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