Pool water pH: range, correction and how often to test
Pool pH is kept between 7.2 and 7.6. In this range chlorine works efficiently, the water does not irritate the eyes, and neither the liner nor the equipment is put under strain.
Why 7.2–7.6
pH shows whether water is acidic (below 7) or basic (above 7). The 7.2–7.6 range is a compromise between three things: the disinfecting power of chlorine, swimmer comfort and protecting the materials. The human eye, incidentally, has a pH of about 7.4.
Free chlorine exists in water in two forms: one very active and one much slower. The higher the pH, the less chlorine is in the active form: at 7.5 about half, at 8 less than a quarter. So with a high pH you may measure the “right” level of pool chlorine and still see the water turning dull.
What happens when pH goes out of range
| pH | What you feel or see | What happens to the pool |
|---|---|---|
| Below 7.0 | Stinging eyes, dry skin | Corrosion of metal parts (ladder, heat exchangers, screws); the liner is put under strain |
| 7.2–7.6 | Comfortable water | Chlorine performs, materials are protected |
| 7.8 and above | Cloudiness, irritation, slippery walls | Scale at the waterline and in the filter, chlorine that cannot keep up with algae |
If the liner sits for a long time in water with low pH and high chlorine, it absorbs moisture and can develop wrinkles. With high pH and hard water, the problem is scale at the waterline.
How to correct pH
Before any correction, check the total alkalinity. If it is out of range, the pH will drift again a few hours after you adjust it.
- pH− (reducer): usually sodium bisulphate granules or liquid acid. This is the most common correction in summer.
- pH+ (increaser): usually sodium carbonate. It is needed less often, e.g. after a lot of rain or with very soft water.
- Work out the pool volume and the dose from the packaging.
- Start with about half the dose: small corrections are safer than one big one.
- Dissolve the granules in a bucket of water and pour the solution in front of the return inlets, with the pump running.
- Let the circulation run for a few hours and test again before adding more.
- Adjust the pH first, then the chlorine.
pH− and chlorine are never added together or mixed in the same bucket, because toxic gas can be released. See safe chemical storage.
What raises and lowers pH
In most pools the pH tends to rise. It is pushed up by liquid chlorine and calcium hypochlorite, salt chlorination, aeration of the water (waterfalls, counter-current swim systems, lots of splashing) and fresh water high in minerals. It is pushed down by trichlor tablets, rain and, of course, pH−. If you know which of these apply to your pool, you know which way the pH will move.
How often to test it
During the swimming season, test the pH at least twice a week, and also after a storm, a pool party or a large top-up of water. At the start and end of the season, once a week is usually enough. Take the sample at about elbow depth, away from the return inlets; all the steps are in the article on testing the water. At holiday homes and guest accommodation, automatic pH and chlorine control keeps the pH stable without daily attention.
Questions
Why does my pool pH keep rising?
The usual causes are high alkalinity, strong aeration of the water, or the use of liquid chlorine and salt chlorination. Check the alkalinity first and correct the pH in small doses.
Can I swim straight after adding pH−?
Better not. Let the circulation mix the water for a few hours, test again and swim once the pH is within 7.2–7.6.
See also
Water alkalinity
Total alkalinity in pool water: how it keeps pH stable, what range to aim for, how to raise or lower it and…
Pool chlorine
Pool chlorine as tablets, granules or liquid: which form suits you, what free and combined chlorine are, and…
Water testing
Testing pool water properly: strips, DPD drops or a photometer, how to take a sample, what to test, how often…