_Reagents._—1. Phenoldisulfonic acid. Dissolve 25 grams of pure white
phenol in 150 cc. of pure concentrated sulfuric acid. Add 75 cc. of
fuming sulfuric acid (15 per cent SO_{3}), stir well, and heat for 2
hours at about 100°C.
2. Potassium hydroxide solution. Prepare an approximately 12 N solution,
10 cc. of which will neutralize about 4 cc. of the phenoldisulfonic
acid.
3. Standard nitrate solution. Dissolve 0.72 gram of pure recrystallized
potassium nitrate in 1 liter of distilled water. Evaporate cautiously to
dryness 10 cc. of the solution on the water bath. Moisten residue
quickly and thoroughly with 2 cc. of phenoldisulfonic acid and dilute to
1 liter. This is the standard solution, 1 cc. of which equals 0.001 mg.
of nitrate nitrogen.
4. Standard silver sulfate solution. Dissolve 4.4 grams of silver
sulfate free from nitrate in 1 liter of water. One cc. of this solution
is equal to 1 mg. of chloride.
_Procedure._—The alkalinity, chloride, and nitrite content, and color of
the sample must first be determined. If the sample is highly colored
decolorize it with freshly precipitated aluminium hydroxide. Measure
into an evaporating dish 100 cc. of the sample, or if nitrate is very
high such volume as will contain about 0.01 mg. of nitrate nitrogen. Add
sufficient N/50 sulfuric acid nearly to neutralize the alkalinity. Then
add sufficient standard silver sulfate to precipitate all but about 0.1
mg. of chloride. The removal of chloride may be omitted if the sample
contains less than 30 parts per million of chloride. Heat the mixture to
boiling, add a little aluminium hydroxide, stir, filter, and wash with
small amounts of hot water. Evaporate the filtrate to dryness, and add 2
cc. of the phenoldisulfonic acid, rubbing with a glass rod to insure
intimate contact. If the residue becomes packed or appears vitreous
because of the presence of much iron, heat the dish on the water bath
for a few minutes. Dilute the mixture with distilled water, and add
slowly a strong solution of potassium hydroxide or ammonium hydroxide
until the maximum color is developed. Transfer the solution to a Nessler
tube, filtering if necessary. If nitrate is present a yellow color will
be formed. Compare the color with that of standards[52][55] made by
adding 2 cc. of strong potassium hydroxide or ammonium hydroxide to
various amounts of standard nitrate solution and diluting them to 50 cc.
in Nessler tubes. The following amounts of standard nitrate solution are
suggested: 0, 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0, 10.0, 15.0, 20.0, and
40.0 cc. These standards may be kept several weeks without
deterioration. If 100 cc. of water is used the number of cubic
centimeters of the standard multiplied by 0.01 is equal to parts per
million of nitrate nitrogen.
Standards that will remain permanent for several years if stored in the
dark may be prepared from tripotassium nitrophenoldisulfonate.[5]
If nitrite nitrogen is present in excess of 1 part per million it should
be oxidized by heating the samples a few minutes with a few drops of
hydrogen peroxide free from nitrate repeatedly added[95] or by adding
dilute potassium permanganate in the cold until a faint pink coloration
appears; the nitrogen equivalent of the nitrite thus oxidized to nitrate
is then subtracted from the final nitrate nitrogen reading.