rising ph levels

[chem geek]

Rising pH
Are you using the all bleach method? If you are, that may be why it is climbing.
Anything that you add to the pool, can change the pH depending on the pH of what you are adding.

Bleach, sodium hypochlorite, has a pH of around 10 or 11, very alkalotic, so the more you add, the higher your pH will go.

Though you are correct that bleach is alkaline (the 12% stuff from pool stores has an even higher pH of 13), chlorine usage is an acidic process so the net effect of chlorine addition and usage is a very small rise in pH due to the extra base that is added to (or kept in) the chlorine bleach (to keep it more stable). When chlorine is broken down by sunlight (UV) or consumed by ammonia to breakpoint or oxidizing organics, the pH goes down by almost the same amount as it goes up when you added it.

[EDIT]
By "chlorine usage" I mean when chlorine gets "used up", not your usage (addition) of chlorine. Sorry for the confusion.
[END-EDIT]

After reading several posts about pH rise, I am coming to the conclusion that it is most often due to outgassing of CO2 which raises pH (and when you add acid to compensate, the alkalinity is lowered and you get back to the original pH -- so the net change of CO2 outgassing plus adding acid is that the alkalinity drops). Aeration of pool water is the most common cause for this. It looks like many people live with a lower alkalinity of around 80 in such environments. Also, keeping your pH closer to 7.5 helps since lower pH accelerates the outgassing process.

Richard