IONIZATION OF POLYBASIC ACIDS 117
When the hydrolysis of the salt of a polybasic acid is considered,
the different H radicals must be treated separately. Thus when 1
mole of tertiary sodium phosphate, Na 3 PO 4 , is dissolved in water,
hydrolysis takes place very extensively as follows:
Na+ Na+ Na+ PO 4 ~~~
H+The solution will have a very strong alkaline reaction, since it
contains a large fraction of 1 mole each of ionized NaOH and of
ionized secondary sodium phosphate, Na 2 HPO 4. The OH" ions
thus formed check the hydrolysis of the secondary sodium phos-
phate; but if solid secondary sodium phosphate is dissolved in
water, hydrolysis of this salt ensues to a sufficient extent to make
the solution alkaline to litmus.
Na+ Na+ HPO 4 "
H 2 O;=±OH~ H+
jr
H2PO4"
When primary sodium phosphate, NaH 2 PO 4 , is dissolved, a
weakly acid solution is obtained, this effect being due to the
tendency of the second hydrogen radical of the acid to ionize.
Na+ H2PO4"
jr
H+
HPO4""
PROBLEMS
When the following salts are dissolved in water, decide from a
consideration of the degree of ionization of the base and acid con-
cerned in each case whether the solution will be neutral, weakly
acidic, strongly acidic, weakly basic, or strongly basic, and give
the explanation and an intersecting ionized equation.
- KNO 3. 58. NaHCO 3.
- Ca(CN) 2. 59. NaHSO 4.
- NH 4 C 2 H 3 O2. 60. NaH 2 PO 4.
- A1CU. 61. Na 2 HPO 4.
- A1(C 2 H 3 O 2 ) 3. 62. Na 3 PO 4.
- Na 3 As0 4. 63. AgNO 3.
- Na 2 CO 3.