Chemistry, Third edition

BUFFERING ACTION OF CARBON DIOXIDE IN WATER 305

Buffering action of carbon dioxide in

water

Carbon dioxide in water

Carbon dioxide is fairly soluble in water, and a small amount of the dissolved CO 2

reacts with water as follows:

CO 2 (aq)2H 2 O(l)\===\HCO 3 (aq)H 3 O(aq) (16.12)

hydrogencarbonate

ion

whereKc(25C)= 4.3  10 ^7 mol dm^3. The production of hydronium ions causes

water in contact with air (for example rain water, tap water and laboratory deionized

water) to be slightly acidic, and experiments show that such water possesses (at 25 °C)

a pH of about 5.6.

However, many natural waters (lakes, ponds and reservoirs) are basic due to the

presence of dissolved solids. For example, water saturated with CO 2 and limestone

(calcium carbonate) at 25 °C contains a higher concentration of hydrogencarbonate

ion than water saturated with CO 2 alone, and is slightly basicwith a pH of 8.3.

If acid is added to a pond, the added H 3 Oions causes the equilibrium concen-

trations of water and carbon dioxide to increase. The CO 2 then bubbles off into the

air. Without such buffering, natural waters would be easily acidified by acid rain.

However, if too much acid is present, the buffer is overwhelmed and the pH of the

water does change.

Buffers in the body

The CO 2 –water equilibrium is also the main buffer present in blood. Excess

H 3 O(aq) in blood, due to the presence of lactic acid produced by the metabolism

of food, is removed by the reaction of the back reaction of equation (16.12). In this

way, the pH of blood is maintained at 7.4. Variations in arterial blood pH of more

than about 0.4 can be fatal.

The hydrogencarbonate buffer in blood would soon become exhausted if there

was not a way for the body to get rid of waste. Excess carbon dioxide and excess acid

are removed by the lungs and kidneys, respectively. In this way, the hydrogencarbon-

ate ions in the blood are released to act as a buffer once again.

16.7

Calculation of the pH of unpolluted rain water

The concentration of dissolved CO 2 in water, [CO 2 (aq)], which has been equilibrated with air

is 1.2  10 ^5 mol of dissolved CO 2 per dm^3 of water at 25 °C.

(i)CO 2 (aq) is a weak acid. Using the expression

Ka(T)

[H 3 O(aq)]^2

[CO 2 (aq)]

show (by calculation) that the pH of water in equilibrium with CO 2 in air is about 5.6.

(ii)Interestingly, concentrated salty water possesses a pH closer to 7 than that of deionized

water in the laboratory – why?

Exercise 16P

For more about

buffers in the body

see Case Study 2

on the website.