How to lower (and raise) pH and alkalinity in a pool

pH and alkalinity move together, mostly

pH is how acidic or basic the water is right now. Total alkalinity (TA) is the water's buffer: how much acid it can absorb before pH drops. The two are linked through dissolved carbon dioxide. In practice, as Trouble Free Pool puts it, TA and pH generally move in tandem: raise one and the other usually rises too.

Targets most home pools aim for:

• pH: the CDC recommends 7.0–7.8 for pools and hot tubs; TFP considers 7.2–8.0 acceptable, and most owners aim for around 7.4–7.6.
• TA: about 50–90 ppm for pools on liquid chlorine or salt (TFP). Below about 50 ppm pH becomes unstable; above about 100 ppm pH keeps climbing.

Why care? As pH goes up, chlorine kills germs more slowly (the CDC notes this especially above 8.0), and high pH with high calcium deposits scale. Low pH and low TA make water corrosive to plaster, grout and metal.

Why pH keeps rising

Pool water with a lot of alkalinity holds dissolved CO₂. That CO₂ slowly escapes into the air, and as it leaves, pH rises while TA stays the same. Anything that agitates the water speeds this up: waterfalls, spa spillovers, jets, rain, heavy swimming. The higher your TA, the more CO₂ is stored, and the faster pH climbs. This is why the long-term fix for constantly rising pH is usually lower TA, not more acid every week.

How to lower pH

Add acid: muriatic (hydrochloric) acid, usually sold at 31.45 % or a milder 14.5 %, or dry acid (sodium bisulfate). Both lower pH and TA at the same time.

How much depends on more than pH. The buffer (TA) decides how hard pH resists, and stabilizer (CYA) and borates add their own buffering. That is why a simple "X ounces per 0.1 pH" table can be noticeably off. A carbonate-chemistry model, the method used by tools such as Pool Math, gives these numbers for a 10,000 US gallon pool (37,850 L), 31.45 % muriatic acid, target pH 7.5:

Starting water	Acid to reach pH 7.5	TA afterwards
pH 8.0, TA 80, CYA 30	≈ 363 mL (12.3 fl oz)	≈ 75 ppm
pH 7.9, TA 100, CYA 40	≈ 399 mL (13.5 fl oz)	≈ 95 ppm

For a 40 m³ pool with pH 8.0, TA 120 and CYA 40, the same model gives about 560 mL of 31.45 % acid. If you use 14.5 % acid, you need a little more than twice the volume.

Adding acid safely

• Add acid to water, never water to acid.
• Pour slowly into the deep end or in front of a return jet with the pump running.
• Never mix acid with chlorine, even in an empty bucket or measuring cup. The two release chlorine gas.
• Wait at least 30 minutes with the pump on before adding any other chemical, then re-test.

How to lower alkalinity: acid plus aeration

There is no chemical that lowers TA alone. The standard method, described by Trouble Free Pool, uses acid to lower both, then air to bring pH back up without bringing TA back:

1. Add acid to bring pH down to 7.0–7.2. This also lowers TA.
2. Aerate: point return jets up, run a waterfall or fountain, use spa jets or a pressure washer, until pH rises back to about 7.4–7.6. CO₂ escapes fastest between pH 7.0 and 7.4.
3. Repeat until TA reaches your target.

Do not use soda ash or other chemicals to raise pH during this process; that puts the alkalinity straight back. Expect it to take days, sometimes weeks, depending on how much aeration you have.

Scale of the effect: with 31.45 % acid, lowering TA by 10 ppm in 10,000 gallons takes about 757 mL (25.6 fl oz). TFP notes that lowering TA by 10 ppm with acid drops pH by roughly 0.6; the carbonate model above gives a similar figure (7.6 to about 7.1).

How to raise pH or alkalinity

Problem	Use	What it does
pH low, TA fine	Aeration, borax or soda ash	Aeration raises pH only; borax raises pH with little TA; soda ash raises pH and TA about twice as much as borax
TA low, pH fine	Baking soda	Raises TA with little pH change: up to about +20 ppm TA moves pH by less than 0.1
Both low	Baking soda first	Fix the buffer first; pH usually comes up with it

Baking soda dose: about 1.68 g per m³ for each ppm of TA, or roughly 1.4 lb per 10,000 gallons for +10 ppm. Example: a 40 m³ pool from TA 60 to 80 needs about 1.34 kg.

Soda ash raises pH fast and can cloud water if you add a lot at once, especially with high calcium. Dissolve it in a bucket of water and add it around the pool with the pump running.

Hot tubs

Spas run hot and are aerated hard every time the jets are on, so pH drifts up quickly. Keep TA on the lower side (around 50–80 ppm) and adjust in small steps: in 1,500 L of water, a few millilitres of acid make a visible difference.

Let the maths follow your water

The acid dose depends on pH, TA and CYA together, which is exactly what makes table lookups unreliable. PoolDose calculates pH and alkalinity corrections with a carbonate equilibrium model: it gives the amount of your muriatic acid (31.45 % or 14.5 %) or dry acid, soda ash, borax or baking soda, shows how the other value will shift, and reminds you to aerate after lowering TA. Follow the safety directions on the product label; the app is a calculator, not a substitute for them.