A Theory About Rising pH in SWG Pools

[chem geek]

Pat,

You do a very good job explaining the chemistry in simpler terms for people to understand. Thank you for doing that. There are a couple of corrections to what you wrote in your thread, but they don't detract from the main points too much except that it is not true that the production AND consumption are an alkaline (basic) process because they are not -- the net of the two (production and consumption) are neutral in pH.

The products from electrolysis are only Hydrogen (H2) gas and Chlorine (Cl2) gas. There is no sodium (Na+) produced nor consumed by this process. There are also no hydroxide ions (OH-) directly produced from the process, but because hydrogen ions (H+) are consumed and due to the balance of the two with water, the net effect is as if hydroxide ions are created (from water).

2H+ + 2e- --> H2(g)
2Cl- --> Cl2(g) + 2e-
Cl2(g) + H2O --> HOCl + H+ + Cl-
H2O --> H+ + OH-
----------------------------------------
Cl- + 2H2O --> H2(g) + HOCl + OH-

Now compare the above with introducing liquid chlorine into the water:

NaOCl + H2O --> Na+ + HOCl + OH-

You can see that both process result in the same "HOCl + OH-" and therefore have the same slightly alkaline (basic) effect on pH. The effect is not strong because HOCl is a weak acid that partly counteracts the OH- (so some product is OCl- + H2O).

If we just stopped here, then one would conclude that both the SWCG process and adding liquid chlorine cause a rise in pH and though that is true, it is not the whole story. The chlorine doesn't just keep growing and growing in concentration in the pool. Instead, it gets used up. There are several processes that use up chlorine and I list some of these in this post, but the net result is that these processes do the following:

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

Note that the processes that consume the chlorine (breakdown from sunlight and oxidation of ammonia and nitrogenous organics) produce hydrogen ion (from HOCl or no pH change from OCl-) that exactly counteracts what was formed during the production of the chlorine. The net result of producing chlorine from a salt cell and then having it consumed is as follows:

2Cl- + 4H2O --> 2H2(g) + 2HOCl + 2OH-
2HOCl --> O2(g) + 2H+ + 2Cl-
----------------------------------------------
2H2O --> 2H2(g) + O2(g)

6Cl- + 12H2O --> 6H2(g) + 6HOCl + 6OH-
4NH3 + 6HOCl -->2N2(g) + 6H+ + 6Cl- + 6H2O
----------------------------------------------
4NH3 --> 6H2(g) + 2N2(g) -or equivalently- 2NH3 --> 3H2(g) + N2(g)

So you can see that the normal process of producing chlorine through an SWCG results in no net change in pH. Instead, Hydrogen and Oxygen gases are produced (when chlorine is broken down by sunlight) or Hydrogen and Nitrogen gases are produced (when chlorine is consumed by ammonia or nitrogenous organics) and I believe other organics can also produce carbon dioxide since the oxidation is similar to burning.

With liquid chlorine, the net result is somewhat similar except that the net result is the production of salt (sodium Na+ and chloride Cl-) along with oxygen gas from the breakdown of chlorine by sunlight or nitrogen gas when chlorine is consumed by ammonia and nitrogenous organics. So using liquid chlorine increases TDS (specifically salt) over time while an SWCG does not (the chloride ion consumed in production and turned into chlorine simply goes back into chloride ion during chlorine consumption).

Richard