Does it reverse to soluble CO2 and then to out-of-solution CO2? In other words, does the pH rise back to the neutral level of regular distilled water? if so, how?
the amount of dissolve CO2 (carbonic acid) will depend on the temperature and the pH of the solution. If it is cold (4 C) and the water neutral, the pH will be around 6.4 though if the partial pressure of CO2 in the atmosphere is increased, the amount of dissolved CO2 will also rise lowering the pH.
The issue with gas sparging (bubbling through the solution) is that it aids in degassing the solution thus decreasing the amount of dissolved gases to a minimum.
Providing you have a CO2 atmosphere above the solution you will always have a weak carbonic acid solution. If you leave the water open to the atmosphere then it will lose CO2 until the concentration of the acid is relative to the partial pressure of CO2 present in the atmosphere.
Are u suggesting that if we do CO2 purging in water at high pH and low temperature, more and more CO2 will be present.
The higher the pH the more the CO2 is trapped as bicarbonate (HCO3^-) or carbonate (CO3^2-) which are more alkaline forms which are salts. The lower the temperature the more soluble CO2 is in water, the higher the partial pressure of CO2 in the atmosphere, the more CO2 will be dissolved. If the solution becomes too acidic then the selection is towards CO2 gas which then bubbles out of the water. Ideally you can achieve a pH range of 4-6.5 from dissolving CO2 dependent on the partial pressure and temperature with a pH of 5.6 to 6.4 normal.
best and easiest way to think about this and explain it is to consider what happens to soda which is left in a warm room. The CO2 bubbles off leaving a flat drink. If you keep the lid on and keep it in the fridge then it stays fizzy. It stays fizzier still if you keep the pressure of CO2 high (using a pressurizer stopper to keep the CO2 partitioned in the fluid)
Thank you all Haralampos, Barry and Mohammad for elaborating for such a valuable discussion, a lot of information has been actually exchanged. A colleague of mine told me that if we heat a solution, being previously purged with CO2 (and later open to the atmosphere), the pH will rise again from nearly 4.3 to the more neutral levels. In other words, accelerating, in reverse, the reaction Haralampos first wrote in his comment.
Thank you again, :)
I apologize for my English.
There is, for example, the method of preparation of distilled water containing no CO2. To do this, distilled water was heated to reflux. Isolating it from the atmosphere beforehand with an alkaline solution.
In experiments, which I conducted by 2005, I used to purge double destilled water with nitrogen. The result always was an increase of the pH form about 7 to about 8.4. Then the experiment was started.
I always always assumed the dissoluted CO2 to be purged out of the water and thus to force the H2CO3 to dissociate to H2O and CO2.
My former boss never disagreed to that theory, but we never prooved it properly. Just thinking about it ... after stoping the purge with CO2 I would expect the H2CO3 to dissociate ...
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