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Selection of a buffer

When selecting a buffer for a particular experimental study, several factors should be

taken into account:

• select the one with a pKaas near as possible to the required experimental pH and
  within the range pKa1, as outside this range there will be too little weak acid or
  weak base present to maintain an effective buffer capacity;


• select an appropriate concentration of buffer to have adequate buffer capacity
  for the particular experiment. Buffers are most commonly used in the range
  0.05–0.5 M;


• ensure that the selected buffer does not form insoluble complexes with any anions
  or cations essential to the reaction being studied (phosphate buffers tend to
  precipitate polyvalent cations, for example, and may be a metabolite or inhibitor
  of the reaction);

Example 3PREPARATION OF A PHOSPHATE BUFFER

Question How would you prepare 1 dm^3 of 0.1 M phosphate buffer, pH 7.1, given that pKa^2
for phosphoric acid is 6.8 and that the atomic masses for Na, P and O are 23, 31 and
16 daltons respectively?

Answer The buffer will be based on the ionisation:

H 2 PO 4 ÐHPO^24 þHþpK^2 a¼ 6 : 8

and will therefore involve the use of solid sodium dihydrogen phosphate (NaH 2 PO 4 )

and disodium hydrogen phosphate (Na 2 HPO 4 ).

Applying the appropriate Henderson–Hasselbalch equation (equation 1.7) gives:

7 : 1 ¼ 6 : 8 þlog½HPO

24 Š

½H 2 PO 4 Š

0 : 3 ¼log½HPO

24 Š

½H 2 PO 4 Š

2 : 0 ¼

½HPO^24 Š

½H 2 PO 4 Š

Since the total concentration of the two species needs to be 0.1 M it follows that

½HPO^24 Šmust be 0.067 M and½H 2 PO 4 Š0.033 M. Their molecular masses are 142 and

120 daltons respectively; hence the weight of each required is 0.067 143 ¼9.46 g

(Na 2 HPO 4 ) and 0.033 120 ¼4.00 g (NaH 2 PO 4 ). These weights would be dissolved

in approximately 800 cm^3 pure water, the pH measured and adjusted as necessary,

and the volume finally made up to 1 dm^3.

13 1.3 Weak electrolytes