How A Buffering Agent Works
The way buffering agents work can be seen by calculating how little the pH of buffer solutions will change after addition of a strong acid or a strong base, whereas the same addition would significantly change the pH of a non-buffed solution. Using the Henderson-Hasselbalch equation we get an equilibrium expression between the acid and conjugate base in terms of the log of the ratio of the acid to conjugate base (the salt of the acid). The concentrations of the weak acid and its salt can change significantly, but the log of their ratio will not. The resulting pH of this combination can be found by using Le Chatelier's principle. If an added strong base halves the, the will double. Thus the pH will be raised by a factor of log4 or 0.6:
For example, if originally the pKa = 7 and =, then the pH will be changed from 7.0 to 7.6 after addition of a strong base that halves and doubles . If a similar addition of 0.001 mol NaOH is made to 1L of non-buffered water, the pH would change from 7 to 11.
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