Ask A Scientist Chemistry Archive

  
name         Ann
status       student
grade        9-12

Question -   What happens when more acid or base is added to a buffer
solution after acid or base has already been added to it?
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A buffer solution is a solution of the salt (or salts) of weak acids and/or
bases. These salts are partially ionized so that there is a concentration
of salt that can convert to/from salt form to/from acid or base form. The
buffer capacity to hold the pH of a solution nearly constant depends upon
the concentration of salt(s) relative to the amount of acid (or base) you
add to the solution. If you add too much of one or the other you consume
and convert the salt form to the acid or base form. When you overwhelm the
capacity of the buffer salt the pH will change.

Vince Calder
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Ann,

There is a nice equation called the Henderson-Hasselbalch equation which 
quantifies this concept. It states that the pH = pka + log ([HA]/[A-]). 
Essentially, this states that the buffer pH is dependent on a constant 
(pka) and the ratio of the molarity of the acid used and the molarity of 
the salt of that acid. When one adds acid or base to a buffer the ratio of 
the two concentrations change. If the initial molarities of the acid and 
salt are very high then essentially the buffer can resist that pH change, 
however if the molarities are low then the buffer pH changes. Note that 
while the ratio of HA/A- controls the pH of the buffer, it is the 
magnitude of the concentrations that define the buffer capacity or its 
ability to resist pH changes.

Greg (Roberto Gregorius)
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