pH
How acidic or basic the water is, on a log scale — 8.0 is ten times less acidic than 7.0. It is not a sanitiser or a nutrient; it is the dial that decides how well every other chemical in the pool works.
Why it matters. pH sets the split between hypochlorous acid (HOCl, the form that actually kills things) and hypochlorite ion (OCl⁻, which barely does). At 7.2 about 63% of your free chlorine is HOCl; at 7.5, 50%; at 7.8, roughly 30%; at 8.0, 22%. Every test reads the same total either way. This is why a pool can show plenty of chlorine and still go cloudy — and it is the single strongest argument for keeping pH down.
▲ What raises it
- Salt cell electrolysisdominant
At the cathode the cell splits water and releases hydrogen gas, leaving hydroxide behind. The chlorine cycle would be pH-neutral if everything stayed in the pool, but the H₂ bubbles off, so the alkalinity does not get consumed back. Rate tracks cell output × pump hours.
- Aeration — the spa spillover above alldominant
Pool water carries more dissolved CO₂ than it would at equilibrium with air. Anything that breaks the surface drives CO₂ out, carbonic acid goes with it, and pH climbs — with no change in alkalinity at all. On this pool the spillover runs the entire time the pump runs, so this is a constant, not an occasional event. The sheer descent and the spa jets stack on top of it.
- Total alkalinityminor
TA is the reservoir of dissolved CO₂ available to be lost, so a high one makes pH rebound fast after acid. At 73 this pool is already inside the 60–90 range a salt pool wants — lower than a chlorine pool would run, precisely because the cell is a permanent upward push. So alkalinity is not what is driving pH up here. The cell and the spillover are.
- Warm water — including both heatersmoderate
CO₂ is less soluble as water warms, so heating drives it out of solution: the same mechanism as aeration arriving through a different door. Nothing the burner produces touches the water, so the effect is purely thermal, which means solar gain does it too — on a sunny day the sun heats this pool harder than the heater does. Water temperature is measured directly, so the fit uses that rather than heater hours as a proxy.
- Cal-hypo shock, soda ash, fresh plasterminor
Cal-hypo slurry is around pH 11.7 and lifts pH as it dissolves. Soda ash (sodium carbonate) is the deliberate way to raise pH, rarely needed on a salt pool. Plaster leaches lime for months after a resurface.
▼ What lowers it
- Muriatic aciddominant
The working tool. Roughly 15 fl oz of 31.45% acid per 0.1 pH in this pool, which also costs about 4 ppm of alkalinity each time. Pour it slowly over a return jet with the pump running, never into the skimmer.
- Acid plus aeration, used deliberatelymoderate
The classic fix for a pool whose pH will not stay down — but not this pool's problem. At TA 73 the buffer is already where a salt pool wants it, and taking it lower trades pH control for pH instability.
- Rain, fill water, trichlorminor
Rain is mildly acidic and dilutes. Trichlor tablets are strongly acidic (pH 2.8) but drag CYA up with them, which is why they are the wrong tool on a pool that already has a cell.
Note. Two forces pull pH in opposite directions here. Chlorine effectiveness wants it low. The saturation index — with calcium at the low end — wants it high. See the tension section below; the resolution is not a pH number, it is more calcium.