300 ELEMENTS OF GROUP Vuntil all the phosphorus is used. Then add 20 cc of 16 N HN0 3 ;
boil the mixture gently in the flask with the condenser in place
until no more red vapor is evolved and until the last specks of
phosphorus have dissolved. If any black residue does persistently
remain filter it off at this point. Transfer the solution to an 8-
inch porcelain dish and boil it under the hood until a rather violent
reaction begins to take place. Remove the flame and let the re-
action proceed; it grows very violent and then suddenly ceases
altogether.
The solution should now be clear and almost colorless, and should
contain no phosphorous acid. Test for phosphorous acid by adding
a few drops of the liquid to 10 cc. of distilled water, adding 1 cc.
of 0.05N AgNC>3 and warming; a dark coloration, or black pre-
cipitate, appearing within 2 minutes indicates phosphorous acid. If
it is found, follow the special procedure given in Note 1; other-
wise transfer the solution to a small casserole and evaporate it
over a low flame until a thermometer whose bulb is immersed in
it stands at 180°. During this final evaporation one must give it
constant attention, for if it is left and the temperature rises
above 180°, not only does the orthophosphoric acid become
changed partially into pyrophosphoric acid, but it attacks the
material of the dish, and the preparation becomes contaminated.
Allow the acid to cool nearly to room temperature and then pour
it into a previously weighed 2-ounce glass-stoppered bottle, and
stopper it tightly. When cool introduce a small crystal of phos-
phoric acid to induce crystallization of the mass.
Note 1. If phosphorous acid was found at the point where the
test was made, it showed that the reaction had been allowed to take
place too violently in the earlier part of the procedure, the heat
driving nitric acid vapor out of the flask. Unless nitric acid is
present in the right amount and at the right concentration when
the secondary reaction takes place, the phosphorous acid is not all
oxidized and it is afterwards extremely difficult to bring about
conditions under which the oxidation can be completed. However,
the following may be tried: Pour the solution back into the large
flask, add 275 cc. of 62V HN0 3 and 10 cc. of 122V HC1 to act as a
catalyzer, insert the condenser in the neck of the flask, and boil
very gently for an hour. Then proceed as before.
Note 2. In the above procedure no provision is made for re-
moving traces of arsenic. If this is to be done, the solution, im-