Expected pH to rise after shocking...but it didn't

rhythm · May 23, 2009

Not that I'm complaining, it was actually a nice surprise not to have to add muriatic acid today as I was expecting.

This week I shocked with liquid chlorine; I think I dumped in a total of 7 gallons over a few days. Everything looks great, and my FC levels are now nearly back to normal. But I am curious about the pH not rising from all of the liquid chlorine. I had the pH at 7.4 before shocking, and it was still right there today when I checked it (it appeared to actually be a bit lower than 7.4, but not as pale as 7.2). My FC was 8 ppm today when I checked the pH.

What factors might keep the pH constant after adding a bunch of liquid chlorine? I fully expected it to be pretty high today.

Other numbers prior to shocking:
CYA 40-45
TA 90
CH normally 350-375 (haven't measured in a few months)
no borates (I plan on changing that soon, though!)

Thanks,
Greg

Ohm_Boy · May 23, 2009

I'm not one of the experts on the chemical reactions, but I think I recall seeing that while bleach is a strong base, the actual sanitizing action, or "using up" of sodium hypochlorite is an acidic process, and that the two pretty much equal each other. So the bottom line of adding bleach and subsequently consuming it nets no real ph change.

duraleigh · May 23, 2009

Ohm_boy has it right....the net effect is practically zero. Why did you expect it to go up?

heikejohn · May 23, 2009

My PH always creeps up, and I have to add acid about once a week or so to keep it in the correct range.
Am I doing something wron here?
By the way, my water looks GREAT.

JasonLion · May 23, 2009

There are many reasons why PH might go up. High TA is the most common one, but there are several others. Bleach does not normally change the PH in the long run, though occasionally there is some PH increase from an off brand that includes extra lye.

rhythm · May 23, 2009

I had thought that liquid chlorine had a pH of around 11, so adding a bunch of it would raise the pH of the pool a bit. I can't remember where I had read or heard this, but I seem to remember thinking that each gallon of liquid chlorine would require about a pint of muriatic acid to keep the pH the same. I guess that perhaps in the past it hasn't been the addition of liquid chlorine that has caused my pH to rise, but other factors. Bottom line is, I thought it would rise because of the high pH of liquid chlorine.

Ohm_Boy's post was very helpful...I didn't realize that was the case. Thanks!

Greg

JasonLion · May 23, 2009

Yes bleach raises the PH, but only for a short time. Note that I said "in the long run". As Ohm_Boy said, adding bleach raises the PH, but then as the chlorine gets used up the PH goes back down by almost exactly the same amount. So in the long run there isn't any change.