_Procedure._—Use an amount of the sample that contains not more than 0.2
mg. of manganese. Add 0.5 cc. of sulfuric acid and evaporate to dryness.
Heat until the sulfuric acid is volatilized and ignite the residue.
Dissolve in 40 cc. of nitric acid, add about 0.5 gram of sodium
bismuthate, and heat until the permanganate color disappears. Add a few
drops of a solution of ammonium or sodium bisulfate to clear the
solution and again boil to expel oxides of nitrogen. Remove from the
source of heat, cool to 20° C., again add 0.5 gram of sodium bismuthate,
and stir. When the maximum permanganate color has developed, filter
through an alundum or Gooch crucible containing an asbestos mat ignited
and washed with potassium permanganate. Wash the precipitate with dilute
sulfuric acid until the washings are colorless. Transfer the filtrate to
a 50 cc. Nessler tube and compare the color of it with that of standards
prepared from the potassium permanganate solution. To prepare the
standards, dilute portions of 0.2, 0.4, 0.6 cc., etc. of the
permanganate solution to 50 cc. with dilute sulfuric acid. The content
of manganese is calculated as described under persulfate method (p. 49).