TA and CO2 outgassing follow up

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Matt said:
Richard,

Are you essentially saying that the only reason we have any carbonates in our pool above 10ppm is that the rate constant of the reaction to form CO2 is so slow? Is it just a kinetics issue and not determined by the change in gibbs free energy of the entire reaction?

Matt
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Matt,

There are two reasons we have carbonates in most pools. First, is that they provide a pH buffer. Second, is that they enable saturation of the water with calcium carbonate (when calcium is also added to the pool water). The saturation of calcium carbonate protects plaster/gunite/grout surfaces from pitting (dissolving). One can have a lower TA and still have the water saturated if the CH or pH (or temperature) is higher.

The flip side of the carbonate portion (not the CYA or borates portion) of TA, however, is that higher TA and lower pH result in faster outgassing of carbon dioxide resulting in a faster rise in pH (or a slower decline, if acidic sources of chlorine are used).

And yes, the lack of equilibrium is due to the slow kinetics of carbon dioxide transfer between water and air.

Richard
 
Is there a drops-based pH test?
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Well... sort of. I use the Taylor K-2006, and I rely heavily on the acid and base demand tests (well, really just the acid one in my case... "base demand... what's that?") There are indicators that can give you a better reading of pH over a wider range than phenol red. Phenol red is used for pools because it works well in the neutral range. There are also electric pH meters that we use in the lab. We put the probe in solution and it gives a continuous pH reading (over a range of 0-14, to 2 decimal places). Not sure what they cost... since I don't personally own one. I believe there are some pool testing electronic versions, but I'm not sure how accurate they are.

You can also try to look for various types of litmus paper that will work in a specific pH range, or look for other indicators. Phenolpthalein, for example, turns from completely clear to bright pink as soon as the pH hits 8.2 (<8.2 it's clear, >=8.2 it's pink). This might be useful in some circumstances. There are other indicators that work at different pH's. Overall though, phenol red is the best single indicator for the ranges used in pools.
 
There are two reasons we have carbonates in most pools.
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Sorry Richard, I think my question was poorly phrased. I wasn't asking why carbonates are desirable... that part I get. I was hypothesizing that the kinetics of the outgassing reaction are the only reason that we are able to physically maintain carbonates in the water above 10ppm... I think you answered that question too though.

I was looking at it from a thermodynamic point of view, when I should have been looking at it from a kinetic point of view. The rate limiting step must be pH dependent then, and facilitated by lower pH...
 
I suppose that should have been obvious to me, as outgassing is spontaneous at all pH's, just at different rates. I'm surprised the thermodynamic equilibrium is as low as 10ppm though.
 
Titanium said:
Umm, this is the first time that I've heard that CYA is a suspected carcinogen.
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There are no reputable studies indicating problems but there have been some concerns, perhaps it would be better to call them fears, that higher levels of CYA could be carcinogenic, and this has resulted in some areas putting upper limits on how high CYA is allowed to go in public pools.
 
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