WHAT IS A SANITISED SWIMMING POOL?

A Sanitised swimming pool is a clear body of WATER containing a required amount of HYPOCHLOROUS ACID at an optimum pH level. Easy to say, not so easy to achieve.

Let’s start with WATER.

The chemical equation for pure water is,

    H₂O            ↔                   H⁺                  +                   OH^-

  (water)                  (hydrogen ion)                (hydroxide ion)

Now, if for example we throw a chemical, say HX into that body of water and it breaks up into H⁺ and X^- ions, then we would have more H⁺ ions than OH^- ions. It’s the difference in the concentration of the H⁺ and the OH^-ions that decides if a body of water is acidic, neutral, or alkaline.

H⁺                        >               OH^-      Acidic Water

H⁺                        =               OH^-    Pure Water

H⁺                        <               OH^-      Alkaline, Basic, Caustic Water

There is a pH Scale which describes how acidic to how alkaline a solution is.

        0                              -                     7                    -                          14

Strong Acid                              Pure Water                           Strong Base

                                                                  ↓

                                                                7.2– 7.8

                                                                Swimming

The third component is HYPOCHLOROUS ACID.

What the hell is it, you may well ask.

Hypochlorous acid is the sanitising agent everyone calls chlorine. It is created when chlorine compounds are added to the pool or when a saltwater chlorinator is running.

While on one hand we are creating hypochlorous acid; on the other hand, hypochlorous acid is decreased by the sunlight, pool usage, suspended materials in the water, bacteria, algae, debris which blows into the pool as well as rubbish sitting in skimmer boxes and filtered ‘dirt’ sitting in the filters.

The battle rages, but why do we use hypochlorous acid in preference to something else. Well, it’s a strong oxidising agent (which kills what we don’t want in a pool) which is readily available and cheap.

Time to look at the equation.

             HOCl                ↔                  H⁺                +             OCl^-

Hypochlorous Acid                                              Hypochlorite Ion

           (Hocal)                                                                       (Ocal)     -  slang that helps us to remember.

• The oxidising agent in this equation is HOCl, the molecule.
• The reaction goes both ways which is continually happening.
• pH controls how much of the molecule converts into its ions.
• At pH = 7.5 the HOCl molecule is at about 54%.
• As the pH increases more and more of the molecules converts into their ions.
• Your DPD test will give you the same result regardless of the pH.
• The higher the pH the weaker the oxidising agent until it is virtually useless.
• This is why pH is so very important.

As just stated, if you look at a pH v’s HOCl graph you will see that at a pH of 7.5 (a common recommended level for a swimming pool) you will see that the HOCl concentration is at about 54%.

If the pH increases to 7.9 which doesn’t look too severe, you will see the HOCl concentration has dropped to 30%.

That is a drop in sanitising power of about 44%. Not too good, is it?

Pool sanitation, controlling it and monitoring it is what FarSeight is about.

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Next Week - How do you maintain the correct pH in a swimming pool?

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