40 INJURY BY SMELTER WASTES. 
solve any precipitate that may have formed. Add a_ sufficient quantity of 
a solution of silver nitrate, which will usually cause a white precipitate of 
silver chlorid, and then cautiously add ammonia until, if arsenic be present, a 
reddish precipitate of silver arsenate appears. If too much ammonia be added 
the precipitate first formed will redissolve and may not be observed at all. 
Cautiously add dilute nitric acid until the red precipitate just redissolves. 
To the faintly acid liquid add a few cubic centimeters of a strong solution of 
sodium acetate to replace the free nitric acid by acetic acid. All the arsenic 
will now at once precipitate as silver arsenate (Ag;ASQ,). 
Heat the precipitated mixture to boiling, cool to room temperature, and filter. 
If the first portions run through turbid, return them to the filter. Test the 
filtrate with a little more silver nitrate and sodium acetate. Wash the precip- 
itate with cold water until a portion of the washings shows only a faint test 
for silver. 3 
Place the original flask under the funnel and dissolve the arsenate on the 
filter with cold 1:1 nitric acid. Wash the filter thoroughly with cold water, 
leaving behind on the filter any silver chlorid that may be present. Dilute the 
filtrate, add 5 ce of a strong solution of ammonium ferric alum and titrate to 
a permanent red tinge with a standard solution of ammonium thiocyanate, 
shaking well, especially toward the end. Multiply the number of cubic centi- 
meters required by the arsenic equivalent of each cubic centimeter. 
Standardize the sulphocyanate solution against a known weight of pure silver 
dissolved in nitric acid. From the equivalent of each cubic centimeter of the 
sulphocyanate in silver the equivalent of each cubic centimeter in arsenic can 
be calculated from the formula AgsASQOu. 
O 
