Swimming Pool Water Chemistry

Swimming Pool Water Chemistry

Swimming pools are a source of endless fun and relaxation, but behind the scenes, proper water chemistry plays a critical role in maintaining a safe and inviting environment. In this article, we will explore the key factors that contribute to balanced water chemistry in swimming pools. By understanding the importance of pH, chlorine, alkalinity, and calcium hardness, pool owners and operators can ensure crystal-clear water that is both safe and comfortable for swimmers of all ages.

Part 1: pH (Acidity)

pH is a crucial factor that directly influences the chemical reactions in swimming pool water. It measures the acidity or basicity of water on a scale of 0 to 14.

- Very acidic = 0
- Neutral = 7
- Very basic (alkaline) = 14

The pH level affects the effectiveness of chlorine in killing microorganisms and oxidizing matter. As pH increases, chlorine's ability to perform these tasks diminishes. Moreover, a pH over 8.0 may lead to scaling (mineral precipitation) and cloudy water. On the other hand, a pH below 7.0 (acidic water) can cause irritation to swimmers' eyes and mucous membranes, as well as damage metal parts and plaster finishes of the pool system.

Part 2: Chlorine (Free and Total)

Chlorine is the most commonly used chemical for treating swimming pool water. It serves a dual purpose by disinfecting the water, killing bacteria and algae, and oxidizing other substances like dirt and chloramines.

When chlorine compounds are added to pool water, they produce hypochlorous acid (HOCl) and hypochlorite ions (OCl-), with their relative proportions determined by the water's pH level.

pH Hypochlorous acid (HOCl) % Hypochlorite ion (OCl–) %
6.0 97 3
7.0 75 25
7.2 63 37
7.5 49 51
7.6 39 61
7.8 28 72
8.0 19 81
8.2 12 88
8.5 5.5 94.5
9.0 0.7 99.3

Hypochlorous acid is a highly effective disinfectant due to its low molecular mass and lack of electrical charge. These properties enable it to penetrate bacterial cell walls and target enzymes, leading to the destruction of microorganisms and their subsequent death.

On the other hand, hypochlorite ions are approximately 100 times less effective as disinfectants compared to hypochlorous acid. The reduced effectiveness is attributed to the negatively charged nature of hypochlorite ions, which hinders their ability to penetrate bacterial cell walls that also carry a negative charge.

To meet regulatory requirements, chlorine must be introduced into pool water using an approved automatic chlorinator. When chlorine, in any form, is added to water, it produces a weak acid called hypochlorous acid. It is this acid, rather than the chlorine itself, that imparts the water with oxidizing and disinfecting abilities. Proper chlorination and filtration processes contribute to the clear and sparkling appearance of pool water.

Different forms of chlorine are available for pool water treatment:

  • Liquid chlorine (sodium hypochlorite): Contains 12-16% available chlorine, higher than the chlorine content in household bleach.
  • Solid chlorine (powder or granules):
    • A. Calcium Hypochlorite: Contains 65-75% available chlorine.
    • B. Chlorinated Isocyanurates: Contains varying percentages of available chlorine.
    • C. Lithium Hypochlorite: Contains 30-35% available chlorine.
  • Gaseous Chlorine: Contains 100% available chlorine. However, due to its high toxicity, special procedures are required when handling gaseous chlorine.
  • Chlorine demand refers to the portion of available chlorine that is consumed during the disinfection and oxidation processes in pool water. The remaining hypochlorous acid, after satisfying the chlorine demand, is known as the free chlorine residual. This residual chlorine level is measured when testing the pool for chlorine.

    For more details, please refer to full swimming pool water chemistry guideline: Download PDF Here