rising ph levels

[chem geek]

chem geek/Richard
By what mechanism is adding bleach an acidic process. I am not doubting you, just curious as to why

Andy

I'll do this in words and in symbols. Adding bleach is a basic process; it is the using up of bleach (chlorine) that is an acidic process so the net result is almost neutral. When I said "chlorine usage" I didn't mean your using chlorine (i.e. adding it) -- I meant when chlorine gets used up by "doing its thing" or "breaking down". Sorry for the confusion I caused.

Adding Chlorine
NaOCl + H2O --> Na+ + HOCl + OH- (+ extra base Na+ + OH-)
HOCl <--> H+ + OCl-
Sodium Hypochlorite (liquid chlorine or bleach) combines with water to produce sodium ions (part of regular table salt) plus disinfecting chlorine plus hydroxyl ion. The hydroxyl ion makes this a basic reaction that raises pH, but because the disinfecting chlorine is a weak acid this overall reaction raises the pH by less than a strong base would. Note that there is a small amount of extra base in the form of Sodium Hydroxide (lye or caustic soda) that comes with Sodium Hypochlorite and is there to help preserve it, but this amount is rather small.

Using Up Of Chlorine
Breakdown of Chlorine by Sunlight (UV)
2HOCl --> O2(g) + 2H+ + 2Cl-
2OCl- --> O2(g) + 2Cl-
Chlorine breaks down in the presence of ultraviolet radiation, such as found in sunlight, and forms oxygen gas and chloride ion (and hydrogen ion, if starting with HOCl hypochlorite). Because a hydrogen ion is produced, this is an acidic process, but since disinfecting chlorine is a weak acid, only some of it breaks down in a way that lowers pH as shown above (i.e. only HOCl produces H+; OCl- does not).

Net Chlorine To Breakpoint (Ammonia "Oxidation")
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
OCl- + H+ --> HOCl
The disinfecting form of chlorine (HOCl) combines with ammonia through a series of reactions (that I have not shown) with the net result being the production of nitrogen gas (which is why it is important to keep your cover off and have good circulation when shocking) plus hydrogen ion and chloride ion. Though by itself this would be a strong acid reaction, there is also OCl- present that will combine with hydrogen ion to form more HOCl since the ratio of HOCl to OCl- will remain constant (and is about 50/50 at pH 7.5). So the net reaction is acidic, but not strongly so.

Overall combination of adding chlorine and having it used up
The net reactions are as follows if you combine the ones I showed above.
2NaOCl --> 2Na+ + 2Cl- + O2(g)
3NaOCl + 2NH3 --> 3Na+ + 3Cl- + N2(g) + 3H2O
So the overall net reaction of adding sodium hypochlorite to your pool and having it used up in its most typical ways is simply to produce salt (yes, sodium chloride or table salt, dissolved in water, of course) and either oxygen or nitrogen gas (and water).

Other things that could happen
If you do not have enough chlorine in your pool relative to your bather load (ammonia demand), then the chlorine may not completely oxidize ammonia and instead you will get chloramines (first, monochloramine). This reaction is basic. However, sunlight may break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (so it's the same as I showed above overall).

It is also possible for chlorine to combine with organic compounds to form chlorinated organics that are hard to breakdown. When people talk about the health problems with chlorine, it is usually about some of these chlorinated organics (Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloramines such as nitrogen trichloride (NCl3) not only smell bad, but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good residual of chlorine you typically don't get these "bad" compounds. If you are really concerned and have money to burn, you can use a constant maintenance level of non-chlorine shock (monopersulfate, MPS) to oxidize organics before chlorine gets a chance, but this is probably overkill for an outdoor pool (though may be a good idea for an indoor pool).

Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reaction:
2H2O + 2Cl- --> Cl2(g) + H2(g) + 2OH-
Cl2(g) + OH- --> HOCl + Cl-
----------------------------------------------
2H2O + Cl- --> HOCl + OH- + H2(g)

Note that the products of HOCl and OH- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

If you have a salt pool and don't use CYA (this isn't normal) then you could also outgas chlorine in the same way that CO2 is outgassed. This is more likely if you are aerating the water (e.g. have water features, slides, fountains, jets pointed up, lots of splashing, ...). This process is strongly basic and greatly increases the pH (HOCl + Cl- --> Cl2(g) + OH-). The reason this would tend to only happen in a salt pool without CYA is that a high concentration of both chloride ion (Cl-) and disinfecting chlorine (HOCl) are needed and it occurs more readily at lower pH.

I know, I know...more than you wanted to know. I hope it helps and that you made it this far...

Richard