Trying to raise alkalinity

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
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Posts: 1
Joined: Thu 05 Sep, 2013 10:47
My Pool: 51x20m pool
1,500,000 liter pool
UV, sodium hypochlorite and hydrochloric acid
Location: united kingdom

Trying to raise alkalinity

Postby ArticFox » Thu 05 Sep, 2013 10:56


Im trying to raise the alkalinity at my swimming pool. Last week the Ta was 30 and ph 7.3,
Over the last week ive beening adding sodium bicarbonate to the pool i have raised the alkalinity to 75, and ph at 7.35 at the moment put in doing so i have gone through roughily an additional 1000liters of hydrochloric acid to counter the sodium bicarbonate. (My pool is 1,500,000 litres) if i stop adding sodium bicarbonate now as it is almost at the correct levels will this stop the amount of acid i am going through or will my auto controler keep pumping acid to the pool until the alkalinity had dropped again.

(Adding 50kgs of sodium bicarbonate every 3 days over space of 2 weeks)

Any suggestions would be great.

chem geek
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Joined: Thu 21 Jun, 2007 21:27
Location: San Rafael, California

Trying to raise alkalinity

Postby chem geek » Thu 05 Sep, 2013 21:11

Your pH target is set too low. It shouldn't be lower than 7.5 and for your pool it perhaps should be at 7.7 or 7.8 instead, especially if you have aeration sources (fountains, spillovers, waterfalls) or if you have a saltwater chlorine generator.

Acid lowers both pH and TA while carbon dioxide outgassing increases pH with no change in TA. You are wasting acid by trying to maintain a lower pH while also keeping the TA up by adding baking soda. TA is not just a pH buffer but is also a SOURCE or rising pH in its own right.

So set your pH setting higher and then see where the TA settles by not adding baking soda (sodium bicarbonate). You should see the amount of acid that you add slow down as the TA gets lower. If it's still too much acid being added, then set a higher pH target, but probably not above 7.8. If you want additional pH buffering without increasing the outgassing of carbon dioxide, use 50 ppm Borates (say, from boric acid).

This chart shows how over-carbonated your pool water is at various pH and TA levels. At a pH of 7.35 and TA of 75 your pool has roughly 10 times more carbon dioxide in it than at equilibrium with the air. If you had your pH target at 7.8, then even at that same TA level the pool would only be over-carbonated by a factor of around 3.5 so would outgas much less and you'd use less than half as much acid.

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