Store in a brown glass bottle in a refrigerator (stable for
months).
3. Prepare reagent I (Tri-sodium citrate solution) by dissolving
240 g tri-sodium citrate dihydrate in about 500 ml ultrapure
water. Add 20 ml 0.5 mol/l sodium hydroxide solution and
anti-bumping granules and boil the solution until the volume is
below 500 ml to remove ammonia. Cool and add ultrapure
water to a final volume of 500 ml. Store in a brown glass bottle.
4. Prepare reagent III (Trione solution) by dissolving 0.25 g
Trione in 100 ml 0.5 mol/l NaOH. Store in a brown glass
bottle in a refrigerator (stable for approximately 3 days).
5. Prepare ammonium standard 140 μg/ml N (10 μmol/ml) by
dissolving 0.1338 g ammonium chloride (pre-dried at 100
C
for 1 h) in ultrapure water and then add ultrapure water to a
final volume of 250 ml. Store in a glass bottle in a refrigerator
(stable for approximately 1 month).
6. On the day of analyses, prepare suitable working solutions of
ammonium (0–100 μg N/l) for calibration by diluting the
standard stock solution (see Step 5) with ultrapure water.
7. Measure the standards manually by a spectrophotometer for a
calibration curve and linearity test (see Note 2).
8. Measure 35 ml of working standards and 35 ml ultrapure water
for blanks with graduated cylinder and pour into reaction
flasks.
9. Add 1 ml citrate solution, 1 ml phenol solution, and 1 ml trione
solution to standards and blanks. Swirl between additions to
mix and close the flasks. Let stand overnight in dark at room
temperature.
10. Measure the absorbance of blanks and standards in a 5 cm
cuvette at 630 nm for the calibration curve (see Note 2).
11. Collect the samples in glass bottles or polyethylene bottles and
analyze the samples immediately after collection. The sample
can be stored in a glass bottle in a refrigerator for about 3 h.
12. Perform manual analyses with a spectrophotometer. Verify
proper operation of the instrument with certified reference
material and include ultrapure water blanks in the run
(see Note 3).
13. Measure two 35 ml aliquots of sample with a graduated cylinder to reaction flasks (sample and reference; see Note 5).
14. Add 1 ml citrate reagent, 1 ml phenol solution, and 1 ml trione
solution to the sample flask. Add 1 ml citrate solution and 2 ml
ultrapure water to reference sample for turbidity. Swirl between
additions to mix and close the flask. Let stand overnight at dark
at room temperature.
76
Jaana Koistinen et al.
months).
3. Prepare reagent I (Tri-sodium citrate solution) by dissolving
240 g tri-sodium citrate dihydrate in about 500 ml ultrapure
water. Add 20 ml 0.5 mol/l sodium hydroxide solution and
anti-bumping granules and boil the solution until the volume is
below 500 ml to remove ammonia. Cool and add ultrapure
water to a final volume of 500 ml. Store in a brown glass bottle.
4. Prepare reagent III (Trione solution) by dissolving 0.25 g
Trione in 100 ml 0.5 mol/l NaOH. Store in a brown glass
bottle in a refrigerator (stable for approximately 3 days).
5. Prepare ammonium standard 140 μg/ml N (10 μmol/ml) by
dissolving 0.1338 g ammonium chloride (pre-dried at 100
C
for 1 h) in ultrapure water and then add ultrapure water to a
final volume of 250 ml. Store in a glass bottle in a refrigerator
(stable for approximately 1 month).
6. On the day of analyses, prepare suitable working solutions of
ammonium (0–100 μg N/l) for calibration by diluting the
standard stock solution (see Step 5) with ultrapure water.
7. Measure the standards manually by a spectrophotometer for a
calibration curve and linearity test (see Note 2).
8. Measure 35 ml of working standards and 35 ml ultrapure water
for blanks with graduated cylinder and pour into reaction
flasks.
9. Add 1 ml citrate solution, 1 ml phenol solution, and 1 ml trione
solution to standards and blanks. Swirl between additions to
mix and close the flasks. Let stand overnight in dark at room
temperature.
10. Measure the absorbance of blanks and standards in a 5 cm
cuvette at 630 nm for the calibration curve (see Note 2).
11. Collect the samples in glass bottles or polyethylene bottles and
analyze the samples immediately after collection. The sample
can be stored in a glass bottle in a refrigerator for about 3 h.
12. Perform manual analyses with a spectrophotometer. Verify
proper operation of the instrument with certified reference
material and include ultrapure water blanks in the run
(see Note 3).
13. Measure two 35 ml aliquots of sample with a graduated cylinder to reaction flasks (sample and reference; see Note 5).
14. Add 1 ml citrate reagent, 1 ml phenol solution, and 1 ml trione
solution to the sample flask. Add 1 ml citrate solution and 2 ml
ultrapure water to reference sample for turbidity. Swirl between
additions to mix and close the flask. Let stand overnight at dark
at room temperature.
76
Jaana Koistinen et al.