PH normal, TA always high

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
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PH normal, TA always high

Postby Adamant » Tue 23 Apr, 2013 04:06

Hi All,

Just wondering if anyone can advise, or shed some light on if I need to worry or not!

I have a small 250 gallon hot tub which I tend to drain and refill every month to 6 weeks depending on use.

I maintain a pretty stable pH of around 7.8 and free Chlorine of 5-6ppm with stabilised chlorine granules.

My TA pretty much just stays constant around 160ppm and never tends to move. Is a TA being slightly high like this likely to cause any damage to heaters / pumps etc?

I heat the tub to 40C constantly if this makes any difference!


chem geek
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PH normal, TA always high

Postby chem geek » Tue 23 Apr, 2013 20:27

Yes, with the pH on the high side and the TA high, you are risking calcium carbonate scaling, especially in the heat exchanger. The calcite saturation index will be greater than 0 if your Calcium Hardness (CH) is higher than 55 ppm.

The reason your pH is fairly stable is that you are balancing the net acidity of Dichlor (chlorinating granules) with the rise in pH from carbon dioxide outgassing that occurs more at the higher TA.

You could more than double the time between water changes if you were to instead use the Dichlor-then-bleach method, but to do that you'd have to add acid and aerate to lower your Total Alkalinity (TA) down to around 50 ppm (that's without CYA in the water, so on your next water change) following the procedure described in this post and would also need to add 50 ppm Borates (usually from boric acid) to provide additional pH buffering. If foaming is an issue, you can increase Calcium Hardness (CH) to 120-150 ppm because your TA is now low enough where that is not a problem.

For every 10 ppm Free Chlorine (FC) added by Dichlor it also increases Cyanuric Acid (CYA) by 9 ppm. As the CYA builds up, the chlorine becomes less effective so eventually is unable to keep up with bather load so the water turns dull/cloudy. This doesn't happen with Dichlor-then-bleach. You just need to add Dichlor about once a month to bring the CYA back up since it slowly gets oxidized by chlorine at roughly the rate of 5 ppm per month. See Dichlor/Bleach Method in a Nutshell, but as I noted target a lower TA and the borates are not optional.

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