Modern inorganic chemistry

184 GROUP IV

12

10

8

6

4

2

H*-** HCOj

HC0 3 + hr-**H 2 C03

Excess

O 20 40 60 80 IOO

cm^3 of 0-1M hydrochloric acid

Figure 8.4. Titration oj a soluble carbonate with hydrochloric acid

carbonate ions, as sodium carbonate solution, are added to a
solution containing the metal ions.
Hydrogencarbonates of sodium, potassium and ammonium are
known in the solid state and show hydrogen bonding in the crystal:

o o o o p O o

C H C H C H C H C

o o o o o o o o

The broken lines indicate hydrogen bonds. The full lines are to
show the structure, they do not simply represent single covaknt
bonds.
Magnesium and calcium hydrogencarbonates are known in
solution and are responsible for temporary hardness in water.

STRUCTURE OF THE CARBONATE ION

The carbonate ion is planar and can be regarded as a resonance
structure between the three forms given below (see also p. 44):