Low pH Brain Teaser

[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

[Spensar]

"21k gallon A/G vinyl"

From reading other threads, I was under the impression that low PH will degrade the vinyl and result in a shorter lifespan of a vinyl pool liner. If so, 4.5 is not good news.

[DavidD]

"21k gallon A/G vinyl"

From reading other threads, I was under the impression that low PH will degrade the vinyl and result in a shorter lifespan of a vinyl pool liner. If so, 4.5 is not good news.

You are correct. Low pH will shorten the life of a Vinyl Liner. I told my buddy that what ever life he would have had left in his 7 year old liner, it has been significantly shortened. BTW, low pH is also bad for the pump motor, variflow valve, sides of the AG pool due to splash out, and pretty much anything else.
Amazing thing is that he has been "running" the pool this way for 7 years and has not had any problems, YET

Dave

[chem geek]

Spensar, thanks for the catch. I was thinking plaster pools since I just did the calculation for dissolving of plaster. Of course low pH is terrible for vinyl as well as metal.