the pH of a mixture containing 50 cm^3 each of 0.1 M KH 2 PO 4 and 0.1 M
Na 2 HPO 4 , which originally has a pH of 7.0. Adding 0.001 moles of acid to
100 cm^3 of water would lower the pH to 2.
The more concentrated the buffer, the greater will be its buffer capacity. This
is the amount of acid (or alkali) that, when added to 1 liter of buffer, will change
its pH by 1 unit. In the above example, the buffer capacity is about 0.04 moles. If
we had used a more concentrated buffer, the capacity would be greater. Very
dilute buffers have little buffer capacity, and hence have limited use.
Table 1 gives a selection of buffers and standard solutions that are useful for
pH control. These solutions and others are often used to calibrate pH meters.pH measurement Two important methods exist for pH measurement: visual, using indicators,
and potentiometric, by means of electrochemical cells.
Indicators for pH measurement are weak acids (or bases) where the color of
the acid form is different from that of the salt.
C4 – pH and its control 77
109 8 7 6 5 4–4 –3 –2 –4 0 1 2 3 4
Millimoles of acid (+) or alkali (–)pHFig. 2. The effect of adding acid (+) or alkali (-) to a phosphate buffer mixture, originally at
pH 7.Table 1. Buffer solutions
Solution pH at 25∞C
0.05 M potassium hydrogen phthalate 4.008
0.1 M ethanoic acid, 0.1 M sodium ethanoate 4.640
0.025 M potassium dihydrogen phosphate, 6.865
0.025 M disodium hydrogen phosphate
0.01 M borax 9.180
0.1 M ammonia, 0.1 M ammonium chloride 9.250
0.025 M sodium bicarbonate, 10.012
0.025 M sodium carbonate
Saturated calcium hydroxide 12.454