A question about pH.

[dtwalters]

I have read here that a free chlorine level above 10ppm will render your pH test inaccurate. I usually test for chlorine and pH twice daily. I add 8.25% Clorox in the evening, shooting for 7.5 to 8.0 ppm. I have noticed that when I test in the morning (when the free chlorine is 7.5 to 8.0 ppm) the pH will be near 7.8. However, when I test in the evening (when the free chlorine is 4.5 to 5 ppm) the pH is usually nearer 7.5. Does a higher free chlorine level actually bring the pH up or is the test not accurate when the free chlorine level is in the 7.5-8.0 ppm range? My pool has been perfectly clear and algae free this year using TFP techniques, thank you!

pH 7.8, fc 7.5, cc .5, ch 100, TA 80, CYA 40

 

[JVTrain]

Yes. Maintaining at a higher FC level will give a higher pH level. I would only worry about adjusting pH if at the 4-5 ppm range your pH was above 7.8.

Adding chlorine bleach is a net zero pH change over the life of the chlorine but when you initially add bleach, the pH goes up. That pH will remain elevated until all the bleach added is consumed (the FC returns to the level it was when the bleach was added). So it's very normal to have a temporary increased pH just because you added bleach.

 

[oldguy70]

Yes. Maintaining at a higher FC level will give a higher pH level. I would only worry about adjusting pH if at the 4-5 ppm range your pH was above 7.8.

Adding chlorine bleach is a net zero pH change over the life of the chlorine but when you initially add bleach, the pH goes up. That pH will remain elevated until all the bleach added is consumed (the FC returns to the level it was when the bleach was added). So it's very normal to have a temporary increased pH just because you added bleach.

I am a bit confused ....I know with FC levels over 10.0 will not give accurate pH readings (pH will read higher), if you add bleach to a level of less than 10.0, are you saying those pH reading will not be accurate until the FC gets back to where it was before you added chlorine?

 

[JVTrain]

It's accurate at any FC level below 10. Adding bleach inherently raises pH temporarily so at a given FC, you'll be measuring the correct pH at that moment. So what you're observing as higher pH at higher FC levels is normal.

There is some good discussion on this topic here: Bleach affects pH?

Bleach itself has a high pH (about 11) but in the process of consuming FC, the process is acidic so over the life of the bleach, it's a net zero change in pH from the initial FC until when it drops back down to that FC.