Total alkalinity: the “brake” that keeps pH steady
Total alkalinity shows how strongly the water resists changes in pH. We usually want it at about 80–120 mg/l and correct it before the pH; otherwise the pH will not stay where you put it.
What alkalinity measures
Total alkalinity is the amount of bicarbonates and carbonates dissolved in the water. It is not the same as pH: pH tells you how acidic or basic the water is right now, while alkalinity tells you how easily that will change.
Think of alkalinity as a shock absorber. With too little, every rain shower and every dose of chlorine or pH− jolts the pH. With too much, the pH “sticks” high and is hard to bring down. It is measured in mg/l (or ppm) as calcium carbonate, with strips, drops or a photometer; see how to test the water.
Low or high alkalinity: the signs
| Alkalinity | How the pH behaves | What else you see |
|---|---|---|
| Below about 80 mg/l | Swings up and down easily, will not settle | More corrosive water for metal parts, eye irritation |
| About 80–120 mg/l | Stable, corrected with small doses | Normal operation |
| Above about 120–150 mg/l | Keeps rising, needs a lot of pH− | Cloudiness and scale, especially with hard water |
Why it is corrected before the pH
Alkalinity determines how the pH reacts to every product you add. If you adjust the pH first while the alkalinity is wrong, within a few hours or days it will have drifted again and you will have used product for nothing. The right order is:
- Alkalinity at about 80–120 mg/l.
- Pool pH at 7.2–7.6.
- Free chlorine at 1–3 mg/l.
Once the alkalinity is right, later pH corrections take only small doses, and they hold.
How to raise it and how to lower it
Low alkalinity
Raise it with sodium bicarbonate, which is also sold as “alkalinity increaser”. As a guide, about 15–20 g per m³ of water raises alkalinity by 10 mg/l, but always follow the dosage on the packaging. Dissolve it in a bucket, pour it in front of the return inlets with the pump running and test again the next day. Bicarbonate raises the pH only slightly, so it is the right choice when you want more alkalinity without sending the pH soaring.
High alkalinity
Lower it with pH−, in small successive doses over a few days. Each dose also lowers the pH, so between doses let circulation and aeration bring it back up: aeration raises the pH without raising the alkalinity. In a liner pool, always dilute the product in water and pour it near the return inlets, never concentrated in one spot on the floor.
Where the problem comes from
Alkalinity depends heavily on the water that goes into the pool. Borehole water or hard mains water usually has high alkalinity, while desalinated water, common on the islands, and rainwater have very low alkalinity. That is why you check it after the first fill, after heavy rain and whenever you add a lot of water.
High alkalinity often goes together with high water hardness, and the two combined with a high pH are among the most common causes of cloudy water and scale at the waterline.
Questions
How often should I test alkalinity?
In season, usually once a week or every two weeks, because it changes more slowly than pH. Test it as well after heavy rain or a large top-up of water.
Is alkalinity the same as hardness?
No. Alkalinity is about the carbonates that stabilize the pH, while hardness is about the calcium that forms scale. They often rise together, though, e.g. in borehole water.
See also
Pool pH
Pool pH: why we keep it at 7.2–7.6, how to raise or lower it with pH+ and pH−, what happens to chlorine and…
Water hardness
Pool water hardness: what calcium does, why scale forms at the waterline, what to expect on islands and with…
Water testing
Testing pool water properly: strips, DPD drops or a photometer, how to take a sample, what to test, how often…