Baking soda (sodium bicarbonate)
Also called: sodium bicarbonate, Alkalinity Increaser, Alkalinity Up, Total Alkalinity Increaser.
Baking soda is sodium bicarbonate, the identical chemical sold at the pool store as "Alkalinity Increaser" or "Alkalinity Up". In a pool it raises total alkalinity and barely touches pH, nudging it toward about 8.3.
Why it matters
Reach for it when total alkalinity is low, not when pH is low. Target TA is 60–80 ppm; if you're already in range, leave it alone, because a high TA makes pH stubborn and prone to climbing. Dose by your real water volume, add about three-quarters of the calculated amount, circulate, and retest before adding more. Broadcast it across the deep end with the pump running rather than piling it in one spot; a little temporary cloudiness clears in an hour or two. The grocery-store box does exactly the same job as the branded bag.
In numbers
About 1.5 lb per 10,000 gallons raises TA ~10 ppm. In a typical 400-gallon spa, about one ounce (two tablespoons) raises TA roughly 10 ppm.