922. THE HENDERSONHASSELBALCH EQUATION
The preparation of laboratory buffers can be accomplished by using the
HendersonHasselbalch 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
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