How much does liquid chlorine raise PH & for how long?

[Catanzaro]

Here are the latest results and ranges for the season. Pool is closed.

CYA 40-50
FC (In range always) 3-6 , based on CYA
CC (99% of the time) 0
TA (90-110)
CH (175-225)
PH 7.5-7.6 (As it rises to around 7.8, I lower back to 7.5-7.6 with about 10 oz. MA weekly). A lot of aeration with 4 octal circulation heads that break water surface.

We all know that liquid chlorine is pretty much PH Neutral. Adding liquid chlorine will raise the PH slightly, but as it gets used up, the PH will drop back down. Given the above conditions, has anyone ever figured out, or is there some type of formula one could use that shows increase time, amount, and decrease time, etc? And, what is the best PH level to target, using the TFPC methodology? Liquid chlorine is added to produce a net effect or increase of 2 ppm daily, and of course if there will be heavy swimmer activity, the amount added is more

How much does PH really rise based on this scenario? How quickly, and in what time frame does it come back to its normal range given no swimmer activity? I understand this may not be a cut and dry type of answer, just like adding numbers and then backing off as well. My testing, each morning is always prior to the addition of liquid chlorine.

 

[ewkearns]

This isn't really what you are asking for, but why not test pH, add chlorine, and after 30 minutes or so, test again.

I don't think the test is accurate enough to reliably show any difference.

 

[chem geek]

It depends some on the quality of the bleach or chlorinating liquid that you use. Clorox 8.25% bleach as a pH of around 11.9 and the lowest percentage of excess lye in it so that after one week the pH with your mid-point numbers the pH would rise only 0.01 units over one week. If chlorine usage/consumption were not acidic, the pH would rise from 14 ppm FC chlorine addition to 8.34. This is why we tell people to lower the pH before doing a SLAM.

With high-quality 12.5% chlorinating liquid that has a pH of 12.5 and somewhat more excess lye than Clorox bleach, the pH would rise by 0.02 units over one week. So still negligible and not measurable at all in your test kit.

The pH rise in your pool comes almost all from carbon dioxide outgassing and that occurs more quickly in your pool because your TA is too high and you have lots of aeration in the water. See this chart that shows you how over-carbonated the pool water is compared to the equilibrium carbon dioxide with water exposed to air. Since you are closing your pool, I presume being in New Jersey with freezing winter weather that you are shutting off your pump so that should eliminate the aeration and that will help considerably, but your high TA will still outgas carbon dioxide and have the pH rise. The pH won't rise above 8.35 since that is the pH of equilibrium with a TA of 100 ppm (with CYA of 45 ppm). The pH rise will slow down considerably before it gets that high.

I suggest you start out testing your pool's pH in one week intervals to get a sense for the rate of rise then pace out testing if the rate is slow. As for what to do if the pH rises, it's hard to control without circulation. As the water gets colder, the rate of carbon dioxide should slow down considerably so that will help. If you are closing with your water temperature cold (50ºF or colder) then that shouldn't have much outgassing and will also significantly slow down algae growth and is why we recommend closing at that temperature (if possible). Using a pool cover also cuts down outgassing rates, though not if using a cover that has holes to let water through.

 

[Catanzaro]

Thank you.