Why high pH is worth fixing
Free chlorine is two chemicals. Hypochlorous acid is the strong, fast one. The hypochlorite ion is far weaker. pH decides how much of each you have. The WHO pool guideline gives the numbers:
pH 8.0
21%
of free chlorine is the strong form
pH 8.5
12%
of free chlorine is the strong form
So a pool at 8.0 with a good chlorine reading can still behave as if it were under-dosed. High pH also helps calcium come out of solution. Together with high hardness, that is one of the causes of cloudy water and scale. WHO sets the band at 7.2 to 7.8 for chlorine pools.
Why pH keeps climbing
Pool water holds dissolved carbon dioxide, and carbon dioxide is an acid. WHO even lists it beside hydrochloric acid as a pH-lowering chemical. Anything that churns the surface lets that carbon dioxide escape into the air, and pH rises as it goes. Your alkalinity reading does not change. That one effect explains most pH drift.
| Cause | What you see | What to do |
|---|---|---|
| Waterfalls, spillovers, jets | Steady rise, alkalinity unchanged | Expect acid on most stops |
| Salt chlorinator | pH up on most visits | Acid each stop, TA checked monthly |
| High total alkalinity | Acid seems to wear off within days | Bring TA down first |
| New plaster | Climbs while the surface cures | Follow the plasterer’s start-up plan |
| Cal-hypo, soda ash overshoot | Jump after a dose | Smaller doses, re-test first |
A salt pool needs acid on most stops, so plan for it rather than treating it as a fault. High alkalinity makes every one of these worse. WHO notes that high alkalinity “can make pH adjustment difficult”, because there is more buffer feeding the rise. If your acid wears off within a few days, fix the alkalinity before you buy more acid.
The acid method
- Test pH and total alkalinity with a drop kit, not a strip, and check free chlorine while you are at it.
- Put the numbers into the pH calculator. It gives muriatic acid in fluid ounces and dry acid in pounds (1 fl oz of muriatic does the work of about 1.34 oz of dry acid). The acid comparison explains the factor.
- Measure half of it into a clean plastic jug. For dry acid, dissolve it in a bucket of pool water first. Always add acid to water, never water to acid.
- With the pump running, pour slowly into the deep end, away from the skimmer and the walls.
- Keep swimmers out until the water has circulated and you have re-tested.
Never on the same pass as chlorine
Acid and chlorine products together release chlorine gas. Dose them on different visits, or hours apart with the pump running, and never pour both into the skimmer.
Dosing in steps, and when to retest
The calculator gives a starting dose. How far a pour moves pH depends on everything buffering the water, so the same amount of acid lands differently in different pools. The calculator is anchored to a product label: HASA’s dry acid label works out to about 6 fl oz of muriatic per 0.1 pH in 20,000 gallons at ordinary alkalinity. Cyanuric acid, borates and a high reading all add buffer the label does not see, which is the reason for the half dose first.
Give it four hours with the pump on, then test again. If pH is still above your target, pour more, scaled to the new reading. Aim for the middle of the band, not the bottom. A pool left at 7.2 has nowhere to go if the next pour is a little heavy. If you overshoot, do not reach for soda ash: the water will aerate back up by itself. Soda ash would also put back the alkalinity the acid just took out. The raise-pH guide covers a genuinely low reading.
How alkalinity and pH pull on each other
Total alkalinity is the buffer: the more of it there is, the harder pH is to move in either direction. Acid moves both readings down together. Aeration moves pH up and leaves alkalinity alone. Those two arrows are all you need.
- pH high, TA high: acid, then aeration, repeated. This walks alkalinity down and keeps pH in band. The alkalinity calculator sets the size of each step.
- pH high, TA in band: small acid doses only. Expect TA to slip over the season, and top it up with bicarb when it drops below 80 ppm.
- pH high, TA low: raise TA first, or the acid will crash pH.
To see how pH, alkalinity, hardness and temperature combine, run the saturation index calculator.