where to add bleach?

[chem geek]

Either way is fine. In my pool, I just pour it in the skimmer while the pump is running.

I know that the basic pH won't harm the copper pipes in the heater, but isn't it a potential problem for a cartridge filter or sand or DE? I don't know, I'm just speculating. If you pour the chlorine quickly into the skimmer, it will raise the pH quite a bit (if you poured at the rate of suction for perhaps a 50/50 mix, then the pH would be only a little less than the bleach pH itself -- pH is logarithmic and doesn't average).

Richard

[mas985]

I don't think it would raise the PH as much as you think it would. It is logarithmic but negative of the PH so high PH has much less effect than low PH. I think bleach has a PH of arround 13 so for non-buffered solutions,

PH = -log10(0.5*10^(-7.5)+0.5*10^(-13)) = 7.8

With buffers as in pool water, it will not change even half of that.

Had to add one more thing. This is the reason that acid works so well when lowering PH and why you don't want to add it to the skimmer. I think I read somewhere that 30% muriatic acid has a PH of -1. So a 50/50 mix would be:

PH = -log10(0.5*10^(-7.5)+0.5*10^(1)) = -0.7

Almost as bad a pure acid. Of course the buffers will resist a change but the PH will still be quite low.

[chem geek]

I don't think it would raise the PH as much as you think it would. It is logarithmic but negative of the PH so high PH has much less effect than low PH.

You are absolute right. My bad. That's what happens when I think off the top of my head and don't do the math. Thanks for catching that!

[EDIT]Hold on a moment...Now that I'm actually working it through, it isn't working out as you showed. Think about diluting Sodium Hydroxide by mixing it with half water to go from a 2 molar solution to a 1 molar solution. It doesn't take on the pH of the neutral distilled water. So hold on a moment while I try to sort this out.[END-EDIT]

[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

[sailork]

I don't agree with the idea that it's going to be a 1 to 1 ratio. My pump supposedly pumps 75 gpm at 50 feet of head. Even if it were only 60 gpm that's one gallon per second. Assuming you dump a 1/2 gallon in in 5 seconds that's 1 part bleach to 10 parts pool water before it hits my pump.

My DE filter contains at least 20 gallons of water so obviously it's never going to see a concentration of more than 40 to 1 if I add a 1/2 gallon of bleach to the skimmer.

Maybe if you dump a gallon of bleach quickly into a small above ground pool skimmer you could do some harm but you'd probably have to pour from something faster than a bleach bottle to do it.

[medvampire]

kjmelak
The greaest concern with pouring through a skimmer is if you have a puck feeder and have ever had pucks in the feeder. The problem come from the diffrent types of chloring mixing. I understand bad things can happen (like puck feeder go boom) by mixing diffrent types of chlorine.
Some members here great luck pouring in to the skimmer but being I do have a puck feeder and use it now and then I pour in front of the returns. As watermom mentioned you migh want to get Bleachcalc you can download it here.
Steve

[chem geek]

Even if it were only 60 gpm that's one gallon per second.

This is true except that you need to divide this rate by the number of intakes which is typically 3 -- two floor drains and one skimmer. So that is still a speedy 1 gallon every 3 seconds so you are right that you wouldn't pour it that fast.

You probably couldn't pour it in much faster than one gallon over 10 seconds and more likely over 15 seconds so that's 1/5th of a 50/50 mixture which is certainly more diluted and reduces the pH increase by about log10(1/5) = 0.7

When this is in the water stream, it is at this elevated pH and hits your filter wherever it is entering your filter. I agree that by the time it spreads around your filter, it's not a problem, but it's that first area of exposure that was of concern. Remember, I was only asking the question, not at all sure that it was a problem.

Anyway, given how most people probably pour more slowly, putting it into the skimmer is probably OK, at least for bleach. For the liquid chlorine at pH 13, that's a little riskier.

Richard

[KirstenHW]

Sadly I cannot comment at all on the chemistry equations - I worked for my B in eleventh grade Regents chem all those years ago. However, I am now nervous as I have on occasion poured the bleach around the entire perimeter of the pool with the pump running - especially at opening. I have an 24 gal 18x42 vinyl ig with a 3 ft shallow end. Of late I have been adding the bleach to one of the 2 skimmers (one closest to non-cartridge DE filter). I add 1.46 gallon jug over the course of about 1-2 minutes time.

I won't do the perimeter thing any more - but was that really bad to have done?

[sailork]

KirstenHW, bleach mixes very quickly with the water when you add it. I think a lot of people add bleach the way you are describing it and it isn't a problem. It's probably better to add it in front of a return, but I'd doubt you've hurt anything by just dumping it in.

chem geek, I appreciate all the math and stoichiometry you present in your posts. It has really helped demystify the chemical processes going on in my pool. (And a big shout out to Evan for the same thing.) I suspect this thread may be getting near the China Shop Event Horizon given Ben's desire to keep from scaring people with too much information in the general forum.