The primary ingredient of chlorine in a pool is sodium hypochlorite (NaClO). Sodium hypochlorite is a chemical compound that releases chlorine gas (Cl2) when dissolved in water. Chlorine gas is a powerful oxidizing agent that kills bacteria, viruses, and other microorganisms in the pool water.

In addition to sodium hypochlorite, other chemicals can be added to pool water to maintain the proper pH level and to prevent the growth of algae. These chemicals include:

* Calcium hypochlorite (Ca(OCl)2)

* Lithium hypochlorite (LiOCl)

* Trichloroisocyanuric acid (TCCA)

* Sodium dichloroisocyanurate (NaDCC)

* Potassium dichloroisocyanurate (KDCC)

The type of chlorine compound used in a pool will depend on the specific needs of the pool.