Lowering alkalinity, Ben's method...

[chem geek]

Steve,

My calculations show that it will take a total of 34.5 cups of Muriatic Acid to lower your TA in your 12,000 gallon pool from 210 to 120 ppm. You obviously do not add this all in at once (more on this later). You will be outgassing nearly half of the carbonates in your water to carbon dioxide through aeration.

However, just to lower your pH from your initial 7.6 to 7.0 you would have needed to add about 13 cups of Muriatic Acid so I'm not sure why your drop test is so different (20 ounces is only 2.5 cups). It turns out that the resistance to changes in pH (in a carbonate buffer system) is greater at lower pH so going from 7.6 to 7.5 (at your 210 ppm TA) only takes 1.4 cups while going from 7.1 to 7.0 (if you started at 210 ppm TA with a pH of 7.1) takes 3.6 cups, but the acid/base demand drop test should show all of that properly.

I suggest you add at least some more Muriatic Acid if you are starting at a pH of 7.6, say 5 cups, then circulate the water for a bit by having the pump running but returns not pointed up (i.e. don't aerate yet -- we aren't trying to get rid of TA, but want to check the pH first), then test the pH after around 10-15 minutes. I predict your pH will be around 7.3. If it is, then my calculations are correct and for some reason your drop test is wrong. Perhaps your acid drops got weakened (though that would be strange since acid doesn't degrade very fast) or your tables are wrong (also strange).

I can walk you through the amount of acid needed at each stage to get from one pH to the other, at a measured TA, but your drop tests should be able to tell you that as well -- very strange indeed!

Richard