Chemistry of Aqueous Solutions – Acids and Bases Part II

Buffers: what exactly are they?

We are taught, in A Level Chemistry, that a buffer solution minimizes pH change when small amounts of acids and bases are added.

We are also taught that there are 2 types of buffer:

1. Acidic buffer: made up of a weak acid and its salt, eg. CH₃COOH + CH₃COONa
2. Alkaline buffer: made up of a weak base and its salt, eg. NH₃ + NH₄Cl.

Let’s take a look at some A level prelim chemistry questions from top JCs.

1. HCI 2010 P3
A buffer solution is made by mixing 500 cm³ of 0.30 moldm^-3 NaH₂PO₄ with 100 cm³ 0.20 moldm^-3 Na₂HPO₄.

For the equilibrium:
H₂PO₄^–(aq) ⇋ H⁺(aq) + HPO₄^2-(aq)       Ka = 6.30 x 10^-7moldm^-3

Calculate the pH of the buffer solution.

2. NJC 2010 P3

Majority of biological samples used in research are made in buffers, especially phosphate buffered saline at pH 7.4. 1 dm³ of this buffer can be made by dissolving 8.00g NaCl, 1.44g Na₂HPO₄and 2.56g NaH₂PO4 in 1 dm³ distilled water.

The pH of this mixture is then adjusted by adding solid NaOH.

Calculate the mass of solid NaOH that must be added to the mixture so that a pH of 7.4 can be achieved. pKa of H₂PO₄^– = 7.2

Notice that in both questions, the buffer system is not made up of the “typical” buffer types as what we have learnt in JC Chemistry.

In fact, the most commonly found buffers in nature do no fall under the “typical” buffer types, for example,

1. Buffers in seawater and fresh water lakes/rivers/wells: HCO₃^–
   and CO₃^2-
2. Buffers in the cellular cytoplasm :H₂PO₄^– and HPO₄^2-
3. Protein buffers: acidic and basic amino acid groups in polypeptides.

To a layman, the natural buffers cited above look like combination of salts, with no “typical” acids or bases present.

So how do we recognize a buffer?

Instead of looking at buffers using conventional definitions, buffers can generally be identified as having the following features

1. Conjugate acid – base pair ( with the exception of polypeptides)
2. No member in this pair should be either a strong acid or base

This definition covers the conventional definitions of acidic and alkaline buffers and also includes buffers without a “typical” acid or base present, but 2 salts, or amino acid residues in polypeptides.

Eg. We might consider HCl and NaCl as a conjugate acid-base pair, but as HCl is a strong acid, it does not have any significant buffering capacity.

A Closer Look at the Henderson-Hassalbalch (HH) Equation.

We are taught that since there are 2 kinds of buffers, there should be 2 different buffer equations:

Acidic buffer:

Alkaline buffer:

However, since we are now looking at buffers in a totally new way, then we should look at the HH equation in a new way too. In fact, the original, generic form of the HH equation is

This generic form of HH equation looks similarly to that of the acidic buffer (Eq.(1)) but some what different from the alkaline buffer (Eq(2)).

However, if one were to deduct the left and right hand sides of Eq. (2) from 14, we will arrive at Eq (3), which means that the Eq (3) alone is sufficient to express the relationship between the pH of the solution and the ratio of the concentrations of the components that make up the buffer!

So why remember 2 equations, when we just need to remember 1?

For more interesting insights into the HH equation and exactly how the additions of small amounts of acids and bases can affect the pH in the HH equation, sign up for A Level and H2 Chemistry Tuition classes to find out!