TA/PH balance with high CH

Wildcat · Sep 9, 2015

High ch, 460, in Arizona. Try to keep CSI low to prevent scaling. Keep PH at 7.2-7.5, TA at 60-75. I can keep it in balance, but as I add muriatic acid to keep PH under control (it has a tendency to rise even though water feature is not run often and low bather agitation) which lowers TA. So then I have to add baking soda to raise TA and then acid to lower PH. Vicious cycle. Is that just the way it is or am I doing something wrong?

CYA 50, FC always within range. No other issues.

JVTrain · Sep 9, 2015

Don't see a SWG in your signature so I assume manual chlorination.

If I plug in your numbers to Pool Math with a slightly higher pH (pH 7.8, TA 60, CH 460, CYA 50, Salt 1500 as you're likely to have some in there, temp 90) I see that your CSI would be about 0.17. Certainly in the safe range and not in danger of scaling. The reason I suggest a higher pH is that it's still safe for not scaling but at higher pH levels, carbon dioxide outgassing is slower, so your pH rise will be slower. Try letting it drift up to 7.8 and only lower it when it rises above that. This should produce lower overall pH rise, less acid additions.

Adding 50 ppm of borates can also help stabilize pH rise if this change is not sufficient.

chem geek · Sep 10, 2015

You could also safely target an even lower TA in addition to the higher pH target. The combination should slow down the rate of pH rise and especially the amount of acid you need to add. As shown in this table a lower pH is significantly more over-carbonated. I wouldn't try lowering the pH below 7.5. Just let the TA fall further with your acid additions. You may find decent stability with a TA of 60 ppm and pH of 7.8 or some similar combination.

Wildcat · Sep 10, 2015

OK, I just thought a PH of around 7.5 was conducive to stable FC levels

chem geek · Sep 10, 2015

In terms of the active chlorine level vs pH, CYA buffers the active chlorine so from a pH of 7.5 to 8.0 the active chlorine level only drops by 15% (see this post for graphs showing this effect).

Now if you are referring to losses from sunlight, it is true that higher pH loses more chlorine from hypochlorite ion, but that's a fairly small amount. With 4 ppm FC and 50 ppm CYA the worst-case (if there were no CYA UV shielding effect) unbound chlorine loss from sunlight at pH 7.2 loses 0.44 ppm FC per day while at pH 7.5 loses 0.66 ppm FC per day while at a pH of 7.8 loses 1.06 ppm FC per day. So from a pH of 7.2 to 7.8 there is about 0.6 ppm FC per day higher loss, again if there were no CYA shielding effect. However, since there is CYA (or CYA-Cl) shielding of UV the different loss rate vs. pH is even smaller.

It's certainly worth experimenting with a lower TA and a higher pH target since it's easy to go back if you decide to do so.

sloop · Sep 10, 2015

Wildcat said:
High ch, 460, in Arizona. Try to keep CSI low to prevent scaling. Keep PH at 7.2-7.5, TA at 60-75. I can keep it in balance, but as I add muriatic acid to keep PH under control (it has a tendency to rise even though water feature is not run often and low bather agitation) which lowers TA. So then I have to add baking soda to raise TA and then acid to lower PH. Vicious cycle. Is that just the way it is or am I doing something wrong?

CYA 50, FC always within range. No other issues.

I am in the same scenario as you. My CH is 450 and I have been keeping my PH towards the lower end after reading a thread from Richard i believe , about managing high CH by keeping his PH lower. My pool is fairly new and PH rises pretty quick with my Ta ending back around 80 but having gone as low as 40. I see on this thread they are saying to let the PH drift up to 7.8 ( Mine goes right on past that ) Before lowering it. My question is, which is correct? Should i be keeping my PH towards the low end as much as possible or just ride it up and then lower it once it goes above 7.8?

JVTrain · Sep 10, 2015

sloop said:
I am in the same scenario as you. My CH is 450 and I have been keeping my PH towards the lower end after reading a thread from Richard i believe , about managing high CH by keeping his PH lower. My pool is fairly new and PH rises pretty quick with my Ta ending back around 80 but having gone as low as 40. I see on this thread they are saying to let the PH drift up to 7.8 ( Mine goes right on past that ) Before lowering it. My question is, which is correct? Should i be keeping my PH towards the low end as much as possible or just ride it up and then lower it once it goes above 7.8?

If you look at the chart that Richard linked, http://troublefreepool.com/~richardfalk/pool/CO2.htm, you can see that for a given TA, the lower pH ranges have a larger number. That is, there is more carbon dioxide dissolved in the water than it would be at equilibrium. This means as said earlier at lower pH levels for a given TA, carbon dioxide outgassing will be faster/more likely to happen than at higher pH levels. The faster the carbon dioxide outgasses, the faster the pH rises.