Some general Bicarbonate/CO2 related questions

[bobfloyd]

Hi,

I have some general questions (and more to come depending on the answers to my first "batch")...

A glass of pure water (pH 7), sitting on the kitchen bench, aerated.

My understanding is that small amount of CO2 will dissolve in the water...

CO2 + H20 = H2CO3 (Carbonic Acid).

So the water will end up with a very slightly acidic pH - just like how "acid rain" forms.

My question is - I understand that the H2CO3, weak acid, will form an associated conjugate base comprised of HC03 (Bicarbonate) and C03 (Carbonate).

In my simple example above, given that a small amount of Carbonic Acid will form, does this also mean that a small amount of Bicarbonate will form (but I guess no Carbonate due to the pH being low)?

So this would therefore mean that the water will have a very low natural Total Alkalinity due to the very slight carbonate hardness?

I know we are talking very small amounts here, but, nevertheless, is my understanding correct?

Cheers.

 

[JoyfulNoise]

Yes, pure water sitting in air will absorb only about 5ppm CO2. This is why rain water has low but measurable TA (if you do the test right). Carbonic acid doesn't really exist in aqueous solution - at equilibrium there 's [EDIT] see Post #4 below for more accurate numbers [END EDIT] more dissolved CO2 than carbonic acid in a pure water solution. At neutral pH, carbonate hardness of any kind is almost entirely composed of the bicarbonate ion. Pool water is over-saturated with CO2 which is why there is always a driving force for CO2 to outgas and cause the pH to rise.


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[JamesW]

The bicarbonate is offset by a hydrogen ion. Therefore, there is no change in TA when carbon dioxide is dissolved in water or when it comes out of solution.

Pure water with a pH of 7 does not have any real alkalinity. Technically, it has a pH over 4.5 due to hydroxide ions, so it will test for some alkalinity.

 

[chem geek]

With CO₂ at 400 ppm in air, pure water would absorb carbon dioxide and result in the following amounts of each carbonate-like species:

CO₂(aq) .... 13.51 mM
H₂CO₃ ........ 0.02 mM
HCO₃^- ........ 2.46 mM
CO₃ ............ 0.000048 mM
H⁺ ............. 2.46 mM (pH 5.6)
OH^- ............ 0.0041 mM

The Total Alkalinity (TA) does not change because bicarbonate and hydrogen ion both rise by the same amount and hydrogen ion counts negatively towards TA.

................ +TA .... -TA
H₂CO₃ ---> HCO₃^- + H⁺
Carbonic Acid ---> Bicarbonate Ion + Hydrogen Ion

Now in practice when doing the TA test one adds acid until the pH indicator in the test changes color which is around a pH of 4.5 so in this case it takes around 4 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons to accomplish this, but this translates into a TA reading of only 1.6 ppm.

The reason for the discrepancy is in this definition of TA for the test which is only a reasonable approximation designed to measure TA with reasonable amounts of carbonate buffer. The pH of 4.5 generally exhausts that buffer, but the test ignores the fact that the water is already starting out acidic -- so much so that the acid amount roughly equals the bicarbonate amount. When one is testing pool water, one usually starts out with a pH that has the hydrogen ion concentration be far, far lower than that of bicarbonate ion.

P.S.
The ratio of aqueous carbon dioxide to carbonic acid is roughly 650 (see this link showing an equilibrium constant ratio of 1.3x10^-3 which inverted is 769 while Wikipedia says 1.7x10^-3 which inverted is 588 while this source gives 0.00159 which inverted is 629 and so on where I use 650 I believe from CRC Themodynamic or other tables).

 

[bobfloyd]

Thanks guys.

So that aqueous CO2 to carbonic acid ratio, that is fixed for a certain temperature, pressure correct?

If I understand correctly, there are also ratios for the carbonic acid, bicarbonate and carbonate as well, and these ratios shift with pH?

Can I have the CO2 bringing the pH down but not the TA bit explained again? Does the co2 reduce the OH- count? I can't see how it would increase the H+ count. Strictly speaking the TA will go up very very slightly as CO2 is "forced" into the water (I believe)....

 

[chem geek]

So that aqueous CO2 to carbonic acid ratio, that is fixed for a certain temperature, pressure correct?

Yes it is. It is an equilibrium constant defined at Standard Temperature and Pressure (STP).

