What Causes TA to Increase?

[HouTex]

I've searched the forum and found at least one post where someone says that TA can't increase on its own.

My pool is 14 months old and after about 11 months of adding about a gallon of MA (in 30-36 oz. portions) every week my pH finally stabilized somewhat. I now go through a gallon of MA in about 3 weeks or so. My TA ranges from 70 to 90 between MA additions, but it always creeps up before I add MA and it goes back down to around 70 (sometimes I get 60) after I bring the pH down to 7.5. Here are my most recent numbers:

FC 4
TC 4
pH 7.5 (after adding 20 oz. of MA yesterday)
TA 70
CH 550
CYA 60
Salt 3000
Borates 50 (maybe slightly less with recent rains)

My question is why does TA continually rise? The only thing that's been added to my pool (other than MA) is rainwater which I thought would lower the TA, not raise it.

 

[bobodaclown]

You could catch some rain water in a plastic container and measure it.

 

[HouTex]

Sure. Everything I've read about the TA of rainwater is that it has zero TA.

When my TA was increasing last year I assumed it was because of the draught and the weekly topping off with 220 TA tap water.

With a fairly steady series of rains this fall and winter, I haven't had to add water in a couple of months so I'm surprised my TA creeps up.

 

[Richard320]

I've searched the forum and found at least one post where someone says that TA can't increase on its own.

My pool is 14 months old and after about 11 months of adding about a gallon of MA (in 30-36 oz. portions) every week my pH finally stabilized somewhat. I now go through a gallon of MA in about 3 weeks or so. My TA ranges from 70 to 90 between MA additions, but it always creeps up before I add MA and it goes back down to around 70 (sometimes I get 60) after I bring the pH down to 7.5. Here are my most recent numbers:

FC 4
TC 4
pH 7.5 (after adding 20 oz. of MA yesterday)
TA 70
CH 550
CYA 60
Salt 3000
Borates 50 (maybe slightly less with recent rains)

My question is why does TA continually rise? The only thing that's been added to my pool (other than MA) is rainwater which I thought would lower the TA, not raise it.

Mine goes up all the time. I expect it to, because I have been working for a couple years to dissolve the calcium scale and it's finally looking like it. My CH climbs, too, but unfortunately, adding acid and aerating doesn't get rid of it.

 

[257WbyMag]

As others have alluded to, TA creeps up whenever one adds water from the tap. Because rain is the result of water evaporation, nothing other than a couple of H's and an O compose it. Yet tap water is almost always the culprit as it comes from reservoirs with mineral exposure in varying degrees.

Most folks here learn to not worry too much about TA. It seems to find its "happy place" when one adds acid to reduce rising pH. Keep your pH in check and your TA should fall in line fairly easily. If it doesn't, there is a process for lowering it.

 

[HouTex]

I've searched the forum and found at least one post where someone says that TA can't increase on its own.

My pool is 14 months old and after about 11 months of adding about a gallon of MA (in 30-36 oz. portions) every week my pH finally stabilized somewhat. I now go through a gallon of MA in about 3 weeks or so. My TA ranges from 70 to 90 between MA additions, but it always creeps up before I add MA and it goes back down to around 70 (sometimes I get 60) after I bring the pH down to 7.5. Here are my most recent numbers:

FC 4
TC 4
pH 7.5 (after adding 20 oz. of MA yesterday)
TA 70
CH 550
CYA 60
Salt 3000
Borates 50 (maybe slightly less with recent rains)

My question is why does TA continually rise? The only thing that's been added to my pool (other than MA) is rainwater which I thought would lower the TA, not raise it.

Mine goes up all the time. I expect it to, because I have been working for a couple years to dissolve the calcium scale and it's finally looking like it. My CH climbs, too, but unfortunately, adding acid and aerating doesn't get rid of it.

So it sounds like my TA is increasing because I'm dissolving scale? I don't think I have scale. Perhaps I'm dissolving the plaster itself.

OnBalance has written extensively here about the white spot etching phenomenon--and I think that's what I have. I've been keeping my CSI slightly negative (-.3 to 0) for most of the time, but I didn't think that level of CSI would be enough to dissolve anything, including scale.

But in addition to increasing TA, my CH has continued to slowly increase (or at least stay the same) even with the considerable rains we've had lately. Whatever it is, it seems to be coming from my plaster--whether it's scale or the plaster itself that's dissolving.

 

[chem geek]

But in addition to increasing TA, my CH has continued to slowly increase (or at least stay the same) even with the considerable rains we've had lately. Whatever it is, it seems to be coming from my plaster--whether it's scale or the plaster itself that's dissolving.

There will still be evaporation and refill going on even when the water is colder though it is obviously a lot slower. As for the rain water, it will likely have zero CH but will have a small amount of TA depending on its pH. At a pH of 7.5, the equilibrium amount of TA relative to the amount of carbon dioxide in air is 8 ppm while at a pH of 8.0 it is 25 ppm. So if your rainwater is higher in pH, then it could have more TA than you think.

