How do you raise pool alkalinity without raising pH?
Use baking soda, not soda ash: 8 lb 6 oz takes a 20,000 gallon pool from 50 to 80 ppm total alkalinity while moving pH only slightly, because baking soda's own pH is about 8.3.
Both products raise alkalinity. The difference is how hard they push pH on the way, and that decides which one belongs in your pool.
Baking soda versus soda ash
Baking soda is sodium bicarbonate, the same chemical sold in pool stores as alkalinity increaser. Soda ash is sodium carbonate, sold as pH increaser. Soda ash raises alkalinity too, but it lifts pH sharply as it does, which is exactly what you are trying to avoid. Baking soda only draws the water gently toward its own pH, so a pool sitting near 7.5 rises a little and settles.
How much to add
About 1 lb 6 oz per 10,000 gallons raises alkalinity 10 ppm, so a 20,000 gallon pool needs 2 lb 13 oz for every 10 ppm. Aim for 60 to 80 ppm with a salt cell or liquid chlorine, and 80 to 120 with trichlor tablets. Add half the dose, let it circulate, retest, then add the rest.
Spreading it
Broadcast it across the surface with the pump running rather than dumping it in the skimmer, where a large dose can clump. The water may cloud for an hour or so. Retest alkalinity after six hours, and pH the next day once everything has mixed.
If pH still climbs
If pH ends up above 7.8, bring it back down with acid in small doses. Acid lowers alkalinity as well, so it takes back a few ppm; that is why you raise alkalinity first and correct pH second, never the other way round. If pH was low to begin with, the small rise from baking soda is welcome. Retest before buying soda ash, since you often will not need it.
Work out your own numbers
Use the alkalinity calculator, pH calculator and the water balance calculator with your own readings. If you do not know your volume yet, measure it with the pool volume calculator.
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