Modern inorganic chemistry

GROUP IV 183

CARBONIC ACID AND CARBONATES

The following equilibria apply to a solution of carbon dioxide in

water :

CO 2 + H 2 O ^H 2 CO 3 ^ H+ + HCO

carbonic

acid

The amount of carbonic acid present, undissociated or dissociated,

is only about 1 % of the total concentration of dissolved carbon

dioxide. Carbonic acid, in respect of its dissociation into hydrogen

and hydrogencarbonate ions, is actually a stronger acid than acetic

acid ; the dissociation constant is :

(cf. Ka - 1.8 x 1CT^5 mol T^1 for acetic acid)

But a solution of carbon dioxide in water behaves as a very weak

acid since the effective dissociation constant K' is given by :

Since carbonic acid is a weak acid, its salts are hydrolysed in

aqueous solution :

CO^23 + H 2 O

HCOJ + H 2 O - OH~ + H 2 CO

Although both these reactions lie largely to the left, soluble carbon-
ates (i.e. those of the alkali metals) are alkaline in aqueous solution,
and the hydrogencarbonates are very feebly alkaline. The equilibria
are displaced to the right on addition of an acid and soluble car-
bonates can therefore be titrated with acids and indeed sodium
carbonate is used as a standard base. The titration curve is given
below for 0.1 M hydrochloric acid being added to 100cm^3 of 0.1 M
alkali metal carbonate {Figure 8.4). At A all the CO^23 ^ has been
converted to HCO^ and at B all the HCO^ has been converted to
CO^". Phenolphthalein changes colour between pH 8.3 and pH
10.0 and can be used to indicate point A whilst methyl orange.
changing colour between pH 3.1 and pH 4.4, indicates point B.
Most metal carbonates are insoluble and they are precipitated
either as the simple carbonate or as the basic carbonate when