If I understand correctly, there are also ratios for the carbonic acid, bicarbonate and carbonate as well, and these ratios shift with pH?

And yes, the ratios for the carbonates shift with pH because they are equilibrium constants that have the hydrogen ion concentration in them.

Ka(1) = [HCO₃^-] * [H⁺] / [H₂CO₃]
Ka(2) = [CO₃^2-] * [H⁺] / [HCO₃^-]

where the carbonic acid concentration by convention includes aqueous carbon dioxide which is the actual dominant species.

Can I have the CO2 bringing the pH down but not the TA bit explained again? Does the co2 reduce the OH- count? I can't see how it would increase the H+ count. Strictly speaking the TA will go up very very slightly as CO2 is "forced" into the water (I believe)....

When you add CO₂ to water, it forms carbonic acid. There is no change in TA with that since you haven't yet created any chemical that can accept a proton (which is what defines alkalinity). The carbonic acid dissociates into bicarbonate ion which increases TA and hydrogen ion which decreases TA so these exactly cancel each other out. I showed you this before in an earlier post where you can see how carbonic acid creates hydrogen ions. Adding or removing a weak acid or base (e.g. sodium carbonate, carbonic acid, boric acid) does not change the TA because the pH is shifted to counter the TA change from the alkaline species. Adding a strong acid or base (e.g. hydrochloric acid or sodium hydroxide) changes the TA as does adding the salt of a weak acid or base (e.g. sodium bicarbonate).

CO₂(g) ---> CO₂(aq)
Carbon Dioxide Gas ---> Aqueous (dissolved) Carbon Dioxide

CO₂(aq) + H₂O ---> H₂CO₃
Aqueous Carbon Dioxide + Water ---> Carbonic Acid

................ +TA .... -TA
H₂CO₃ ---> HCO₃^- + H⁺
Carbonic Acid ---> Bicarbonate Ion + Hydrogen Ion

So adding (or removing) carbon dioxide does not change the TA. It only changes the pH.

............................ +TA
NaHCO₃ ---> Na⁺ + HCO₃^-
Sodium Bicarbonate (baking soda) ---> Sodium Ion + Bicarbonate Ion

So adding baking soda increases TA.

.......... -TA
HCl ---> H⁺ + Cl^-
Hydrochloric Acid ---> Hydrogen Ion + Chloride Ion

So adding hydrochloric acid decreases TA.

Total Alkalinity in chemistry (not exactly what the TA test kit does) is defined as the sum of chemical species that can accept one or more protons where you count twice if two protons can be accepted. Hydroxyl ion (OH^-) counts positively towards TA while hydrogen ion (H⁺) counts negatively towards TA (since it's a proton itself). The [OH^-]-[H⁺] difference is zero at pH 7.0 so the TA of pure water is 0. If you were to add a strong acid, the TA would be negative, while if you were to add a strong base, the TA would be positive. Technically one must have some sort of pH cutoff for this definition so that one does not include the salts of strong acids as being able to accept a proton (e.g. chloride ion for hydrochloric acid) and the TA test uses a pH of 4.5 for that, but the problem is that the test itself does not account for hydrogen ions being negative TA when you start at a lower pH, but this is a minor error as I indicated before.

 

[JoyfulNoise]

You might be interested in Googling the term Bjerrum Plot (specifically for carbonate equilibrium). It shows how all of the species of dissolved inorganic carbon (DIC) vary with pH. It's quite clear from a Bjerrum Plot that, at pool pH, the dominant carbon species is bicarbonate and the second most dominant species is dissolved CO2 gas. Neither carbonic acid nor carbonates factor in. It also shows why you keep you pool pH lower than 8 as that is the point where carbonates begin to form and scaling can occur.


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[chem geek]

The Bjerrum Plot is useful, but also confusing because it is wrong when showing the relative amounts of aqueous carbon dioxide and carbonate where the plot implies that they are equal near a pH of 7.5. At normal salinity found in pools, the balance is between a pH of 8.2 and 8.3. In ocean water the balance point is lower, but still closer to 8.0 than to 7.5. The slides in this link and this link show the correct plots for fresh water where you can see the correct crossover point that is above pH 8.

 

[steve57765]

Wow- this thread should be in "the deep end"

I still like reading all chem geeks posts,