pH problem

[punkjay]

i'm a pool technician and one problem i am stumped with is with only a few swimming pools the pH is always low and no matter what method of trying to correct it nothing works. The next visit i turn up to the customers pool ie. every 2 weeks i go to test the water its always sitting around 6.8 or bellow. The chlorine, alkalinity, calcium hardness etc are within the required levels. The pools have some form of heating ie. electric heat pump or solar and most of them are fiberglass pools and one vinyl pool that are using tri chlor tablets and the water is town supply, has any of those things have anything to contribute to this problem? does anyone have any suggestions?

[chem geek]

I answered this on poolspaforum(dot)com here. I am repeating it below.

Trichlor is very acidic, especially when accounting for chlorine usage (that is, when chlorine gets used up from sunlight or oxidizing organics, this is also acidic). If a pool is using 2 ppm Free Chlorine (FC) per day and having this added by Trichlor tabs, then for every 10,000 gallons it takes about 2-1/2 3" 8-ounce tablets/pucks of Trichlor per week (to get the 14 ppm FC) and this will lower the pH from 7.5 to 7.06 assuming one starts with 120 ppm TA and 30 ppm CYA. Also, for every 10 ppm FC added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

So, depending on the chlorine usage per day, it is not surprising for the pH to be dropping rapidly. If you raise the TA level higher, then this will increase the amount of carbon dioxide outgassing and that can help counter the drop in pH since outgassing of carbon dioxide raises pH with no change in TA. Even if you have the TA at 120 ppm, if the CYA level is very high than some of this TA is from CYA which means less is in the form of carbonates that would help counter the pH rise (by outgassing carbon dioxide). The pools in question probably do not have water aeration features that would help outgas the carbon dioxide.

Though increasing the TA may help the pH situation, the increase in CYA means that the chlorine will be less effective so if you don't use a regular weekly algaecide (PolyQuat 60 or a phosphate remover), these pools will risk getting algae. To prevent algae growth by chlorine alone requires the FC level to be at least 7.5% of the CYA level at all times (this rule works up to a phosphate level of around 3000 ppb which is quite high).

Richard