236

QL'A\TITAT1\'E AGRICULTURAL

soils nitrates have accumulated in such, amounts as to interfere
with plant growth.1

The plienoldisulphoiiic acid method is used for the deter-
mination of nitrates, the following equations representing the
reactions :

2KNOS -» 2HNOa + K2SO4; (1)

; (2)

C6H3OH(S03H)2 + HNOs-> kQH2OH-N02(SO3H)2 -h

Pbcnoldisulphonic acid A * Nitrophenoldisulplionic acid

C6H2OH-N02(S03H)2 4- 3NH4OH->C6HsONH4(S

+ 3H20. (3)

The ammonium salt of nitrophenoldisulphonic acid, thus
formed, is intensely yellow and the color so produced is compared
with that formed from a standard nitrate solution.

FIG. 51.—Mixing machine.
Determination of ITitrate Nitrogen.—Prepare the following reagents:
(a) Phenoldisulphonic Add.—Mix 30 gm of pure crystallized phenol with
370 gm of concentrated sulphuric acid. Immerse the flask in boiling water
for six hours. When cool store in an amber colored battle. A smaller
quantity of the solution may be made, if desired.
(b) Standard Color Solution,.—Prepare a standard solution .of potassium
nitrate by dissolving 0.7215 gm of dried pure potassium nitrate in distilled
water and diluting to 1 liter. Each cubic centimeter of this solution will
contain 0.1 mg of nitrogen. Pipette 10 cc of this solution, into a dish and
evaporate to dryness over a steam bath. Cool and moisten the dry nitrate
with 2 cc of phenoldisulphonic acid, rubbing together with a glass rod,
1 Colo. Exp. Star. BitlL, 178