It is true that when CO2 is added to H2O the pH will drop because of carbonic acid (H2CO3) which has an acid-dissociation constant (Ka) of 4.3E-7 at 25 degree celsius. However, H2CO3 is unstable and therefore will react with the other H20 molecules in and conjugate acid-base pair equilibria. Thus, forming the weak base, bicarbonate (HCO3-) which has a base-dissociation constant (Kb) of 2.3E-8. The very reason for this is what I mentioned before about the oxygen molecule in H20 being a strongly polarising atom, H20 will therefore act as a nucleophile and 'attack' the electrophilic H2CO3 in a conjugate acid-base pair equilibria. Despite HCO3- being a weak base, it will still raise the pH slightly.
NB. The larger the value of Ka, the stronger the acid, and therefore the lower the pH
You can see a second description of the calculation of the pH of water under normal atmospheric conditions
here just open the word document or click
here for the google html version. The effect of atmospheric CO2 is to acidify water not make it alkaline. You are adding carbonic acid to the water, despite the fact it is polyprotic and conjugate bases are formed the water does not become basic. The formation of conjugate bases also produces hydronium ions lowering pH.
Some of these bases in their equilibrium react with hydronium to form their conjugate acid raising the pH, but this is calculated for above. The effect of these comparitively weak bases does not stop acids from acidifying.
When you add sulfuric acid to water you acidify it not basify it despite the fact that H2SO4 can deprotinate twice forming the base SO4-2 (and HSO4-1). In fact this is the acidifying action of acids. They donate protons. If the conjugate bases weren't formed then they wouldn't be making anything acidic.
Sodium hydroxide (NaOH) is a very strong base and can be sold in pet stores to raise pH. I have a Wardley 'pH up' which utilises sodium bicarbonate (NaHCO3) as its active constituent. But to say that NaHCO3 is a buffer is misleading because HCO3- is the buffer, not NaHCO3 itself. This can be seen in the major buffer system in blood: the carbonic acid-bicarbonate buffer system. NaHCO3 is used as an acid neutraliser in acid spills. It is also found in antacids (predominately the product Alka-Seltzer) to neutralise HCl, the active acid when referring to gastric juice which has a Ka of 1E-1, corresponding to pH=1. Furthermore, the solubility constant for NaHCO3 is 10.3 g/100g at 25 degree celsius, which makes it hardly soluble. Thus, in water NaHCO3 will not readily dissiociate into HCO3- and Na+ to allow the HCO3- act as a buffer. Also, the tank temperature will obviously be less than 25 degrees celsius which also hinders the dissociation of NaHCO3. The lower the temperature, the fewer collisional interactions between the solvent (water) and solute (NaHCO3). Therefore, NaHCO3 is a base.
Yes, it's a base but also a weak buffer. HCO3- will go to equilibrium with it's conjugate acid, CO3-2 (damn the lack of superscripts!), making the acid/base pair of a buffering solution. The buffer isn't any one component it's the acid/base pair that makes the buffer. It's a very commonly seen buffer along with the phosphate buffering system.
As to solubility, how can you define sodium bicarbonate as "hardly soluble?" In a ten gallon aquarium you could dissolve nearly 4 kg of NaHCO3.
The way you worded things is odd too. If NaHCO3 dissolves it will form it's ionic components. The undissolved stuff of a supersaturated solution will not, of course.
Also pH is the negative log base 10 of the aquated hydrogen ion concentration.
Therefore pH = -log [H+/H30+]
Sure pH is associated with acidity but when measuring pH, you do not measure the [acid], you are measuring the [H+/H30+] because as pH decreases [H+] increases.
Yes, I misspoke, it is indeed the negative log. I was simplifying by saying acid concentration but yes, more truly it is [H+/H3O+].
