Alkalinity reduction

[bizbad]

My Taylor book claims if you slug muratic acid in the pool rather than spreading it and splashing it, you will lower TA rather than reducing ph. Is there any truth to this?

[chem geek]

No, there is very little truth to this. The theory is that the slug approach is so highly acidic in a concentrated area that it drives the carbon dioxide from that area into bubbles that come out of the water. The problem is that simple dilution math shows that there simply isn't enough TA in a small volume of water to make a difference even if ALL of the carbon dioxide was driven out. To make any real difference, you need the entire volume of pool water, and specifically the surface of the pool water, to be involved in getting rid of dissolved carbon dioxide.

For example, if you added enough acid to make a pool with a TA of 100 go from a pH of 7.5 to 7.2, then after the acid has diluted with 10% of the pool water (and it doesn't dilute thoroughly anyway, but let's assume that it did), the pH would be about 6 and would not drive out all of the carbon dioxide. Essentially, to get the pH low enough to instantly form bubbles of carbon dioxide, similar to a carbonated beverage, you have to add enough acid to completely overwhelm the TA in an area and that comes to a pool water volume of around 6.5% and even then the pH is only down to around 4.5. It's only for pool water volumes 5% and lower where any bubbling effect might occur so at best the slug approach would lower the TA by no more than 5% for a significant slug (2 cups per 10,000 gallons). In practice, you don't get anywhere near that kind of TA reduction (after accounting for the pH drop -- see below) and you can prove this to yourself by seeing if you see any bubbles coming from the water when you pour in a slug of acid (pouring smoothly so as not to create the bubbles yourself from splashing).

Adding acid will lower both the pH and the TA making one think that one has actually lowered the TA when in fact they have not, if one restores the pH back to where it was. The only way to truly lower TA (without changing the pH) is to outgas significant amounts of carbon dioxide from the water (which raises the pH) and add acid to bring the pH back down. Doing this at low pH and with lots of aeration makes the process go much faster. See this thread for details on the proper procedure.

Richard