Borax

[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