Getting TA under control.

[cschnurr]

Every 3-4 months I change the water in my hot tub.
The TA of my tap water is around 250ppm, pH greater than 8.0.
So I have to drop the TA to get the pH under control.

I use the pool calculator to drop the pH to 7.0-7.2.
I find this difficult because my pH is over 8.0, and climbs above 8.0 very quickly.
My test kit only measures up to 8.0, so I can't determine easily how much acid to add.
I've been assuming 8.0, add amount of acid necessary to lower from 8.0 to 7.0, wait and remeasure. Then repeat this 3-4 times until I can get the TA down to a reasonable level.
I find myself questioning the validity of the measurements and process.

Is there an easier way?
I was thinking of measuring pH above 8.0 would give me a better starting point, or is there a calculator that will determine the total amount of acid I will need to lower the TA from 250 to 100.

I took advantage of a warm spell yesterday to change the hot tub water but we had a major temperature swing. I changed the water yesterday and it was +50F (+10C), now I'm trying to adjust the chemicals in -4F (10 to -20C).

Thanks

 

[jseyfert3]

You can do an acid demand test. My Taylor K2006 kit came with it. You do a pH test, then drip an acid solution into the pH tube and mix. You count the drops needed to change the pH in the sample to whatever, then use the chart to see how much acid you need to add to drop your pH to that level.

You don't want to add enough acid to drop from 250 to 100 all at once, because that would take your pH very very low which could damage stuff.

 

[JoyfulNoise]

Which test kit are you using? The Taylor test kits will give you all the tests you need to determine pH and TA accurately. You could also purchase an inexpensive pH meter (they last about a year or so with good care) if you really want to know the exact pH value.

Once your TA drops below 80ppm, the pH rise will get slower. You should aim for 50ppm TA and then consider adding 50ppm borates to your water (there’s a commercial product called Gentle Spa that adds borates if you can’t get boric acid where you are on Canada). The borates will act as a secondary, high pH buffer that will help to keep the pH stable.

The acid/aeration process works as described and is your only viable method for TA reduction outside of some very complicated and expensive water filtration processes (ion exchange membranes using expensive anionic exchange resins or reverse osmosis filtration).

 

[cschnurr]

Thanks, I have a same test kit. I totally forgot about the acid demand test.
My main goal was to reduce the number of measurements, accurately measuring pH helps a lot.

It would be nice if there were a TA calculator that would determine the total amount of acid required to to drop from 250 to 100. That way I could have a reasonable idea how much acid I will need to add, and do so in doses to keep my pH above 7.

 

[JoyfulNoise]

Just use the Effects of Adding Chemicals section at the bottom of Pool Math. As long as your tub volume is set correctly, you can select the acid you’re using and then increase the amount added until the display results say your TA is reduced by 150ppm. You can’t trust the pH drop results because the calculator is not exact for large pH changes but that’s ok because you won’t be adding that much acid all at once anyway.