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Chemistry of Aqueous Solutions – Acids and Bases Part II
  • focuschemistry
  • September 22, 2018
  • Chemistry of Aqueous Solutions

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) + HPO42-(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 CO32-
  2. Buffers in the cellular cytoplasm :H2PO4 and HPO42-
  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:

pH = pKa + lg[salt]
——–
[acid]
…(1)

Alkaline buffer:

pOH = pKb + lg[salt]
——–
[base]
…(2)

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

pH = pKa + lg[conj base]
————
[acid]
…(3)

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!