284
QUANTITATIVE AGRICULTURAL ANALYSIS
,*'

digesting with sulphuric acid. In order to avoid this loss the
Kjeldahl method may be modified by adding benzoic acid, then
using permanganates to oxidize the nitrobenzoic acid to ammonia.
Phenolsulphonic acid may be substituted for benzoic acid, the
nitrophenolsulphonic acids formed being then reduced to amino-
phenolsulphonic acid by zinc dust. This compound is then
oxidized by heating with sulphuric acid.
Salicylic acid has now superseded both benzoic acid and
phenolsulphonic acid. The reducing agent is either sodium
thiosulphate or zinc dust:

/
/
2KN0_3
HN0_3 + C_6H_4

H_2S0_4
OH

K_2S0_4 + 2HN0_3,
OH
2H_20,

(i)
(2)

The nitro compound is then reduced by nascent hydrogen
from zinc and sulphuric acid:

xOH
-COOH •
OH
6H + C_6H_3~COOH -» CeHsf-COOH + 2H_20,	(3)

•IK
^(i
•j
hi

f f
11

or by sodium thiosulphate:
Na_2S_20_3 + H_2S0_4 -» Na_2SO_4 + H_2S0_3+S,	(4)
/OH	/OH
3H_2S0_3 + CeHs^COOH + H_2O-H>3H_2S0_4 + C_8H_3^-COOH.  (5)
\NH_2
The amino acid is then oxidized by concentrated sulphuric acid,
ammonium sulphate resulting.
Determination  of  Total  Nitrogen in  Materials   Containing  Nitrates:
Modified Kjeldahl Method.—Weigh 2 gm of fertilizer and place in a Kjeldahl
flask. Add 30 cc of concentrated sulphuric acid containing 2 gm of sali-
cylic acid (these must be added together) and mix by shaking vigorously.
After 30 minutes add 5 gm of sodium thiosulphate or 2 gm of zinc dust. If
zinc dust is used it must be added gradually, shaking the flask after each
addition. Heat gently until frothing ceases then boil for 10 minutes. Add
0.7 gm of mercury oxide or 0.3 gm of copper sulphate and continue the diges-
tion, distillation and titration as in the Kjeldahl method. Make a blank
determination for nitrogen in the reagents, using sugar as already directed.
Calculate the per cent of total nitrogen in the fertilizer.