The Chemistry of Swimming Pools

 

One of the first things I noticed when I bought a pool is that pool chemistry-terms are confusing to both the lay person and to a person with a chemistry background. There is a lot of jargon, which means nothing to a non-chemist, and terms like “alkalinity” and “free chlorine” can be easily misinterpreted by someone who knows their textbook definitions.

 

1.         Chlorine and pH

 

When pool manufacturers use the term “free chlorine”, what they are actually referring to is the chemical hypochlorous acid, HClO, which exists in aqueous solution. This is the actual substance that kills bacteria. By doing so, HClO prevents diarrhea, swimmer's ear (a nasty earache) and various respiratory, skin and infections of minor cuts. Cl₂, diatomic chlorine, would also do the trick, but it is more dangerous.

 

So when you add, say, Ca(ClO)₂ to your pool, the product will slowly dissolve in water to produce OCl^-_(aq).

But this ion creates an equilibrium with water, in other words a pair of reactions that occur at the same rate:

 

ClO^- + H₂O = HOCl + OH^-

 

If the pH is too high the reverse reaction will be encouraged leading to a higher concentration of ClO^- , but lowering the amount of essential HOCl. Although a lower pH(more acid) will discourage the forward reaction (by destroying OH^-), it also makes the water more irritating and more corrosive. Copper for example, which is used in some electric heating coils and in less expensive heating pumps, can withstand the action of HOCl alone but copper metal will dissolve in the presence of HOCl and acid. The right balance is a pH of 7.2.

 

Of course, although a bit of chlorine kills bacteria, too much would irritate eyes, lungs and so forth. After all it was used as a chemical weapon in World War I. The ideal amount is about 2 parts per million, meaning about 2 mg of calcium hypochlorite per litre of water. You normally start with a little more because UV will break down chlorine compounds and convert them to HCl. If you dissolve 120 g in a 40 000 L pool, that’s equivalent to 3 g per 1000L, which is 0.003 g/L or 3 mg per L.

 

On average such an amount will last two days. But factors such as temperature, use, presence of pool cover(which filters UV) will either increase or decrease HOCl’s residence-time.

 

This is important because trichlor tablets are sold with high levels of cyanuric acid, which is included to drive the HOCl-equilibrium to the right, but unless the owner constantly adds carbonate ion(so-called pH+), the acid will take its toll on pool equipment. For this reason I prefer Ca(ClO)₂, but it too has its side effects, as calcium deposits can accumulate, if dilution and backwashes are not routinely performed.

 

So this is why it is important to constantly monitor pH and chlorine levels in one’s swimming pool. Too much chlorine is toxic; too little leads to problems. Too little acid (high pH) will not let HOCl do its job, and too much will be corrosive.

 

2.         Alkalinity

 

Alkalinity to a chemist means base-level. A base is an acid’s counterpart. But to a poolster, alkalinity refers to a specific base, namely hydrogen carbonate ion (HCO₃^-). The reason it’s important to have a decent amount of this in your pool, is that it acts as a buffer and stabilizes the pH level.

 

3.         Measuring pH

 

The typical pH paper provided in some kits is not at all accurate. It can easily be off by more than 1 unit, and a single unit on the pH scale, a logarithmic one, is really a factor of 10 in terms of H⁺. Some kits come with phenol red, but it is only useful between pH’s of 7 and 8. If the water is more acidic than 7---tap water and rainwater typically are----you will first need bromothymol blue. Take a small sample of tap water and one of pool water. Since the indicator bromothymol blue is dissolved in NaOH, adding four drops to a small vial containing an unbuffered sample will not give the expected green colour for tap water. It will be blue. If your pool water plus four drops of the same indicator is a lighter blue, or greenish blue, it will obviously be more acidic than tap water. It’s time add carbonate in proportion to the volume of the pool, and it will correspond to a pH of <6.8 in the chart in the back of the pH+ bottle. The after 12 hours remeasure the pH using phenol red indicator. If it’s stillbelow pH= 7.2 add more carbonate. If you accidentally overshot the desired value, it will be time to buy pH(-), and you are on your way to having your own lab.

 

4. Measuring Chlorine

 

A common agent used to measure free chlorine is ortho-tolidine, which turns either a light or dark shade of yellow depending on the concentration of HOCl. Ortho-tolidine is unfortunately a cancer suspect agent.

 

 

References:

 

http://www.rhtubs.com/chlorine.htm#trichlor

 

http://en.wikipedia.org/wiki/Calcium_hypochlorite

 

http://www.chemicalland21.com/specialtychem/finechem/o,o'-TOLIDINE.htm