When chlorine enters water, ...

When chlorine enters water, it immediately begins to react with compounds found in the water. The chlorine will react with organic compounds and form trihalomethanes. It will also react with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite ions. Let's consider one example, in which chlorine reacts with hydrogen sulfide in water. Two different reactions can occur: Hydrogen Sulfide + Chlorine + Oxygen Ion → Elemental Sulfur + Water + Chloride Ions H2S + Cl2 + O2- → S + H2O + 2Cl- Hydrogen Sulfide + Chlorine + Water → Sulfuric Acid + Hydrochloric Acid H2S + 4Cl2 + 4 H2O → H2SO4 + 8 HCl I have written each reaction using both the chemical formula and the English name of each compound. In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create elemental sulfur, water, and chloride ions. The elemental sulfur precipitates out of the water and can cause odor problems. In the second reaction, hydrogen sulfide reactions with chlorine and water to create sulfuric acid and hydrochloric acid. Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric acid which have no disinfecting properties. The total amount of chlorine which is used up in reactions with compounds in the water is known as the chlorine demand. A sufficient quantity of chlorine must be added to the water so that, after the chlorine demand is met, there is still some chlorine left to kill microorganisms in the water.