Buffer Capacity and Ranges – Acid-Base Reactions

00:01 Concentrations of the components should exceed Ka by at least 100 times.

00:02 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.

00:16 This will happen before one of the... one of the buffer components is completely changed to the other.

00:20 The more concentrated the buffer, the greater its capacity for buffering.

00:27 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 [Unaware 00:40:47].

00:41 The buffer will be effective one unit either side of this point.

00:45 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 reasonably effective.

01:00 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 to 5.74.

01:14 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 conditions.

01:31 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.

01:59 Thank you very much.