_Reagents._—1. Nitric acid. Dilute 1 part of concentrated nitric acid
with 4 parts of distilled water. Free the dilute acid from brown oxides
of nitrogen by aeration.
2. Sulfuric acid. Dilute 1 part of concentrated sulfuric acid with 3
parts of distilled water.
3. Dilute sulfuric acid. Dilute 25 cc. of concentrated acid to 1 liter
with distilled water. Add enough permanganate solution to color faintly
the dilute acid.
4. Standard manganous sulfate. The standard solution of manganous
sulfate prepared as described under persulfate method (p. 48) should be
used and the standards should be prepared by following the same
procedure as is used for the sample. This solution is more permanent
than a solution of potassium permanganate, which may, however, be used.
To prepare it dissolve 0.288 gram of potassium permanganate in distilled
water and dilute the solution to 1 liter.