Is There A Relationship Between Cyanuric Acid And Ph

[macgd016]

I have had my pool for 6 years but 2 years ago I installed a liquid chlorine dosing system. One of my reasons for fitting the chlorine pump was that my pool is in southern Spain where they have perpetual water shortages so pumping out water to reduce cyanuric acid did was not an acceptable option. Before I installed the chlorine pump my pH was rock steady but the cyanuric acid massively high, I estimated it was over 750. The pool is fairly small holding 50,000 litres, a typical weekly top up in the summer is 1000 litres.

After two winters of above average rainfall my cyanuric acid is now down to 80 but now my pH constantly drifts upwards. From what I can see there is a perpetual fight between high TA and rising pH. Typically before my weekly maintenance the pH will be around 7.8 and the TA 150, after topping up with 1000 litres I add 8x75ml scoops (600g?)of pH minus, 24 hours later the pH will be down to 7.6 and the TA to 50, but a week later they are back to 7.8 and 150 or there abouts. I have a very basic understanding of the relationship between TA & pH so I can see why they are fluctuating although I would prefer to have them more stable but I do wonder why the pH used to be rock steady but now fluctuates, was it the cyanuric acid?

[waterbear]

No, there is no relationship between CYA and pH but there is a relationship between TA and pH. When you were adding trichor you were also constanlty adding acid because trichlor is very acidic (very acidic on addition and even more acidic when the chlorine is consumed). Liquid chlorine is basically pH neutral on use (alkaline on addition and acidic when the chlorine is consumed). The primary cause of pH rise in ANY pool or spa is outgassing of CO2 and the higher the TA the faster the outgassing. (This does not include new plaster pools with curing plaster that put a lot of alkaline material in the water for the first 6 months or so.) The faster the outgassing the faster the pH rise.

Get your TA down to about 80 ppm and you will find that the pH becomes much more stable. Also, when you lower the pH do not lower it below about 7.6 and wait until it climbs to above 7.8 before lowering it again. (but you do need to monitor it so it does not go to 8.0 or above and stay there for an extended time.) It's really easier than it sounds.

I would strongly suggest reading this post and if you have any more questions about what TA is, how it effects pH or how to PROPERLY lower it then please post them.

The reason your pH used to be steady and now rises is because your high TA was constantly driving your pH up to around 8.4 but you were adding a lot of acid via the trichlor without realizing it.

If you lower the TA your pH will not rise as fast

[chem geek]

Typically before my weekly maintenance the pH will be around 7.8 and the TA 150, after topping up with 1000 litres I add 8x75ml scoops (600g?)of pH minus, 24 hours later the pH will be down to 7.6 and the TA to 50, but a week later they are back to 7.8 and 150 or there abouts.

600 ml (about 864 grams) of sodium bisulfate dry acid in 50,000 liters lowers the TA by about 7 ppm. Going from a TA of 150 to a TA of 50 ppm makes absolutely no sense whatsoever. Are you using test strips? If so, they are practically useless. Your TA is probably high the whole time which, as waterbear said, explains your pH rise.

[macgd016]

Typically before my weekly maintenance the pH will be around 7.8 and the TA 150, after topping up with 1000 litres I add 8x75ml scoops (600g?)of pH minus, 24 hours later the pH will be down to 7.6 and the TA to 50, but a week later they are back to 7.8 and 150 or there abouts.

600 ml (about 864 grams) of sodium bisulfate dry acid in 50,000 liters lowers the TA by about 7 ppm. Going from a TA of 150 to a TA of 50 ppm makes absolutely no sense whatsoever. Are you using test strips? If so, they are practically useless. Your TA is probably high the whole time which, as waterbear said, explains your pH rise.

I use a tablet test kit not strips. I believe the mains water has a very high TA (240+) and pH of more than 8 which is one reason why the pH and TA rise each time I top up the pool. I repeated the test above three time and got very similar results each time. Last winter (no topping up) I did manage to get the TA down to 80 and pH to 7.4 and it stayed there nicely until the pool was first topped up so I will see whether I can do the same this winter and then check what happens when the pool is topped up in the Spring. Hopefully I will have a Taylor K-2006 by then so I can trust the test results.