Acid-base equilibria
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Is it true that when adding more alkali/acid, the pH of the buffer changes slightly? If that's so how is a buffer a buffer?
Igloo writes ...
You’ve missed the point. A buffer is not a system which is totally resistant to changes in pH. A buffer simply “cushions” the effect, rather like the buffers at mainline railway terminals.
Let’s have a look at an example. If a few drops of concentrated hydrochloric acid are added to a dilute solution of ethanoic acid contained in a beaker, the pH drops like a stone. But if the ethanoic acid has already been buffered, e.g. by stirring in some sodium ethanoate beforehand, the pH drops only slightly, at least initially. Eventually, when enough hydrochloric acid has been added, the buffer is destroyed, and the pH drops “normally”. The extent of buffer protection depends in part on the quantity of sodium ethanoate used initially.
A buffer solution is a practical application of Le Chatelier's principle. A typical buffer is an equilibrium, in aqueous solution, between a weak acid and one of its salts. On adding a little acid (tending to increase the hydrogen ion concentration) the position of equilibrium shifts to try to compensate (thus tending to reduce the hydrogen ion concentration nearly back to where it was at the start).
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updated: 22 January 2006