However, since you are seeing a rise in both TA and CH or that the CH is not dropping with water dilution, then I think you are right that you might be dissolving calcium carbonate from plaster. If your pH was rising a lot, then that would make this scenario even more likely. Dissolving calcium carbonate to get a 10 ppm increase in CH and TA (they go together with calcium carbonate dissolving or scaling) would have the pH rise from 7.5 to 8.0 assuming a TA of around 80 ppm (with 30 ppm CYA). Unlike the pH rise from carbon dioxide outgassing, it won't slow down as the pH approaches 8.2 so 8.0 would go to 8.5 though it would slow down above that due to higher pH buffering from the carbonate buffer system.

I wouldn't expect a slightly negative CSI to be that aggressive. Though it would theoretically be safer to be closer to zero or perhaps even slightly positive, in practice except for fresh plaster in the first month or so where initially the hydration and subsequent hardening are most critical, plaster degradation would be expected to be very slow except in much more aggressive waters, especially at lower pH.

 

[RobbieH]

Because rain is the result of water evaporation, nothing other than a couple of H's and an O compose it. .

I disagree. Rain is slightly acidic when unpolluted, and can be even more acidic in the presence of pollution. Look up acid rain.

 

[HouTex]

But in addition to increasing TA, my CH has continued to slowly increase (or at least stay the same) even with the considerable rains we've had lately. Whatever it is, it seems to be coming from my plaster--whether it's scale or the plaster itself that's dissolving.

Dissolving calcium carbonate to get a 10 ppm increase in CH and TA (they go together with calcium carbonate dissolving or scaling) would have the pH rise from 7.5 to 8.0 assuming a TA of around 80 ppm (with 30 ppm CYA).

That's about where my pH ends up (around 8.0) if I go about 10 days without adding MA. I've yet to get a purple result showing a pH of 8.2--but I've never let it go that long without adding at least a little MA.

So what should I do? If I target a CSI of near zero that pretty much means I can't let pH get below 7.6 with a TA of 70. With continued mandatory water rationing (despite recent rains) I'm destined to have high CH in the 500-550 range.

 

[chem geek]

Because rain is the result of water evaporation, nothing other than a couple of H's and an O compose it. .

I disagree. Rain is slightly acidic when unpolluted, and can be even more acidic in the presence of pollution. Look up acid rain.

Rain is not pure water because it picks up other atmospheric components. Pure water would become acidic even without pollution because carbon dioxide in the atmosphere dissolving into pure water would make such water acidic. The pH would be around 5.7 in that case alone though the TA does not change solely from carbon dioxide dissolving into water. pH that is lower can result from pollutants such as sulfur dioxide. pH that is higher and that also increases TA can result from pollutants such as calcium oxide or sodium hydroxide.

 

[chem geek]

That's about where my pH ends up (around 8.0) if I go about 10 days without adding MA. I've yet to get a purple result showing a pH of 8.2--but I've never let it go that long without adding at least a little MA.

So what should I do? If I target a CSI of near zero that pretty much means I can't let pH get below 7.6 with a TA of 70. With continued mandatory water rationing (despite recent rains) I'm destined to have high CH in the 500-550 range.

First of all, don't panic. We don't know if you really have a plaster dissolving problem or not. Since your pH seems to slow down when it gets towards 8.0 it's not really definitive, but with a spill-over spa and an SWCG you are getting a lot of aeration and possibly some chlorine outgassing as well so those factors may explain the pH rise. You are already using borates to help slow down the rate of that rise.

Most certainly, there is absolutely nothing wrong with a high CH if your CSI isn't high. There are quite a few people on this forum who manage their pools in very high CH areas with CH in their pools around 1000 ppm and just keep their TA lower and it works out to avoid scaling. You probably don't want to get too much below 7.5 anyway since the lower the pH the faster the rate of pH rise. In fact, if there were no risk of metal staining (say, because your water is essentially metal free), there really wouldn't be a problem having the pH in the 7.8-8.0 range most of the time, but for SWCG pools we normally say to target around 7.7 anyway because we know that the pH tends to rise and that 7.5 is unrealistic for many SWCG pools.

The bottom line is that you have a lot of pH rise that you are trying to minimize so you are keeping your TA low as we normally recommend. The counter-balance to that is to have a higher CH level. The fact that this goes outside the recommended range for a specific parameter, namely CH, says more about our recommended range table for SWCGs not being quite right than it does about what you should do. The recommended ranges are a simplistic approach for those that don't have outlier situations or don't want to calculate the CSI. In your case, there is absolutely nothing wrong with your CH level or even a higher CH if that is stable for you and doesn't start resulting in scaling in your SWCG cell. With your use of borates, the risk of scaling in the salt cell is lowered.

Unless you have some visible or physically felt signs of plaster deterioration or pitting, I wouldn't worry about it and also wouldn't worry about the CH being higher so long as you don't see scaling in your salt cell since you would likely see it there long before you see it anywhere else.

 

[HouTex]

Chem geek, thanks for the advice and comforting words.

My plaster looks and feels fine except for the so-called white spot etching, which is not horrible in my case.

By the way, I inspected my salt cell a couple of months ago and it looked brand new--as I suppose it should after only 11 months.