How to lower or raise pool pH with acid and soda ash
pH drives comfort, chlorine effectiveness and equipment life. Here is how to nudge it up or down — and why alkalinity comes first.
Safety: dosing figures are estimates. Follow the product label, add one chemical at a time, never mix pool chemicals, add chemical to water (never water to chemical), keep chemicals away from children, and re-test before swimming. Add about ¾ of a dose, re-test, then top up. Not a substitute for professional pool service.
pH measures how acidic or basic your water is, on a scale where 7 is neutral. Pool water is usually kept slightly basic (around 7.4–7.6): too low and the water turns corrosive and stings eyes; too high and chlorine loses its punch while scale forms on surfaces and equipment. Adjusting pH is straightforward, but it is tangled up with total alkalinity, so it is best treated as an estimate you approach in steps.
Lowering pH — muriatic acid
To bring pH down you add acid, usually muriatic acid (hydrochloric acid, commonly ~31.45%). A practical rule of thumb, at a total alkalinity near 100 ppm, is:
≈ 10.7 fl oz of 31.45% muriatic acid per 10,000 gallons lowers pH by about 0.1
This is approximate because how far pH drops depends on the water’s buffering — its total alkalinity. The same acid moves pH more in a low-alkalinity pool and less in a high-alkalinity one. Dry acid (sodium bisulfate) is an alternative that is easier to handle but adds sulfates over time.
Worked example
10,000-gallon pool, lower pH from 7.8 to 7.5 at TA 100: that is a 0.3 drop, so roughly 3 × 10.7 ≈ 32 fl oz of muriatic acid (estimate). Add about three-quarters of it, circulate, re-test, then finish the adjustment.
Raising pH — soda ash or aeration
To raise pH you add soda ash (sodium carbonate). Soda ash is potent and also nudges total alkalinity up, so add it in small amounts and re-test. If your alkalinity is already fine but pH is low, you can often raise pH simply by aerating the water — running features, fountains or returns aimed upward — which off-gasses carbon dioxide and lifts pH without adding anything. Borax is another option that raises pH with less effect on alkalinity than soda ash.
Why alkalinity comes first
Total alkalinity (TA) is the water’s resistance to pH change — its buffer. If TA is wrong, pH will not hold: too-low TA lets pH bounce around unpredictably, and too-high TA makes pH stubborn and prone to drifting upward. Balance total alkalinity into range before you fine-tune pH. The two are adjusted with related chemicals — baking soda raises TA with only a small pH bump, while acid lowers both — so plan them together.
Handling and testing
Acid demands respect: add it to water (never water to acid), pour slowly into the deep end away from the skimmer with the pump running, avoid splashing, and keep it well clear of chlorine products. Never mix acid and chlorine. Because pH adjustment is an estimate, always under-dose, circulate for a few hours, and re-test before adding more. Salt chlorine generators tend to push pH up over time, so salt pools often need periodic small acid additions.
Keeping pH stable over the season
pH rarely sits still, and knowing which way it tends to move saves a lot of chemical chasing. It drifts up when water is aerated (fountains, waterfalls, splashing, a salt generator’s gas production) and when a new plaster surface leaches lime in its first months. It drifts down with acidic rain and with trichlor tablets, which are acidic. Fresh fill water can push pH either way depending on your source. Because these forces are constant, the winning strategy is small, frequent corrections rather than big swings: test a couple of times a week in season, nudge pH back toward 7.4–7.6, and it will spend most of its time in range.
Balance, not just a single number
pH does not act alone. Together with total alkalinity, calcium hardness and temperature it determines whether water is balanced, scaling or corrosive. High pH with high calcium encourages scale on surfaces and heaters; low pH turns water aggressive toward plaster, grout and metal. So while you adjust pH for comfort and chlorine efficiency, keep an eye on the whole picture: get total alkalinity into range first as the buffer, keep calcium hardness sensible, and pH becomes far easier to hold. If you fight the same pH problem week after week, the fix is almost always in the alkalinity, not the acid bottle.
The bottom line
pH is a comfort-and-efficiency dial you nudge, not a number you slam. Lower it with muriatic acid — roughly 10.7 fluid ounces of 31.45% per 10,000 gallons drops pH about 0.1 at a normal alkalinity — and raise it with soda ash, borax, or simple aeration when alkalinity is already fine. Because acid demand depends on the water’s buffering, every dose is an estimate: add about three-quarters, circulate, and re-test before finishing. Set total alkalinity into range first, because it is the buffer that keeps pH from wandering, and aim for roughly 7.4 to 7.6. Handle acid with respect — add it to water, pour slowly into the deep end, and keep it well away from chlorine. Small, frequent corrections beat big, reactive swings every time.
Estimate an acid or soda-ash dose for your readings with the pH calculator, get alkalinity in range first with the alkalinity calculator, and check scale-forming calcium hardness too.