Pool Chemistry

You may not have realised it, but looking after a pool requires a lot of chemistry. Most people do not understand the full interactions of all the chemicals, but just add chemical X when measurement Y falls below 5. My report outlines the chemistry behind saltwater pools, the interactions of different chemicals and chemical species, and sample calculations for determining how much of each chemical needs to be added to a new saltwater pool to maintain a healthy pool.

It is very intriguing considering the chemical interactions – many of them have to do with the pH. Some of the most important equilibrium reactions are outlined below.

Equilibrium of hydronium and hydroxide ions:
2H2O(l) ⇋ H3O+(aq) + OHˉ(aq)                       Kw = 1 × 10-14Production of chlorine in a chlorine generator:
2Clˉ(aq) + 2H2O(l) → Cl2(g) + H2(g) + 2OHˉ(aq)

Dissolution of chlorine gas into the water:
Cl2(g) + H2O(l) ⇋ HOCl(aq) + H+(aq) + Clˉ(aq)

Dissolution of hypochlorous acid (HOCl):
HOCl(aq) + OHˉ(aq) ⇋ + OClˉ(aq) + H2O(l)

Decomposition of hypochlorous acid and the hypochlorite ion in UV rays:
2HOCl(aq) + UV light → 2Clˉ(aq) + 2H+(aq) + O2(g)
2OClˉ(aq) + UV light → 2Clˉ(aq) + O2(g)

Reaction of hypochlorous acid and hypochlorite with nitrogenous wastes:
HOCl(aq) + NH3(aq) → NH2Cl(aq) + H2O(l)
HOCl(aq) + NH2Cl(aq) → NHCl2(aq) + H2O(l)

The sodium bicarbonate buffer system:
CO32ˉ(aq) + 2 H+(aq) ⇋ HCO3ˉ(aq) + H+(aq) ⇋ H2CO3(aq) ⇋ CO2(g) + H2O(l)

 

If these sorts of things excite you, then there’s plenty more equilibrium reactions and explanations in my full report.

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