PooloraPool pH and alkalinity: what is the difference?
pH tells you how acidic the water is right now. Alkalinity tells you how hard it is to change that. Adjust them in the right order and you will use less acid and less soda.
- pH: 7.2–7.8. Total alkalinity: 80–100 ppm with liquid chlorine, bleach, cal-hypo or salt; 100–120 ppm with tablets.
- Baking soda raises alkalinity: about 1.4 lb (22 oz · 635 g) per 10 ppm in 10,000 gallons.
- The same acid dose moves pH less when alkalinity is high. Fix alkalinity first, then pH.
Two different measurements
pH is a log scale of acidity. Below about 7.2 the water starts to sting eyes and attack plaster and metal; above about 7.8 chlorine works less well, the water can turn cloudy and scale forms more easily.
Total alkalinity (TA) is the water's buffer, reported in ppm "as calcium carbonate". It does not tell you whether the water is acidic; it tells you how much acid or base it takes to move the pH, and how fast pH will drift on its own. High TA makes pH creep up; low TA makes pH bounce around.
The right ranges depend on your chlorine
| Range | Source | |
|---|---|---|
| pH | 7.2–7.8 (CDC accepts 7.0–7.8) | ANSI/APSP/ICC-11, CDC |
| TA with liquid chlorine, bleach, cal-hypo or a salt system | 80–100 ppm | PHTA |
| TA with trichlor tablets or dichlor | 100–120 ppm | PHTA |
| TA absolute limits | 60–180 ppm | ANSI/APSP/ICC-11 |
Why two TA ranges? Liquid chlorine, bleach, cal-hypo and salt generators push pH up over time, so a lower alkalinity slows that rise. Tablets are acidic and pull pH and alkalinity down, so a bit more buffer protects the water.
Why the same acid dose moves pH differently
Because alkalinity resists change, the acid needed to lower pH depends on TA. Poolora calculates it with a carbonate-equilibrium model instead of a fixed rule. Here is muriatic acid 31.45% to lower pH from 7.8 to 7.5 in 10,000 gallons (water at 80 °F, calcium 300, stabilizer 40):
| Total alkalinity | Muriatic acid 31.45% | TA after |
|---|---|---|
| 80 ppm | 9 fl oz 270 mL | 76 ppm |
| 100 ppm | 11 fl oz 320 mL | 96 ppm |
| 120 ppm | 12.5 fl oz 375 mL | 115 ppm |
Notice the acid also lowers alkalinity a little. Repeat that every week and TA slides down, which is how pools end up with pH that won't stay put.
Raising alkalinity: baking soda
Baking soda (sodium bicarbonate) raises alkalinity with only a small effect on pH, so it is the tool when TA is low. Per 10,000 gallons:
| Raise TA by | Baking soda |
|---|---|
| 10 ppm | 1.4 lb 635 g |
| 20 ppm | 2.8 lb 1.25 kg |
| 30 ppm | 4.2 lb 1.9 kg |
Spread it over the deep end with the pump running. Large amounts are easier to dissolve in two or three additions, retesting in between.
Raising pH: soda ash or air
If TA is fine but pH is low, use soda ash (sodium carbonate). It raises pH strongly and alkalinity too. Example from the engine: 10,000 gallons at pH 7.0 and TA 80 need about 1.8 lb (29 oz · 820 g) of soda ash to reach pH 7.4, and alkalinity ends near 100 ppm.
Aeration (fountains, jets pointed up, waterfalls) raises pH without adding alkalinity, because it lets dissolved carbon dioxide escape. It is slow but free.
Lowering alkalinity: acid plus aeration
Acid lowers both pH and alkalinity. To lower TA by 10 ppm in 10,000 gallons takes about 3.25 cups (26 fl oz · 760 mL) of 31.45% muriatic acid, or 7.5 cups (60 fl oz · 1.8 L) of a 14.5% product. Pool owners usually lower TA in steps: add acid to bring pH down to around 7.2, aerate to bring pH back up without raising TA, and repeat until alkalinity reaches the target. Never let pH fall below 7.0.
What to adjust first
- If pH or TA is high, add acid first.
- If TA is low, raise it with baking soda.
- Only then, if pH is still low, raise it with soda ash or aeration.
- Leave chlorine for last.
Safety: always add acid to water, never water to acid. Pour it slowly in front of a return jet with the pump running, keep it away from chlorine, and wear gloves and eye protection.
Common questions
Why does my pH keep rising?
High alkalinity, new plaster, aeration and salt systems all push pH up. Keeping alkalinity at the low end of your range (80–100 ppm with liquid chlorine or salt) slows the climb.
Can I use baking soda to raise pH?
Only a little. Baking soda is for alkalinity; it barely moves pH. To raise pH use soda ash or aeration.
Is dry acid the same as muriatic acid?
Dry acid (sodium bisulfate) lowers pH and alkalinity like muriatic acid, but it is a powder and adds sulfate to the water. Lowering TA by 10 ppm in 10,000 gallons takes about 2.2 lb (34 oz · 975 g) of 93% dry acid.
Do I adjust pH before or after chlorine?
Before. Acid goes first and chlorine last, never together, with time to circulate in between.
Do it with your own products
Poolora turns your test into the exact amount of the products on your shelf, in cups and ounces, in the right order, and tells you when it's safe to swim. Free to start; every chemical, water balance and history are part of Pro.
Keep going
Sources
- ANSI/APSP/ICC-11, American National Standard for Water Quality in Public Pools and Spas (pH 7.2–7.8; TA 60–180 ppm)
- Pool & Hot Tub Alliance (PHTA): alkalinity fact sheet (80–100 ppm with pH-raising sanitizers, 100–120 ppm with pH-lowering ones)
- CDC: Home pool and hot tub water treatment and testing (pH 7.0–7.8)
- Acid and soda ash doses: carbonate-equilibrium model of the Poolora engine (constants from the USGS PHREEQC database, results checked against PHREEQC)