Total nitrogen
In the absence of nitrates
7.2—7.2.1 Transfer by pipette into a 300 ml Kjeldahl flask a portion of the filtrate (7.1), containing a maximum of 100 mg nitrogen. Add 15 ml concentrated sulphuric acid (4.9), 0.4 g copper oxide or 1.25 g copper sulphate (4.24), and a few glass beads to control boiling. Heat moderately at first in order to initiate the reaction, then more strongly until the liquid becomes colourless or slightly greenish and white fumes appear. After cooling, transfer the solution into the distillation flask, dilute to about 500 ml with water and add a few granules of pumice stone (4.25). Connect the flask to the distillation apparatus (5.1) and carry out the determination as described in Method 8a, 7.1.1.2.
In the presence of nitrates
7.2.2 Transfer by pipette into a 500 ml Erlenmeyer flask a portion of the filtrate (7.1) containing not more than 40 mg nitric nitrogen. At this stage of the analysis, the total quantity of nitrogen is unimportant. Add 10 ml 30% sulphuric acid (4.12), 5 g reduced iron (1.2), and immediately cover the Erlenmeyer flask with a watch glass. Heat gently until the reaction becomes strong but not violent. Stop heating and allow to stand for at least 3 hours at ambient temperature. Transfer the liquid quantitatively to a 250 ml graduated flask, ignoring undissolved iron. Make up to the mark with water and mix carefully. Transfer by pipette a portion containing a maximum of 100 mg nitrogen into a 300 ml Kjeldahl flask. Add 15 ml concentrated sulphuric acid (4.9), 0.4 g copper oxide or 1.25 g copper sulphate (4.24), and a few glass beads.
Heat moderately at first in order to initiate the reaction, then more strongly until the liquid becomes colourless or slightly greenish and white fumes appear. After cooling, transfer the solution quantitatively to the distillation flask, dilute to about 500 ml with water, and add a few granules of pumice stone (4.25). Connect the flask to the distillation apparatus (5.1) and continue the determination as described in Method 8a, 7.1.1.2.
Blank test
7.2.3 Make a blank test in the same conditions (omitting only the sample), and use this value in the calculation of the final result.
Expression of result
7.2.4
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out the same conditions as the analysis.
A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.
M = weight of the sample, in grams, present in the aliquot part taken for analysis.
where:
a = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the blank, carried out the same conditions as the analysis.
A = ml of standard solution of sodium or potassium hydroxide (0.2 N) used for the analysis.
M = weight of the sample, in grams, present in the aliquot part taken for analysis.