(l)  ↔ HOCl (aq) + NaOH (aq)

> Solid calcium hypochlorite (Ca(OCl)2) forms hypochlorous acid and calcium hydroxide (Ca(OH)2) when added to water:

Ca(OCl)2(s) + 2H2O(l)  ↔ 2HOCl(aq) + Ca(OH)2(aq)

> Hypochlorous acid further dissociates into the hypochlorite ion (OCl-) and a proton (H+), which is pH-dependent:

HOCl(aq)  ↔ OCl-(aq) + H+(aq)

> Free chlorine is the sum of hypochlorous acid, hypochlorite ion, and molecular chlorine (Cl2):

Free chlorine = HOCl + OCl- + Cl2