rising ph levels

[wolfcue]

What does the check mark after my post mean? Just curious.

[SoCalBoo]

wolfcue - you don't have an swg by chance, do you?

[waterbear]

What does the check mark after my post mean? Just curious.

It means you are subscribed to the thread and you can check settings in your control panel to get email notifications of new posts in the thread.

[azimmer1]

Rising pH
Are you using the all bleach method? If you are, that may be why it is climbing.
Anything that you add to the pool, can change the pH depending on the pH of what you are adding.

Bleach, sodium hypochlorite, has a pH of around 10 or 11, very alkalotic, so the more you add, the higher your pH will go.
I have found that in my pool which is open all year round, I ONLY ADD 2 chemicals!!!!!!!!

Bleach and Acid.
Bleach for my source of chlorine and acid once in a while to bring the pH down.
That is it. Once your pool is "tuned" it can be that simple.

[chem geek]

Rising pH
Are you using the all bleach method? If you are, that may be why it is climbing.
Anything that you add to the pool, can change the pH depending on the pH of what you are adding.

Bleach, sodium hypochlorite, has a pH of around 10 or 11, very alkalotic, so the more you add, the higher your pH will go.

Though you are correct that bleach is alkaline (the 12% stuff from pool stores has an even higher pH of 13), chlorine usage is an acidic process so the net effect of chlorine addition and usage is a very small rise in pH due to the extra base that is added to (or kept in) the chlorine bleach (to keep it more stable). When chlorine is broken down by sunlight (UV) or consumed by ammonia to breakpoint or oxidizing organics, the pH goes down by almost the same amount as it goes up when you added it.

[EDIT]
By "chlorine usage" I mean when chlorine gets "used up", not your usage (addition) of chlorine. Sorry for the confusion.
[END-EDIT]

After reading several posts about pH rise, I am coming to the conclusion that it is most often due to outgassing of CO2 which raises pH (and when you add acid to compensate, the alkalinity is lowered and you get back to the original pH -- so the net change of CO2 outgassing plus adding acid is that the alkalinity drops). Aeration of pool water is the most common cause for this. It looks like many people live with a lower alkalinity of around 80 in such environments. Also, keeping your pH closer to 7.5 helps since lower pH accelerates the outgassing process.

Richard

[wolfcue]

Thanks to everyone for the help.

[azimmer1]

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

Andy

[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