A Theory About Rising pH in SWG Pools

[chem geek]

I thought the rising PH was due to the chemical reactions of the SWG as indicated here: http://www.goldlinecontrols.com/Elec...orination.aspx

You are correct that the generation of hypochlorous acid from the SWCG is a basic (increasing pH) reaction and is equivalent to adding sodium hypochlorite to your pool, except without the sodium (chloride ion is removed instead so there is no net charge difference). However, the chlorine gets used up mostly through the breakdown from sunlight (UV) and some from the disinfection and oxidation of organics and ammonia. The net equation for the entire process is first

2Cl- + 2H2O --> Cl2(g) + H2(g) + 2OH- --> HOCl + Cl- + H2(g) + OH-
so the net of this is
Cl- + 2H2O --> HOCl + H2(g) + OH-

which is the basic (increasing pH) process you are referring to, though HOCl is a weak acid so this is weakly basic overall. Then the using up of chlorine produces the following:

2HOCl + (UV) --> O2(g) + 2H+ + 2Cl-
-or- 3HOCl + 2NH3 --> N2(g) + 3H2O + 3H+ + 3Cl-

These are weakly acidic processes since HOCl is a weak acid so using it up results in the following reaction to keep HOCl and OCl- in balance:

OCl- + H+ --> HOCl

The net reaction for the production and destruction of chlorine is:

2H2O --> 2H2(g) + O2(g)
-or-
2NH3 + 3H2O --> N2(g) + 3H2(g)

Details of all of this may be found at this thread.

This is why using chlorine bleach with a pH of 11 does not cause a rise in pH over time (nor using chlorinating liquid with a pH of 13 though there is some extra base added in this case for stability so this will increase pH a little bit). This is also why using Di-Chlor (1% solution is 6.8) in fact lowers the pH more than one would think and why Tri-Chlor (1% solution is pH 2.8) is even worse (more acidic) that it first appears.

Now if you were to build up combined chlorine such as chlorinated organic compounds that didn't fully oxidize, then yes you would lose the "acid" part of the balance and rise in pH, but I assume your CC is staying near 0.

mas985, I'm curious. What is your current TA? If it's not unusually high, then indeed something in the SWCG systems would seem to be generating more base than it should. Is your SWCG in clear plastic so that the plates are visible to you? If so, can you see gas being generated vigorously at both plates or is it mostly just at one plate (hydrogen) therefore indicating an efficient dissolving of chlorine gas in water. If you find gas bubbles on both plates and that they seem to stream off of both plates without dissolving, then the salt cell is not operating very well in dissolving the chlorine gas into the water. Some of the gas on the chlorine side could be oxygen gas (a competing reaction), but mostly it should be chlorine gas if designed correctly. When oxygen gas is produced, there is no net change in pH which is why I think the culprit is undissolved chlorine gas.

There is also an increase in outgassing of chlorine gas in salt pools, but with the high CYA and relatively low 3-5 ppm chlorine levels, the rise in pH over a week shouldn't be as high as what you and others are seeing (it's about a 0.1 rise in pH over a week). Now hot tubs are an entirely different matter where there is lots of aeration and the temperature is a lot hotter, but they also are typically covered most of the time. Lots of variables, lots of questions, and we may never find the answers, but we can try.

Richard