9-22. THE HENDERSON-HASSELBALCH EQUATION
The preparation of laboratory buffers can be accomplished by using the
Henderson-Hasselbalch equation. The equation is derived from the ionization constant
of weak acids and bases.
+
-
[H ] [A ]
K = --------
[HA]
The hydrogen ion concentration can be calculated by rearranging the equation.
Ka X [HA]
+
[H ] = ------------
-
[A ]
+
Because [H ] is usually expressed as pH, it is usually more convenient to
express all concentration in this equation in logarithmic form.
[HA]
+
log [H ] = log Ka + log --------
-
[A ]
+
Since pH = -log [H ] multiply both sides of the equation by -1.
[HA]
+
-log [H ] = -log Ka - log ------
-
[A ]
Restated:
-
[A ]
+
-log [H ] = -log Ka + log ------
[HA]
[salt]
pH = pKa + log --------
[acid]
The expression relating pOH to the components of a buffer may be developed
similarly.
[salt]
pOH = pKb + log ------
[base]
MD0837
9-15