How can I maintain a steady PH level?

[meatloaf]

I have an inground plaster pool and my PH seems to rise daily. I have to add 4.5 oz of 31% muriatic acid daily to maintain 7.5. What could the problem be?
CL 3
TA 70
CH 550
CYA 40
Temp 74*
Thanks

[chem geek]

Do you have a saltwater chlorine generator (SWG)? Do you have a lot of aeration of your water such as waterfalls, fountains, spillovers, etc.? What is your source of chlorine, assuming that is what you are using for sanitation?

[meatloaf]

I don't have a SWG, using 10% liquid chlorine and none of the fountains, waterfall or areators are turned on.

[chem geek]

Has it been raining? If not, then I'm not sure why your pH is rising so quickly unless the chlorinating liquid you are using has a very high amount of excess lye in it. There is another possibility, however. How old is your plaster pool or if it was replastered, how recently was that?

[meatloaf]

It hasn't rained for a couple of weeks. The pool is about 15 years old and has never been replastered. I have the same problem when I use Tri-chlor 3" pucks.

[chem geek]

The pH rises even when you use Trichlor pucks? That is very strange indeed since they are quite acidic and usually people have to add pH Up to restore pH and TA.

Is it possible that your TA reading is wrong? How are you making these measurements? Are you using a Taylor K-2006 test kit or are you using test strips or getting readings from a pool store? Are you adding any chemicals to the water other than chlorine?

[meatloaf]

I am using the Taylor K-2005 test kit. I am not adding any other chemicals.

[chem geek]

Well, I've run out of ideas. You could try letting the TA drop down to 50 ppm and see if that changes the rate of pH rise. If it does, then it's carbon dioxide outgassing, though without aeration I don't see why it would be high. If you do find that a lower TA is more stable or requires less acid to compensate for the pH, then you could raise the CH level to compensate for the saturation index and you can use 50 ppm Borates as an additional pH buffer (which will slow down pH rise, but won't lower the amount of acid you need to add so is only useful if the lower TA results in less total acid needed per time).

[meatloaf]

I just tested the TA again using my magnetic stirrer and came up with the same number, 70. I'll try dropping the TA to 60 and see if that helps. If not I'll take it down to 50.
Thanks for all your help!

[SteveM1800]

In order to make everthing else balanced the ph needs to be right first. When you get the ph between 7.2 to 7.8 try adding alkalinity. Its is a ph buffer which means that if you ph drops down the alkalinity will raise it back up to normal again that way your not raising the ph on a daily basis. Check it out on our website 1800pools.com.

[chem geek]

SteveM1800,

The problem noted by meatloaf is that of rising pH, not falling pH. Your suggestion would be correct if the pH tends to drop over time since a higher TA will not only buffer the pH more but will tend to force the pH to rise or at least not fall so quickly over time.

However, since meatloaf's situation is that of rising pH, adding TA will only make that problem worse. This is because TA, specifically carbonates and more specifically carbon dioxide, is a SOURCE of rising pH itself since outgassing of carbon dioxide causes the pH to rise (with no change in TA for technical reasons I won't get into here).

Richard