A mixture of 300 ml. of water and the
p-nitrobenzenediazonium borofluoride1 obtained from
0.25 mole of p-nitroaniline (Note
2) is added during a period of 1 hour to the
sodium arsenite solution. The foaming that accompanies the evolution of
nitrogen is readily controlled by the occasional addition of small amounts of
ether or
benzene (Note
3). As the reaction proceeds,
100 ml. of 10% sodium hydroxide solution (0.25 mole) is added in 20-ml. portions. Stirring is continued for another hour, and then the mixture is warmed to 60° for 30 minutes and filtered with suction through a
sintered-glass funnel. The residue on the funnel is washed with two 50-ml. portions of water. To the combined filtrate and washings is added concentrated
hydrochloric acid (sp. gr. 1.19) until the solution is acid to litmus paper. The mixture is filtered, activated charcoal is added to the filtrate, and the solution is concentrated to about 350 ml. After the hot solution has been filtered with suction, concentrated
hydrochloric acid is added until the solution is acid to Congo red paper. The solution is placed in a
refrigerator overnight, and the crystals are collected on a
Büchner funnel and washed twice with 20-ml. portions of ice water. The combined filtrates and washings are concentrated to about 150 ml., chilled, and filtered. The total quantity of crystals is dissolved in
10% ammonium hydroxide solution; the solution is filtered and again made acid to Congo red paper with concentrated
hydrochloric acid. After the solution has been thoroughly chilled (preferably overnight), the
p-nitrophenylarsonic acid is filtered on a Büchner funnel and washed with small portions of ice water until free of
ammonium chloride. After drying, the yellow crystals melt with decomposition at
298–300.° The yield is
44–48.5 g. (
71–79%) (Note
4).