Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

pH log 4 , 510

2      1. 35

Once again we see that the contribution to the H 3 O+ concentration from the autoprotolytic contribution of

water is negligible compared to the calculated x-value why this contribution of 10-7 M in practice may be

omitted.

5.3 Polyprotic acids

Some acids such as e.g. H 3 SO 4 and H 3 PO 4 may provide more than one H+ ion. Such acids are called

polyprotic acids, and these acids provide their H+ ions in steps one proton at the time. For each step it is

possible to determine a Ka value. The diprotic acid H 2 CO 3 provide thereby its two protons in the following

two steps with corresponding equilibrium expression:

H 2 CO 3 (aq)  H+(aq) + HCO 3 - (aq)

 

 2 3

4 , 3107 3

HCO

H HCO

Ka M

^

HCO 3 - (aq)  H+(aq) + CO 3 2-(aq)

 

^

^

3

2

5 , 6 1011 3

HCO

H CO

Ka M

Please note how the corresponding base from the first step (hydrogen carbonate, HCO 3 - ) becomes the acid in

step number two. Such a property is characterised for an amfolyte. We shall see in the example below how

pH may be calculated when we have a solution of a polyprotic acid.

Acids and bases

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