Low pH Brain Teaser

[DavidD]

It's not that I have an abundant amount of free time or try to push the BBB method on anyone but when they ask, I try to explain. I've been helping a friend straighten up his pool after he admired how "sparkling" my water was. Week before last, here is what I discovered:

21k gallon A/G vinyl
FC .5
CC 1
pH >6.8
ALK 0
CH 0
CYA <100
Algae was present around the seam throughout most of the perimeter (and probably under steps), water semi cloudy but the bottom was visible.

I started the whole "education" by having him read the FAQ's at Pool Solutions, Downloading Michael's Bleach Calc and Ben's Best Guess Chart to his desktop and signing him up at Trouble Free Pool so he could post there if he'd like. He ordered Dave's (Duraleigh's) test kit. I told him he had two problems (low pH and High CYA) and I'd attack them both at the same time by draining half the water then refilling. He was a little hesitant and wanted to attack the low pH first. He ended up adding 4 boxes of Borax and it didn't even touch the pH. He then decided to try it my way and drain half and refill. He then added 4 MORE boxes of Borax and here is what he's got:

FC 9
CC 0
pH 7.5
Alk 80
CH 0
CYA <100

He's adding baking soda tonight to creep up to 100 ppm of ALK and keeping on top of the FC and we'll dilute the CYA sample tonight to try and get a guestimate of where it is.

Now, to the jest of my post:
21k gallons + 4 boxes (76 oz each) Borax - 10.5k gallons of water + 10.5k gallons of water @ pH 7.2 (fill water) + 4 boxes (76 oz each) Borax = 7.5 pH.
How low could his pH have been?

Dave

[KurtV]

Have to know the pH of the fill water to even hazard a guess.

[chem geek]

You wrote that initially the pH was > 6.8, but I suspect you meant < 6.8. Same with CYA that you wrote < 100 when I think you meant > 100. To measure 0 for TA, which means it turned red immediately, that implies a pH below 4.5 because that's the transition pH for the two indicators used in the TA test.

Diluting very acidic water with close to neutral or even basic/alkaline water just results in the hydrogen ion concentration getting cut in half and that just increases the pH by 0.3 units.

I'll assume an initial CYA of 250 since cutting this in half still resulted in > 100. If I work backwards by ending up with pH of 7.5, TA of 80, CYA of 125 I find that if I got here by adding 4 boxes of Borax, then I must have started with 12.3 ppm Borates (from the first 4 box addition), pH 6.7, TA 50. I then go back before the 1:1 dilution and figure the pH 6.4 (assuming the fill water is not acidic, which is usually the case -- it's usually 7.5 to 8.5), TA unknown, 12.3 ppm Borates, CYA of 250. To get to here from adding 4 boxes of Borates I had to start with a pH of around 4.8 and a TA of 2 and no Borates to end up with a pH of 6.4 and a TA of 30 (the additional TA to get to 50 can come from the fill water).

So we know it's below 4.5, but my best guess is that it isn't much below 4.5 and is probably somewhere in the range of 4 to 5.

If I start with normal pool water of pH 7.5, TA 100, CYA 30, then the addition of 60 3" 8-ounce Trichlor tabs/pucks (for 157 ppm FC over time; that's only 1 ppm FC per day for a little over 5 months) plus some outgassing of carbon dioxide results in a pH of 3.7 and a CYA of 125. Since his CYA was much higher, I suspect that he used more tablets than I described and since his pH wasn't quite that low I suspect he added either pH Up or Alkalinity Up at some point, but obviously not enough. Either that or he corroded (dissolved) 10 ppm calcium carbonate equivalent of plaster (about 1.75 pounds) since that results in a pH of 4.4 with the scenario just given (so if more pucks were used, then even more plaster could have been corroded).

Richard

[DavidD]

Have to know the pH of the fill water to even hazard a guess.

+ 10.5k gallons of water @ pH 7.2 (fill water)

You wrote that initially the pH was > 6.8, but I suspect you meant < 6.8. Same with CYA that you wrote < 100 when I think you meant > 100. To measure 0 for TA, which means it turned red immediately, that implies a pH below 4.5 because that's the transition pH for the two indicators used in the TA test.

Yes Richard, you are correct in that I had my less than & Greater than signs all backwards. And yes, the TA test turned red immediately. Thanks for taking a stab at it. I made the same "assumption" that his pH was 4.5 or less based upon the pH not being affected by the initial 4 boxes of Borax AND the TA test turning Red immediately. I vaguely remembered you or someone else once making this statement in the past. Scary thing is that this doesn't seem to be that unusual. I know of at least one other pool that is probably in the same situation. I tried to offer a little advice but got "What I'm doing is working" so I backed off.
Is this 4.5pH dangerous to humans?

Dave

PS: I forgot to list it but the fill water has a TA of 80 ppm and the pH of 7.2 is actually a little higher based on the slightly darker shade.

[chem geek]

A pH of 4 or above will very likely sting the eyes if you open them underwater. Other than that, you may not notice it unless you are sensitive. Skin is an amazingly tolerant "organ". If you look at the Wikipedia entry for pH, you will find a range of substances and their pH here. The pH of this pool was in between beer and orange juice. Skin seems to tolerate, in terms of "sensing", acidic conditions a little better than basic/alkaline conditions, though extremes of either are very harsh.

I think the bigger problem with the low pH would be dissolving of plaster. This pool measured 0 CH so is it a vinyl pool? Also, that's strange to measure no CH at all -- even the fill water should have some even if it's "soft".

Richard

[DavidD]

This pool measured 0 CH so is it a vinyl pool? Also, that's strange to measure no CH at all -- even the fill water should have some even if it's "soft".

Richard

You'd think this was my first post or something. It initially showed CH as 0ppm (low pH maybe?) however the subsequent test showed 100 ppm and the fill water shows 70 ppm.

Speaking of "soft" water, any reason not to top off my vinyl liner IG with soft water other then the expense of the softener having to regenerate more frequently?

Dave