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Carbon Dioxoide #3 of 4

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CO2 in Pool Water #3 of 4
While some of the following information has already been mentioned, let’s specifically address the question raised at the beginning of this update series. Why does the pH eventually rise after acid initially makes it drop? Again, the answer lies in the fact that whenever the pH is below 8.2-8.3, there is generally more CO2 in water than its natural equilibrium level with the atmosphere. Because of this, the extra CO2 will off-gas into the atmosphere over time. The pH rises because CO2 is off-gassing from the pool water. Unless checked, the pH will continue to rise until the CO2 reaches its equilibrium or about 8.3. The more alkalinity (over 80 ppm), the stronger pull to a pH of 8.3. This is one of the difficulties that pool service techs have to deal with. Generally, there is no getting around this.

But there are exceptions to the above rule. For instance, a common but occasionally unrecognized factor which interferes with CO2 off-gassing or absorption is a pool cover. When pools are covered with non-gas permeable covers, such as the common blue bubble solar blankets or solid vinyl or plastic automatic safety covers, the exchange of gas from water to air and air to water is blocked.

With newer plaster pools, a vast supply of hydroxide (a component of the plaster surface) is exposed to the water and its chemistry. If a pool cover blocks the otherwise natural process of CO2 off gassing, the CO2 reacts with the plaster surface and, together with hydroxide, form carbonate, thereby reducing CO2 in the water. Eventually, all aqueous CO2 could be depleted, causing the pH to climb to 8.4, and the pool cover would not allow more CO2 from the atmosphere to dissolve into the water to keep the pH from rising even higher than 8.4. At this point, dissolved calcium in the water would probably begin to precipitate and produce scale on the floor and walls.

In vinyl, painted, and fiberglass pools, on the other hand, no ready source of hydroxide is available, so pool covers on these pools can keep the pH artificially low when inhibiting the ability of CO2 to off-gas. When acid or an acidic sanitizer is added to this type of pool that has a non-permeable cover on it, the CO2 generated (by the acid) will stay in the water, and will not be able to off-gas. Therefore, the pH will probably remain low and unchanged until either other chemicals are added, or the cover is removed.

When non-permeable covers are used, pH needs to be watched carefully. Ideally, the cover should be removed for enough time to allow gases to equilibrate (perhaps 6 to 8 hours, twice a week for residential pools). When this is not an option, careful control of pH using acids and bases must be maintained. How will you know if the pool contains the right amount of CO2? The pH will be balanced.

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Comment by Richard A. Falk on July 14, 2009 at 7:22pm
I don't think the point is to run the pool at a high pH, but rather that a pool with "normal" amounts of TA typically recommended will tend to rise towards that higher pH. I think one main takeaway from the series of articles is that ironically a lower TA can lead to much greater pH stability. Yes, this is counterintuitive, but is very true when you are using hypochlorite sources of chlorine. The reason is that TA is mostly a measure of bicarbonate and that carbonates in the water (including dissolved carbon dioxide) do TWO things: they act as a pH buffer AND they are a source of rising pH themselves due to carbon dioxide outgassing. The latter effect overwhelms the former effect when the TA is higher so it turns out that lowering the TA slows down the rate of pH rise.

As an extreme example, if you were to raise the TA to 200 ppm (and assuming the CH is low so you don't get scaling) and just wait, you would notice the pH rising over time far more rapidly than if the TA were at 100 ppm. If the pH tends to rise at 100 ppm, then having it at 50 ppm would have such rise slow down considerably. The following chart shows how far out of equilibrium (that is, how "over-carbonated") pools are at various TA and pH levels. Just like a carbonated beverage, pools will outgas their excess carbon dioxide and this process causes the pH to rise (with no change in TA for technical reasons I won't get into here).
Comment by Alan Lewis on July 14, 2009 at 3:18pm
If this discussion s refering to public or large commercial type pools, then I ask the qestion: Why are we discussing what happens when the pH rise 8.3-8.4? Surely in a public pool it is highly counter-productive to run the pH so high. Remembering that he whole point of buffering is to avoid pH rising (exponentially) as Hypochlorite is dosed, - then having reached a pH so high you now need to dose 3 or 4 times the amount of Hypochlorite to achieve the same ORP. What logic is there in this this? The more chlorine you need to disinfect the water - the worse the situation becomes and the greater the level of undesirable chloramines in the pool.
Comment by Richard A. Falk on June 29, 2009 at 1:20pm
Yes, a higher TA level is at equilibrium with CO2 in the air at higher pH, but at a TA of 200 the pH at equilibrium is indeed around 8.7 to 8.8. If a pool had a TA of 500 ppm, then the pH (with sufficient aeration to accelerate movement towards equilibrium) would go towards around 9.1. What I said before in terms of equilibrium "pairs" of pH and TA is consistent with what I said below. A higher pH does indeed support a higher equilibrium TA, but that means the converse is also true that at higher TA the equilibrium pH is higher as well.

This is all rather academic since most pools don't get that high in TA and they significantly slow down in their outgassing rates long before such equilibrium pH values. It's just that this all encompassing idea of there being a single pH that carbonate buffer systems migrate to is simply not true as it depends on the TA level itself since (at a given pH) it is proportional to the amount of carbon dioxide in the water.

Remember that equilibrium occurs when the carbon dioxide in the air and in the water are in a ratio corresponding to Henry's Law constant. The concentration in the air isn't changing and the amount in the water is proportional to TA and also greatly affected by pH. If the TA is higher, the equilibrium pH must be higher for there to be the same amount of aqueous carbon dioxide in the water.
Comment by Kim Skinner on June 25, 2009 at 3:11pm
I am in agreement with most of your comments with the exception that the pH does not tend towards 8.7 or 8.8 at an alkalinity of 200. As you stated in another post comment, the higher the alkalinity, the higher amount of CO2 can be dissolved into the water. Therefore, with increasing alkalinity, more CO2 will be taken in from the atmosphere, and the pH will tend to stay at 8.3 -8.4.
Comment by Richard A. Falk on June 25, 2009 at 1:13pm
As noted in another comment to a different post, it is not true that the pH always migrates to 8.3 or 8.4. It depends on the TA level. The pH will remain stable at pH 7.5 with no net carbon dioxide outgassing if the TA level were around 10 ppm (with no CYA) since that is roughly the equilibrium amount of carbon dioxide in water at that pH. In practice, at normal TA of 50 ppm or above, the pH will tend to rise as you described and the 8.3 to 8.4 rule is correct for a TA in the ballpark of 100 ppm. At a TA of 200 ppm, for example, the pH will tend towards 8.7 or 8.8. At a TA of 50 ppm, it will tend towards 8.1 or 8.2.

It is counter-intuitive, but true, that a lower TA level will result in greater pH stability (slower pH rise) in pools using hypochlorite sources of chlorine.

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