3 Method
Use acid-washed glassware (soaked for 4 h in 6% HCl and rinsed
with deionized water).
3.1 Total Nitrogen
The procedure follows that of Koroleff in the first edition of [1, 3]
(excluding the reduction and reaction to the azo dye that is performed here with a combined reagent, containing VCl 3 reduction
solution as in [2]).
1. Prepare 0.075 mol/l sodium hydroxide solution by dissolving
3 g sodium hydroxide in 1000 ml of ultrapure water (see
Note 1).
2. Prepare oxidizing reagent by dissolving 10 g potassium peroxodisulfate and 6 g boric acid in 1 l 0.075 mol/l sodium
hydroxide solution (see Notes 2 and 3). Store in a brown
glass bottle in a refrigerator (stable for several weeks).
3. Prepare sulfanilamide solution (nitrite reagent I) by dissolving
1.0 g sulfanilamide in a mixture of 85 ml ultrapure water and
14.5 ml concentrated HCl. Store in a glass bottle at room
temperature.
4. Prepare
N-(1-naphtyl)-ethylenediamine
dihydrochloride
(NED) solution (nitrite reagent II) by dissolving 0.07 g NED
in 100 ml ultrapure water. Store in a brown glass bottle in a
refrigerator (renew if the color is brown; stable about
1 month).
5. Prepare vanadium chloride solution by dissolving 1.6 g vanadium chloride in a mixture of 170 ml ultrapure water and
16.8 ml concentrated HCl, and then add ultrapure water to a
final total volume of 200 ml. Store in glass bottle in a refrigerator (stable about 1 month).
6. Prepare TN mix reagent by mixing 200 ml vanadium chloride
solution with 40 ml sulfanilamide solution and 40 ml amine
solution. Store in a refrigerator or freeze (prolongs stability).
7. Prepare total nitrogen standard 140 μg/ml N (10 μmol/ml N)
by dissolving 0.1862 g Na 2 -EDTA in ultrapure water, and then
add ultrapure water to a final total volume of 100 ml. Store in a
glass bottle in a refrigerator (stable for several months).
8. On the day of analyses, prepare working solutions of total
nitrogen (0–600 μg/l) for calibration by diluting the standard
stock solution (see step 7) with ultrapure water (e.g., 1 ml to
50 ml for 2800 μg/l solution and then, e.g., 10 ml of this
dilution to 50 ml ultrapure water for 560 μg/l solution).
Determining Total Nitrogen
83
Use acid-washed glassware (soaked for 4 h in 6% HCl and rinsed
with deionized water).
3.1 Total Nitrogen
The procedure follows that of Koroleff in the first edition of [1, 3]
(excluding the reduction and reaction to the azo dye that is performed here with a combined reagent, containing VCl 3 reduction
solution as in [2]).
1. Prepare 0.075 mol/l sodium hydroxide solution by dissolving
3 g sodium hydroxide in 1000 ml of ultrapure water (see
Note 1).
2. Prepare oxidizing reagent by dissolving 10 g potassium peroxodisulfate and 6 g boric acid in 1 l 0.075 mol/l sodium
hydroxide solution (see Notes 2 and 3). Store in a brown
glass bottle in a refrigerator (stable for several weeks).
3. Prepare sulfanilamide solution (nitrite reagent I) by dissolving
1.0 g sulfanilamide in a mixture of 85 ml ultrapure water and
14.5 ml concentrated HCl. Store in a glass bottle at room
temperature.
4. Prepare
N-(1-naphtyl)-ethylenediamine
dihydrochloride
(NED) solution (nitrite reagent II) by dissolving 0.07 g NED
in 100 ml ultrapure water. Store in a brown glass bottle in a
refrigerator (renew if the color is brown; stable about
1 month).
5. Prepare vanadium chloride solution by dissolving 1.6 g vanadium chloride in a mixture of 170 ml ultrapure water and
16.8 ml concentrated HCl, and then add ultrapure water to a
final total volume of 200 ml. Store in glass bottle in a refrigerator (stable about 1 month).
6. Prepare TN mix reagent by mixing 200 ml vanadium chloride
solution with 40 ml sulfanilamide solution and 40 ml amine
solution. Store in a refrigerator or freeze (prolongs stability).
7. Prepare total nitrogen standard 140 μg/ml N (10 μmol/ml N)
by dissolving 0.1862 g Na 2 -EDTA in ultrapure water, and then
add ultrapure water to a final total volume of 100 ml. Store in a
glass bottle in a refrigerator (stable for several months).
8. On the day of analyses, prepare working solutions of total
nitrogen (0–600 μg/l) for calibration by diluting the standard
stock solution (see step 7) with ultrapure water (e.g., 1 ml to
50 ml for 2800 μg/l solution and then, e.g., 10 ml of this
dilution to 50 ml ultrapure water for 560 μg/l solution).
Determining Total Nitrogen
83