DISODIUM PHOSPHATE 301mediately after the violent secondary reaction has ceased, is
poured into a flask, diluted to about a liter with water, saturated
with hydrogen sulphide gas, stoppered, and allowed to stand over
night. If, the next morning, the contents of the flask smell
strongly of hydrogen sulphide, the precipitate of arsenic sulphide
is filtered off; if not, the solution is again treated with hydrogen
sulphide in the same manner as before. Evaporate the filtrate
until its temperature has risen to 125°, and proceed as above.
Note 3. If no crystallized phosphoric acid is at hand for use
as seed crystals the sirupy acid can be made to crystallize if it is
heated to 180° and then cooled with a freezing mixture.QUESTIONS- Write the reaction by which phosphoric acid can be prepared
from calcium phosphate. - How can phosphoric anhydride, pyrophosphoric acid, and
metaphosphoric acid be prepared? Give formulas. Why cannot
the anhydride be prepared by heating orthophosphoric acid?
For what practical purpose is phosphoric anhydride used? - Compare the acid strength of phosphoric acid with that of
other common acids. Do all three hydrogen ions of H3PO4 dis-
sociate with equal readiness? - Give the formulas of primary, secondary, and tertiary
sodium phosphates. State how the solution of each behaves with
litmus. - Write the reaction for the precipitation which occurs when
magnesium chloride and a large excess of NH4OH are added to a
solution of phosphoric acid. This precipitate constitutes one of
the most important tests for a phosphate. - Give an example of phosphorous acid acting as a reducing
agent.
PREPARATION 53
DISODIUM PHOSPHATE, NaJIPCV^H^O
The raw material from which phosphorus and its compounds are
prepared is calcium phosphate, either in the form of natural " phos-
phate rock" or in bone ash. For laboratory preparations bone
ash is preferable since it is practically free from fluorides and com-
pounds of iron. This bone ash may be assumed to be 80 per cent