MadSci Network: Chemistry |
Peter, Sorry about the delay on the answer. Your question concerns the nature of diprotic acids and bases. This means that they have more than one dissociable location. Sodium Hydrogen Carbonate, (Sodium Bicarbonate) has the formula NaHCO3 The parent acid is Carbonic Acid, H2CO3. When the salt of a weak acid is added to water, some portion of the salt acquires a hydrogen ion from the water. This leaves an excess of hydroxide ions and thus, the pH of the solution goes up. The interesting thing that you have found is that when the pH is higher than that of the salt + water alone, then the second dissociable hydrogen of the Bicarbonate ion will act as an acid and lower the pH. This is called a BUFFER system. The solution really does not want to change pH very much at all, and it requires fairly large amounts of acid or base to change pH. There is a lot of information out on the web about buffers and Acid Base chemistry. Try running a search using those as keywords. You may wish to look at a first year collegiate chemistry textbook. The discussion on polyprotic acids and bases as well as buffers will give you plenty of information to satisfy your curiosity. As an aside, that is about the highest pH of tap water that I have ever heard of. You probably live in an area with limestone or chalk deposits underground. Keep down the pH Michael M. Gallagher Ph.D. Senior Research Chemist J.R. Simplot Co.
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