_First procedure._—Evaporate 100 cc. of the water to dryness, or use the
residue left after the determination of residue on evaporation (p. 29).
Ignite the residue at a low red heat taking care not to heat it hot
enough to make the iron difficultly soluble. Cool the dish and add 5 cc.
of concentrated hydrochloric acid. Moisten the inner surface of the
dish. Warm the solution for two or three minutes, and again moisten the
inner surface of the dish by permitting the hot acid to flow over it.
Wash the hot solution from the dish into a 50 cc. Nessler tube,
filtering if necessary through paper that has been washed with hot
water. Dilute to 50 cc., and add 3 drops of potassium permanganate
solution. Add 5 cc. of potassium sulfocyanide solution, mix, and compare
with standards.
If it is not convenient to use the residue on evaporation and if the
sample is relatively free from organic matter, boil 50 cc. of the sample
with 5 cc. of 5N nitric acid for five minutes. Add a few drops of
permanganate and 5 cc. of potassium sulfocyanide and compare with
standards, using nitric acid in place of hydrochloric acid in the
standards. This method is excellent for ground waters. The permanganate
and acid liberate chlorine in water high in chloride, and produce a
permanent yellow color which interferes with the determination, unless
the sample is first diluted to 50 cc. An excess of permanganate,
reacting with hydrochloric acid, causes similar trouble. The amounts of
hydrochloric acid, 5 cc., and of sulfocyanide, 5 cc., should be
approximately measured because more acid lightens the color whereas more
sulfocyanide deepens it. This is especially important if permanent
standards are used.
_Second procedure._—For surface waters containing small amounts of
organic matter, the method of Klut[59] is recommended. Samples
containing small amounts of iron should be concentrated, if possible,
until at least 0.5 mg. of iron is present in the volume tested. Boil the
sample in a beaker with 2 to 3 cc. of concentrated nitric acid free from
iron, adding permanganate if necessary to destroy the organic matter. To
the hot liquid add ammonia in slight excess and warm until the smell of
ammonia is hardly discernible. Filter and wash with water at 70° to 80°
C. containing a little ammonia. Dissolve the iron in the beaker and on
the filter paper in 5 cc. of concentrated hydrochloric acid, and wash
with hot water until the iron is all dissolved, collecting the filtrate
in a 50 cc. Nessler tube. Dilute to 50 cc. Add potassium sulfocyanide
and determine the iron by comparison with standards.