Diffusing CO2 to control PH

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

Diffusing CO2 to control PH

Postby Plutonium » Wed 23 Sep, 2009 03:18

OK, based on titrating with a strong acid down to a pH of 4.2, then I will agree that the hydroxyl ion will count toward the total alkalinity and you are correct on both counts.

I had considered Total Alkalinity to be based on any movement of the pH.


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Diffusing CO2 to control PH

Postby chem geek » Wed 23 Sep, 2009 10:18

Yes, that's exactly how the TA test works -- titrating with a strong acid down to a pH of around 4.5 when the indicator dye changes color. The indicator dye is typically a combination of methyl red and bromocresol green that goes from green to red and stops changing at around a pH of 4.5.

However, it's not just the specifics of the test but rather the technical definition of TA itself. Though I don't always like Wikipedia since it is sometimes inaccurate, it currently does describe the definition reasonably well here . You can see that the definition includes the terms [OH-]-[H+] since hydroxyl ion can (weakly) buffer against acid addition while hydrogen ion itself is the opposite and represents negative alkalinity. The difference between these is the pH buffering capacity of pure water which is zero at a pH of 7.0.

Technically, in pure water, the TA test is off since one drop of titrant in the test would change the sample from green to red and be recorded as 10 ppm though further analysis would show that an extremely small amount of titrant would do the same so in practice the TA is near zero in the test. If one adds TA indicator dye to a water sample and it immediately turns red, then this means that the pH is at or below 4.5 and the TA is zero (technically, it's negative).
Plutonium

Diffusing CO2 to control PH

Postby Plutonium » Wed 23 Sep, 2009 14:05

OK, some references use a double endpoint titration method where the sample is titrated to a pH of 4.5 and then to 4.2 for extra accuracy. That's where I got the pH of 4.2 as shown in these references.

epa wrote: Total alkalinity is measured by measuring the amount of acid (e.g., sulfuric acid) needed to bring the sample to a pH of 4.2.

At pH 4.5, it is certain that all carbonate and bicarbonate are converted to carbonic acid. Below this pH, the water is unable to neutralize the sulfuric acid and there is a linear relationship between the amount of sulfuric acid added to the sample and the change in the pH of the sample. So, additional sulfuric acid is added to the sample to reduce the pH of 4.5 by exactly 0.3 pH units (which corresponds to an exact doubling of the pH) to a pH of 4.2.

http://www.epa.gov/volunteer/stream/vms510.html


The United Nations Geographical Information Working Group wrote: A known volume of the sample aliquot is titrated with a standardized solution of H2SO4 (or HCl), to pH=4.5 then to pH=4.2, using an automatic titrator and a pH meter calibrated for 25oC. The total alkalinity is found from both titration volumes. A two endpoint technique is employed to determine the actual inflection point.
http://www.ungiwg.org/openwater/?q=book/export/html/113


umass.edu wrote: Total alkalinity is measured by collecting a water sample, and measuring the amount of acid needed to bring the sample to a pH of 4.2.
http://www.umass.edu/tei/mwwp/phalk.html


I'm assuming that the standard pool test kit probably uses a pH of about 4.5 or just slightly below to ensure accuracy.
Plutonium

Diffusing CO2 to control PH

Postby Plutonium » Wed 23 Sep, 2009 14:20

The Wikipedia reference that you give contains this:

Wikipedia wrote:Dissolution of carbonate rock
Addition of CO2 to a solution in contact with a solid can affect the alkalinity, especially for carbonate minerals in contact with groundwater or seawater . The dissolution (or precipitation) of carbonate rock has a strong influence on the alkalinity. This is because carbonate rock is composed of CaCO3 and its dissociation will add Ca+2 and CO3−2 into solution. Ca+2 will not influence alkalinity, but CO3−2 will increase alkalinity by 2 units.


This could explain some of the increase in alkalinity reported by some pool operators. If some of the calcium carbonate or calcium hydroxide in the plaster is dissolved, then the total alkalinity could increase.
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Diffusing CO2 to control PH

Postby chem geek » Wed 23 Sep, 2009 18:40

Thanks for the links and explaining where the 4.2 came from. The standard TA tests in pool kits use a single transition point at 4.5 and are only accurate to within one drop (i.e. 10 ppm). That's more than good enough for the purposes of calculating the saturation index.

If the rise in TA is from the dissolving of plaster (which would also raise the pH) then that's not a good thing, of course. The saturation index should be kept close to zero and adding carbon dioxide to the pool should not be done with the carbon dioxide getting too close to plaster surfaces. This would be similar to adding acid near such surfaces.

Fill water almost always has a reasonably high amount of TA in it -- usually at least 50 ppm. My tap water has close to 80 ppm. If there is evaporation and refill, then whatever is in the fill water gets added to the pool (evaporation does not remove anything but water from the pool) so generally that causes an increase in both TA and CH, though this can be a slow rise. My own pool tends to slowly have the TA rise over a season and this is probably the reason -- it's that slow because I have a pool cover so the pool is only open for 1-2 hours every weekday plus longer on weekends. My tap water has a CH of 50 ppm so the rise in CH is not as noticeable and splash-out/refill (I have a cartridge filter, so no backwashing) would tend to lower the CH so the net is pretty constant.

You can see annual pan evaporation rates here where 50" (more than 4 feet) is not uncommon. The average depth of a typical 6 foot deep pool is around 4.5 feet so that's around 90% of the pool volume being added with fill water so roughly increasing the pool's TA by that of the fill water. Even indoor pools evaporate quite a lot of water since this has a lot to do with warm water and drier air.

The use of carbon dioxide for lowering the pH would make any sources of rising TA more noticeable. When using a strong acid to lower the pH, the TA would get lowered as well, usually faster than such sources of rising pH so that baking soda needs to be added on occasion to raise the TA.

Richard
johns fountains

Diffusing CO2 to control PH

Postby johns fountains » Wed 14 Dec, 2011 03:07

The key is to use c02 with a mixer to add some muriatic acid: Balance the two: If the TA gets to high simply turn off the c02 and use acid: This makes the use cost effective: This is how it is done with large institutional swimming pools: Injecting a small amount of muriatic acid will normally keep the TA down: The water will become cloudy if just C02 is used to reduce the PH:

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