Borax

[abessellieu]

To Chem Geek: And let us know your results.

I am new at all this, and recently lowered my pH too much (to 7.2) and my plasterer recommended I keep it as close to 7.4 as possible. Since my TA was already 120, I wanted to raise my pH slightly w/o affecting TA, so I used your table to calculate how much Borax to use.

I have a newly (5 days ago) replastered pool that holds about 35,000 gal. By my calculations that is 2.19x larger than the 16,000 gal base you used.

According to your table 1 lb Borax with TA of 120 and pH of 7.2 will give a rise in pH of .07.

So I calculated 4 lb Borax would give me a 1.3 bump up in ph:

4/2.19 = 1.83 x .07 = 1.3

So I added 4 lb Borax at about 10 pm last night and at 8 am this morning my pH was 7.4. This was a little more than I expected, but I thought it might be due to the curing plaster. Doesn't it cause an upward creep in pH?

If this is the case it looks like your table is pretty accurate, but please let me know if you see any errors in my calculations or my logic.

I am still struggling with all of this, but each step I take seems to clear the fog a bit more. I really appreciate your taking the time with the tables and all, because at least it gives me a starting point. Otherwise I'd be totally clueless instead of mildly confused.

[chem geek]

Your calculations were correct.

Well, unless you are using an accurate pH meter, the colorimetric pH test is only accurate to within perhaps 0.1 if you're really good so I would say that you were in the ballpark. And yes, during the curing of plaster (gunite, cement, concrete, etc.) the plaster essentially absorbs water and releases calcium hydroxide ( Ca(OH)2 ) which is quite alkaline (basic) and causes the pH and CH to rise. Also, at your TA of 120 and pH of 7.2 your pool water had a somewhat strong tendency to outgas carbon dioxide (relative rate 24.6 where 15 is when many complain of pH rising) which would also cause your pH to rise. So between both factors, it's not surprising that your pH would rise even if you had not added any Borax at all.

[EDIT] One more factor to consider is that in the table you will notice that as you go higher in pH, the rise in pH due to 1 pound of Borax becomes greater. so instead of a .07 pH move you would start to get a .08 and then .09 pH move which when multiplied by your 1.3 factor becomes .10 to .12 instead of .09 though I believe the abovementioned pH rise factors are the more likely cause for the small discrepency. To really test the table, you'd have to have more significant pH moves in which case you should use the midpoint value from the table in the middle range between your starting and ending pH values. [END-EDIT]

Of course, if you or anyone else finds that my tables "seem wrong", please let us know. At this point, it seems that they may be correct.

Thanks,
Richard