where to add bleach?

[chem geek]

OK, I've got this figured out. You can't just look at a single species such as H+ when you are dealing with water because there is the equilibrium going on:

..... H2O <--> H+ + OH-

With the equilibrium constant Kw=[H+][OH-]=10^(-14)

Since H+ and OH- can get consumed into H2O, the easiest way to do the mass balance is to forcibly consume the smaller of the two components into H2O so that the quantity [H+]-[OH-] represents the excess H+ or if this is negative then this represents the excess OH-.

Now when you mix the two solutions together, all you have to do is add the two "excesses" and you almost have your answer. This will tell you the rough extra amount of one species over the other, but then water will dissociate to fill in some more in order to get to equilibrium.

So, when you add bleach, which is only about 11 pH or so (its liquid chlorine that's 13), we have the following:

Pool solution of pH 7.5: [OH-]-[H+] = 10^(-14)/10^(-7.5) - 10^(-7.5) = 2.85x10^(-7) excess OH-
Bleach: [OH-]-[H+] = 10^(-14)/10^(-11) - 10^(-11) = 10^(-3) excess OH-

Adding the two together gives roughly the excess concentration of OH- over H+ of the Bleach or 10^(-3). A little water dissociates so that [OH-] is about 10^(-3) while [H+] becomes 10^(-11).

Now you made a point about the buffer solution and you are correct that this will an effect except I believe the buffer gets overwhelmed with this amount of base. I have a spreadsheet with all the equations in it and when I mix an equal liquid volume of bleach with water I get a pH of 9.86 so my guess about the buffer being overwhelmed is correct.

[EDIT]The way to think about what happened to the H+ in the pool when the two liquids were mixed is that the 10^(-7.5) concentration of H+ in the pool gets mixed with this very basic solution with all of this extra OH- hanging around so to maintain equilibrium most of this H+ combines with this newly introduced OH- and turns into water.[END-EDIT]

Now a pH of 10 isn't awful. Putting in chlorinating liquid that starts at a pH of 13 would be much worse as it would result in a pH of about 12.5

Anyway, I don't know that this pH is bad for the filter, I just was asking.

Richard