Explainer: The chemistry that keeps swimming pools clean and safe

It’s the middle of summer with record high temperatures throughout Europe, the US and elsewhere, and people everywhere are turning to outdoor swimming pools for a respite. We take it for granted that somebody somewhere is monitoring and balancing chemicals so that the whole experience is pleasant and safe, rarely thinking about the chemistry protecting us.

Why does pool water need to be chemically treated?

Pathogenic microorganisms – including bacteria like Escherichia coli  or Salmonella  and viruses – end up in swimming pool water, often introduced by swimmers themselves. Disinfectants are required to kill these pathogens, and a chlorine -based disinfectant – sodium hypochlorite (NaOCl) or bleach – is the most commonly used and effective of these compounds. When sodium hypochlorite dissolves in water, it forms a weak acid, known as hypochlorous acid, which is a powerful microbicidal agent and active against bacteria, viruses, algae and fungi.