136 THIO-ACIDS AND SALTS.If barium hydroxide is chosen as the base, it is possible to obtain the crys-
tallized salt, after precipitating the excess of the base with carbon dioxide
and evaporating the filtrate. Hypophosphites are strong reducing agents.Heat a solution of 120 g. crystallized barium hydroxide in
1200 c.c. of water with 30 g. of yellow phosphorus (Caution) in
a round-bottomed flask on a Babo funnel. After about four hours
, nearly all of the phosphorus will have disappeared. The process
should be carried out in a well-ventilated hood, since phosphine
gas escapes freely. Filter the solution through a plaited filter
into a large porcelain dish; heat and pass in carbon dioxide until
the excess of barium hydroxide is precipitated. Filter, rinse the
precipitate with boiling water, and evaporate the solution to one-
half its volume. After again filtering, concentrate further — at
the last in a beaker — until crystals begin to separate; then add
alcohol and leave the solution to crystallize. Collect the product
on a filter and evaporate the mother-liquor to obtain more crystals.
Purify the entire product by recrystallization. Yield, 40 to 60 g.
of colorless flaky crystals.
Reactions: 1. Treat a sample of the product with concentrated
sulphuric acid and heat it to boiling; a large amount of sulphur
dioxide escapes and sulphur distils to the cooler part of the test-
tube.- On adding hypophosphite to a dilute solution of gold chloride,
H[AuCl 4 ], and warming gently, a blue-violet coloration (cf. No. 25)
and later a violet-red precipitate of gold is obtained. - With silver nitrate a dark brown separation of silver takes
place, slowly at the room temperature and more rapidly when
warm; with mercurous salts a separation of mercury occurs. - If a dry sample of barium hypophosphite is heated in a test-
tube, it turns red with loss of water and further decomposition:
phosphorus distils off and phosphine escapes.
Dependent preparation: Copper Hydride, No. 33. - Phosphorous Acid.
Phosphorous acid, which is sometimes used in quantitative analysis, is
prepared by the hydrolysis of phosphorus trichloride (No. 46):
PC1 3 + 3 H 2 O = H3PO3 + 3 HC1.
Pour 100 c.c. of water into a 200 c.c. beaker, place the beaker
in ice water under the hood, and slowly add 25 g. (15 c.c.) of