Synthetic Inorganic Chemistry

302 ELEMENTS OF GROUP V

Ca 3 (PO 4 ) 2 , 10 per cent CaCO 3 , and 10 per cent inert material.

The following reactions with sulphuric acid may take place:

CaCO 3 + H 2 SO 4 -»• CaSO 4 1 + CO 2 + H 2 O (1)

Ca 3 (PO 4 ) 2 + 3H 2 SO 4 -»• 3CaSO 4 1 + 2H3PO4 soluble (2)

Ca 3 (PO 4 ) 2 + 2H 2 SO 4 -»• 2CaSO 4 j + Ca(H 2 PO 4 ) 2 soluble. (3)

Ca 3 (PO 4 ) 2 + H 2 SO 4 -»• CaSO 4 1 + 2CaHPO 4 1 insoluble (4)

The inert material does not react at all with the acid.

It is obvious that if all the phosphate is to be dissolved, such an

amount of sulphuric acid should be taken that after all the cal-

cium carbonate has reacted there will be more than 2 F.W. of

H 2 SO 4 for each formula weight of Ca 3 (PO 4 ) 2. Furthermore, an

amount of acid in excess of that required for reaction (2) is to be

avoided since the excess would remain in solution. A quantity is

chosen, therefore, which is more than sufficient to complete re-

action (3) but insufficient to complete reaction (2). All the

phosphate is converted into soluble form, and all the sulphate

appears as insoluble calcium sulphate.

After the removal of the insoluble calcium sulphate the filtrate

is treated with sodium carbonate in sufficient amount to satisfy

the equations:

Na 2 CO 3 -» Na 2 HPO 4 + CO 2 + H 2 O

Ca(H 2 PO 4 ) 2 + 2Na 2 CO 3 -»2Na 2 HPO 4 + CaCO 3 1 + CO 2 + H 2 O

An excess of sodium carbonate does not react to convert Na 2 HPO4
to Na 3 (PO 4 ). On page 100 it is seen that the acidic ionization of
HCO 3 ~ is greater than that of HPO 4 ~~. Since a stronger acid
displaces a weaker acid, the reaction NaHCO 3 + Na 3 PO 4 —•
Na 2 HPO 4 + Na 2 CO 3 would take place rather than the reverse
reaction.

Materials: bone ash, 80 per cent Ca 3 (PO 4 ) 2 ,100 grams.

36iVH 2 SO 4 ,47cc.

anhydrous sodium carbonate, Na 2 CO 3.

Apparatus: 8-inch porcelain dish.

suction filter and trap bottle.

8-inch crystallizing dish,

iron ring and ring stand.

Bunsen burner.