How to lower pool pH and alkalinity (and raise them)
pH and total alkalinity are linked: alkalinity is the buffer that holds pH steady. Most pool owners spend their time pushing pH down, because it tends to drift upward. Here's why that happens and how to correct both numbers without chasing them back and forth.
How pH and alkalinity relate
- pH is how acidic or basic the water is right now. The CDC recommends keeping pool pH between 7.0 and 7.8, and many owners aim for 7.4–7.6.
- Total alkalinity measures the dissolved bicarbonate (and a few other buffers) that soak up acid. Common targets are 60–120 ppm.
Higher alkalinity makes pH harder to move in either direction. That sounds good, but it has a side effect: pool water with high alkalinity holds a lot of dissolved carbon dioxide, which escapes into the air. As CO₂ escapes, pH rises, and the higher the alkalinity, the faster it rises. So high alkalinity causes the very pH drift people add more acid to fix.
Why pH keeps rising
- CO₂ escaping, especially with high alkalinity, waterfalls, spillovers, jets aimed upward and heavy splashing.
- Saltwater chlorine generators. Chlorine produced in the cell is pH-neutral overall once it's used up, but the cell's bubbling drives off CO₂, so salt pools typically see pH climb.
- New plaster, which releases alkaline material for months after it's applied.
- Liquid chlorine and cal-hypo raise pH a little when added, though most of that reverses as the chlorine is used up.
Things that lower pH: trichlor tablets and dichlor (both acidic), acid, and rain in some areas.
How to lower pH
- Test pH and alkalinity, and work out the dose with the pH calculator. It accounts for alkalinity, which changes the dose a lot.
- Use muriatic acid (hydrochloric acid, usually 31.45% or a 14.5% "low-fume" version) or dry acid (sodium bisulfate).
- With the pump running, pour the acid slowly into the deep end or in front of a return jet, close to the water surface. Don't pour it into the skimmer, and keep it away from chlorine.
- Add about three-quarters of the dose, circulate for at least 30 minutes, retest, then top up.
How to lower total alkalinity
Adding acid lowers alkalinity and pH together. Aeration raises pH but leaves alkalinity unchanged. Combining them lowers alkalinity while pH ends where it started:
- Add acid to bring pH down to about 7.0–7.2, not lower.
- Aerate the water: aim returns up at the surface, run waterfalls, fountains or a spa spillover, and leave covers off. pH will climb over a day or two.
- Repeat until alkalinity reaches your target. In 10,000 gallons, each 10 ppm of alkalinity takes about 25.6 fl oz of 31.45% muriatic acid. The alkalinity calculator gives the total for your pool.
Lowering alkalinity into the 60–80 ppm range is often what finally stops a pool's constant pH rise, especially in saltwater pools.
How to raise pH
- If alkalinity is already in range or high: aerate. It raises pH without adding anything to the water.
- If alkalinity is low too: soda ash (sodium carbonate, sold as "pH up") raises both, with more effect on pH. The pH calculator estimates the dose.
- Borax also raises pH, with less effect on alkalinity, and adds borates that help buffer pH. It's sold by some pool brands for that purpose.
How to raise total alkalinity
Use baking soda (sodium bicarbonate). It raises alkalinity with only a small change in pH. In 10,000 gallons, about 1.4 lb raises alkalinity by 10 ppm. Broadcast it over the water with the pump running, add no more than about half the dose at once, and retest after a few hours. The alkalinity calculator gives the amount for your pool.
Common mistakes
- Overshooting. Adding the full calculated dose at once, then correcting the other way. Test kits read pH in steps of 0.1–0.2, so small errors add up. Add part, retest, then finish.
- Reading pH during a shock. Very high chlorine can make common pH test reagents read falsely high. Wait until chlorine falls back before trusting the pH reading.
- Fighting high alkalinity with pH up. If pH is low but alkalinity is high, aerate instead of adding soda ash.
- Mixing acid and chlorine. Never add them together, in a bucket, a feeder or the skimmer. They release chlorine gas.
Frequently asked questions
What happens if pH is too high?
Chlorine works less well (the CDC notes the effect is strongest above 8.0), calcium scale forms more easily on tile, heaters and salt cells, and the water can turn cloudy.
What happens if pH is too low?
The water stings eyes, etches plaster and grout, corrodes metal parts, and can wrinkle vinyl liners over time.
Can I use vinegar to lower pH?
Vinegar is a weak acid, so you'd need a very large amount, and it adds organic material that uses up chlorine. Muriatic acid or dry acid are the practical choices.