Lowering alkalinity, Ben's method...

[steveinaz]

Correct, the 20oz's of muriatic acid is dropping ph to about 7.1, then the aireation is helping ph climb back into a "normal" zone while lowering TA...

The PH numbers above are before the addition of the muriatic acid, for clarification. I'm using the acid demand test to figure out how much muriatic I'll need to get to 7.1.

My acid demand was about 8oz per day, now I'm running 20oz every 12 hours (well, so far), I was afraid to go any heavier on the acid for a 12,000 gallon pool.

am I missing something? Do I need to verify how low ph is going? If so, how long should I wait after adding the acid to test? I want to do this right the first time...

[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

[steveinaz]

There are alot of variables, maybe I'm misreading the drops needed to get to the PH I want, or possibly my pool is closer to 15,000 gallons than it is 10,000 gallons (the values on the chart). I've been using the 10k chart to be safe. The PB estimated it at 12,000 gals.

Anywho, some things have come up that aren't going to allow me to finish this now--properly, but probably sometime after the first week in April I'll give it another go.

The acid demand was not too bad prior to this, I was only needing a little over a cup to get my PH a perfect 7.5. Overall I'm really pleased with the numbers, and how easy it has been to keep them in the "good" zone. This mornng my numbers were:

FC=5.5
CC=0
PH=7.6
TA=190
CYA=45

The trend for the last 5 days has been: at 4pm I add 2 pints of Clorox (adds approx 1.1ppm FC) and 1 cup of muriatic acid, and the water is crystal clear. I'm losing about 1ppm of chlorine per 24hr period.