The Great Tetraborate Experiment!

[chem geek]

OK, I finally put in the full Boric Acid equations into my spreadsheet and here's what I found out (assuming I didn't make an error). [EDIT] I initially made a mistake, but have corrected it and corrected the numbers below. [END-EDIT]

One box of 10 Mule Team Borax which you say is 4 pounds and 12 ounces (76 ounces total) and which I've been told is sodium tetraborate decahydrate, will add 9.78 ppm Borate (measured as ppm Boron) so it would take about 5 boxes to get to 50 ppm.

So, to get to 50 ppm in your 6,600 gallon pool, you need 388.6 ounces (24 pounds, 4.6 ounces -- or about 5 boxes plus 8.6 ounces) and 193.9 ounces (1 gallon, 8 cups, 1.9 ounces) of Muriatic Acid or 1.19 quarts per Borax box (you should alternate adding some Borax and then Muriatic Acid back and forth so as not to wildly swing your pH in your pool). This will increase your TA by 7.0 ppm CaCO3 equivalent and will increase your TDS by 416.1 ppm (77.0 from chloride ions from the Muriatic Acid and 339.1 from Sodium and Boric Acid from the Sodium Tetraborate) though some of this TDS increase (about 270 ppm) will not generally be seen in TDS measurements because some of this increase is with the neutral ion B(OH)3 and the standard TDS measurment is made using a conductivity test (which does not measure neutral ions).

It would appear that the rise in TA with Borax is more like a 7:1 ratio of ppm Boron to TA rise, not the 3:1 that Proteam claimed. The 7:1 is also consistent with my earlier calculation, but then I'm the same person doing both calculations so I certainly could have made the same mistake twice!
Perhaps the relatively small increase in TA is why you didn't notice it. It's less than 1 drop on the TA test and you had other things going in your pool in the meantime.

As for the buffering ability of 50 ppm Borate (Boron) in your pool, here's an example. Let's say that we use your numbers and have 2 moles (about 4%) of the total carbonate in your pool outgas as carbon dioxide. This would cause the pH to rise from 7.6 to 7.86 with no Borates in your pool. With 50 ppm Borate, the pH rise is only from 7.6 to 7.71 which is much less. It appears that the Boric Acid / Borate buffer system is quite efficient and is a good compliment to the Carbonate buffer system without having the side effect of outgassing carbon dioxide. [EDIT] Remember that TA is measured in ppm CaCO3 (100.0892 g/mole) while Total Borates are measured in ppm Boron (10.8117 g/mole) and that CaCO3 counts twice for alkalinity (because it can consume two hydrogen ions). So the Total Borates measured in the same units as TA would be 100.0892/10.8117/2 = 4.63 times larger so 50 ppm of Borates has a buffering equivalent closer to 230 which is why it is so effective (it doesn't increase measured TA by that much because most of it is Boric Acid which is uncharged -- this just shows that TA isn't really a measure of buffering capacity unless you use carbonates at a typical pool pH where the HCO3- species dominates). [END-EDIT]

This means that you should be able to run your pool with much lower TA and still get good buffering. For example, you can lower your TA to 80 and without the 50 ppm Borate your pH would rise to 7.97 but with 50 ppm Borate it would only rise to 7.73 so about the same as with the higher TA.

Richard