By The Zagosa Team. Published .
Reviewed or updated .
Total alkalinity is the buffer that holds pH steady. When alkalinity is too low, pH lurches with every dose, every aeration cycle and every bather. When alkalinity is right, pH mostly looks after itself. Nearly all pH trouble traces back to alkalinity being out of band.
What alkalinity is doing
Alkalinity is the water's resistance to pH change. In chemical terms it is the carbonate and bicarbonate content that soaks up hydrogen ions before they can shift the pH.
Picture alkalinity as a cushion. A small dose of acid against a thin cushion swings pH a long way. The same dose against a thick cushion barely moves it.
The hot tub target is 80 to 120 ppm. At those levels, pH usually stays within 0.2 of where you set it for days on end.
The tell-tale signs
pH that swings between sessions, high after bathing and low the next morning, is a classic low-alkalinity pattern.
pH you can adjust but which returns to the same wrong value within a day. That is the buffer dragging it back to its natural equilibrium.
pH that reacts wildly to tiny doses, where a teaspoon of pH down moves it half a point. That is low alkalinity for certain.
The reset
Test alkalinity. If it is under 80 ppm, dose alkalinity increaser (sodium bicarbonate) per the bottle, wait an hour and retest.
If alkalinity is over 120 ppm, dose pH down (sodium bisulphate) carefully. The same product lowers both pH and alkalinity, but the alkalinity drop comes first if you add it slowly with the jets running.
Once alkalinity is in band, leave pH alone for an hour then test. It will usually have settled into a comfortable range with no further work.
FAQ
Can I just keep dosing pH?
You can, but you will burn through product and never reach a stable result. Sorting alkalinity once a month is far less work than chasing pH every day.
Is high alkalinity ever a good thing?
Above 150 ppm in a hot tub it tends to scale, and there is no benefit to going past 120 ppm. The buffer is more than strong enough at that level.