_Procedure._—Use an amount of the sample that contains not more than 0.2
mg. of manganese. Add 2 cc. of nitric acid and boil down to about 50 cc.
Precipitate the chloride with silver nitrate solution, adding at least 1
cc. in excess. Shake and heat to coagulate the precipitate, and filter.
A sample that contains much chloride should be evaporated with a few
drops of sulfuric acid until white fumes appear and then diluted before
the nitric acid and silver nitrate are added as directed above. If the
sample is highly colored by organic matter it should be evaporated with
sulfuric acid, and the residue ignited and dissolved in dilute nitric
acid. Add about 0.5 gram of ammonium persulfate crystals and warm the
solution until the maximum permanganate color is developed. This usually
takes about ten minutes. At the same time prepare standards by diluting
portions of 0.2, 0.4, 0.6 cc., etc. of the standard manganous sulfate
solution to about 50 cc. and treating them exactly as the sample was
treated. Transfer the sample and the standards to 50 cc. Nessler tubes,
and compare the colors immediately. Manganese in parts per million is
equal to the number of cubic centimeters of standard manganous sulfate
solution in the tube that the sample matches multiplied by 100, divided
by the number of cubic centimeters of the sample used.