Can't keep pH up!

[chem geek]

Though the use of Tri-Chlor tablets would certainly seem to be the problem, and I used to have to add Soda Ash regularly when I (foolishly) used Tri-Chlor myself, something in your numbers just doesn't add up.

The Tri-Chlor tablets I used were 3" round and weighed 8 ounces. I know that some are 6 ounces so factor this into this discussion accordingly. Adding one 8 ounce Tri-Chlor tablet (and having it almost fully dissolve) into your pool size of 15,000 gallons adds 3.66 ppm of chlorine (FC) and 2.2 ppm of CYA. The amount that pH is lowered is a function of your starting pH and TA levels. If you were at a TA of 100 and a pH of 7.5, then your pH would drop by 0.07 while at a TA of 116 and a pH of 6.66 your pH would drop by only 0.04 instead. How many Tri-Chlor tablets do you use per unit time (per week, for example)?

Also, adding 4 pounds of soda ash to a TA of 116 and pH of 6.66 would increase the pH by 0.25 and the TA by 30.2. It would take 25 Tri-Chlor tablets plus the outgassing of carbon dioxide representing about 9.9% of your carbonate in your pool in order to get back down to the original pH and TA (given your low pH, it is likely you are outgassing a lot of CO2). You see, if you are truly using Soda Ash, then this is Sodium Carbonate (check the ingredients on your Soda Ash, or pH Up, to be sure) and the increase in TA will eventually outgas as carbon dioxide and this will cause the pH to rise, so the Tri-Chlor needs to lower the pH even more than you might think if BOTH the pH and the TA are to get restored to their original levels.

So though it is certainly true that Tri-Chlor is acidic, I would guess that something else acidic is also entering into your pool. Does it rain a lot where you are? Rain can sometimes be quite acidic. The shocking with Potassium Monopersulfate (MPS) is acidic. If you use 1.5 pounds of shock, then starting at a pH of 6.66 and with TA of 116 the MPS shock would lower your pH by 0.04 and your TA by 2.9. However, adding this once a month does not fully explain your situation.

If you switch to using liquid chlorine, please report back to see if you still have to add soda ash, even if it's not as much, since normally you shouldn't have to add any at all when using liquid chlorine or bleach.

[EDIT] If I use 4 pounds of Borax instead of Soda Ash in my calculations, then it only takes 5.5 Tri-Chlor 8 ounce pucks to compensate and no outgassing of carbon dioxide is needed. That sounds more reasonable, but you said you were adding Soda Ash and not Borax. [END-EDIT]

Richard