Concentrations of the components should exceed
Ka by at least 100 times.
Buffers do not have a limitless ability to
resist change in pH. Addition of sufficient
acid or base will overcome the buffering capacity
of a solution and a change in pH will be observed.
This will happen before one of the... one
of the buffer components is completely changed
to the other.
The more concentrated the buffer, the greater
its capacity for buffering.
And the more effective buffer will be one
which has the identical concentrations of
acid per conjugate base such as in the case
pH = pKa. So, therefore, the other terms are
The buffer will be effective one unit either
side of this point.
So, for example, if we take the log of the
conjugate base over the log of the weak acid,
as long as the value falls within one unit,
-1 or +1, we know that it’s going to be
The acetic acid or ethanoic buffer gave a
pH of 4.74 with equal concentrations of acid
and a conjugate base. So, ethanoic buffers
will be effective at their pH range of 3.74
As you can appreciate from what I said, a
pH of 7.4 is typically considered to be acceptable
physiological pH, for example, for blood plasma.
And so, often sodium acetic, sorry, sodium
ethanoate and ethanoic acid buffers are not
particularly useful in mimicking biological
Now, buffer recipes can be found in books
to ensure sufficient capacity, giving solutions
of varying pH. Obviously, what I’ve just
showed you there was to do with an acetic
acid and sodium acetate buffer, which gives
a relatively acidic pH buffer. And when you’re
looking at biological and medical applications,
it’s also important to consider the potential
toxicity of the ingredients. That’s one
of the reasons for the use of Tris or HEPES
buffers and also, if you require it, some
of the high pH buffers like borate.
Thank you very much.