Concentrate (1)[D] rapidly by boiling in a 7–inch porcelain dish over a
free flame 3 or 4 liters of the sample to be tested, or more if very
small amounts of the metals are present, to a volume of about 30 cc. Add
10 or 15 cc. of ammonium chloride solution to assist in the separation
of the sulfides, then add a few drops of concentrated ammonium
hydroxide, and saturate with hydrogen sulfide. Allow to stand some time,
preferably over night, add a little more ammonium hydroxide and hydrogen
sulfide, boil the contents of the dish a few minutes, and filter. The
precipitate (2) may consist of lead, zinc, copper, and iron sulfides and
the suspended organic matter. The soluble coloring matter is in the
filtrate (3). Wash the precipitate a few times with hot water, place the
precipitate and the filter paper in the original dish and boil with
dilute nitric acid, rubbing down the sides of the dish, if necessary, to
detach any adhering sulfide precipitate. After again filtering and
washing several times with hot water, evaporate the filtrate and
washings in the original dish to a bulk of 10 to 15 cc., cool, add 5 cc.
of concentrated sulfuric acid, and heat until copious fumes of sulfuric
acid are evolved.
Footnote D:
The numbers in parentheses refer to tables 10–12, pages 55–56.
If lead is present dilute the contents of the dish slightly with water,
and treat them with 150 cc. of 50 per cent alcohol, in which the lead
sulfate is insoluble. Allow to stand some time, preferably over night,
filter off the lead sulfate, and wash it with 50 per cent alcohol. Save
the filtrate for the determination of zinc.
Dissolve the precipitate of lead sulfate by boiling the filter
containing it in ammonium acetate solution in a porcelain dish. (4).
Filter into a 50 cc. Nessler tube and wash the filter with boiling water
containing a little ammonium acetate. Divide this filtrate in halves and
treat one-half with saturated hydrogen sulfide water in order to get an
approximation of the amount of lead present. To the other half, or an
aliquot portion, if a large amount of lead is present, add a few drops
of acetic acid, then an excess of saturated hydrogen sulfide solution,
and compare the color with that of standards made by treating known
amounts of the standard lead solution with a little acetic acid,
ammonium acetate, and hydrogen sulfide.