90
Exercise 7
The method is applicable in a range of about 1 to 500 flg P0 4 -P/I, with a precision
level of ± 1 flg P04-P/1. If natural concentrations are low, an extraction method is
necessary to increase sensitivity [see below and Strickland and Parsons (1968)].
Sample Storage. The water samples should be filtered immediately through glass
fiber filters (Reeve Angel 984H, 0.5-flm pore size or Whatman GF/F, 0.6 to 0.7 flm pore
size) and stored at 5°C. Analyses must be performed within 2 h, preferably within a halfhour of collection. When the analyses must be delayed for more than a few hours, they
should be quick frozen (40% glycol bath at - 20°C) in screw-capped polyethylene
bottles.
Reagents
1. Ammonium molybdate solution: Dissolve 15 g of ammonium paramolybdate
(NH4)6M070z4-4HzO in distilled water and dilute to 500ml. Store in an amber
polyethylene bottle.
2. Sulfuric acid: Add 140 ml of concentrated sulfuric acid to 900 ml of distilled water.
Store in a glass-stoppered bottle.
3. Ascorbic acid solution: Dissolve 27 g of L-ascorbic acid in 500 ml of distilled water.
Make fresh daily or store solution frozen in a plastic bottle; thaw for use and refreeze
immediately. The solution is stable for a few days if refrigerated.
4. Potassium antimonyl-tartrate solution: Dissolve 0.34 g of potassium antimonyltartrate [K(SbO)C 4 H 4 0 6 ·tHzO] in 250 ml of distilled water, warming if necessary.
Store in a glass bottle.
5. Composite reagent: Mix together 100 ml ammonium molybdate, 250 ml sulfuric acid,
100 ml ascorbic acid, and 50 ml potassium antimonyl-tartrate solutions. Prepare
daily for use and discard any excess.
6. Phosphate standard solution: Dissolve 0.2197 g of oven-dried (105°C, 24 h) potassium
dihydrogen phosphate (KH z P0 4 ) in distilled water and dilute to 1 liter. Store in
dark bottle with 1 ml of chloroform:
1.00ml = 50.0jlgP0 4 -P
Dilute with distilled water as necessary to make appropriate standards.
Procedures for concentrations greater than 10pg P0 4 -P/I
1. Obtain water samples of greater than 100 ml and warm, if necessary, to between 15
and 30°C.
2. Because certain waters are colored or turbid, measure thc absorbance (ODturb ) of a
subsample of the water to obtain a turbidity correction.
3. To 100 ml of the sample add 10 ± 0.5 ml of the composite reagent from a 25-ml
graduated cylinder; mix thoroughly at once.
4. After 10 min, but within 2 h, measure the extinction coefficient of the solution in a Ito 10-cm cell at a wavelength of885 nm. Adjust the photometer to zero reading using
distilled water before measuring the extinction coefficient of the sample.
5. Measure the absorbance of a reagent blank (OD b ) (steps 3 and 4 above using distilled
water in place of the sample).
6. Subtract the extinction value of the reagent blank and the turbidity-color correction
from the value for the sample extinction to obtain a corrected sample extinction:
ODcorr = ODs - (ODb + ODturb)
Exercise 7
The method is applicable in a range of about 1 to 500 flg P0 4 -P/I, with a precision
level of ± 1 flg P04-P/1. If natural concentrations are low, an extraction method is
necessary to increase sensitivity [see below and Strickland and Parsons (1968)].
Sample Storage. The water samples should be filtered immediately through glass
fiber filters (Reeve Angel 984H, 0.5-flm pore size or Whatman GF/F, 0.6 to 0.7 flm pore
size) and stored at 5°C. Analyses must be performed within 2 h, preferably within a halfhour of collection. When the analyses must be delayed for more than a few hours, they
should be quick frozen (40% glycol bath at - 20°C) in screw-capped polyethylene
bottles.
Reagents
1. Ammonium molybdate solution: Dissolve 15 g of ammonium paramolybdate
(NH4)6M070z4-4HzO in distilled water and dilute to 500ml. Store in an amber
polyethylene bottle.
2. Sulfuric acid: Add 140 ml of concentrated sulfuric acid to 900 ml of distilled water.
Store in a glass-stoppered bottle.
3. Ascorbic acid solution: Dissolve 27 g of L-ascorbic acid in 500 ml of distilled water.
Make fresh daily or store solution frozen in a plastic bottle; thaw for use and refreeze
immediately. The solution is stable for a few days if refrigerated.
4. Potassium antimonyl-tartrate solution: Dissolve 0.34 g of potassium antimonyltartrate [K(SbO)C 4 H 4 0 6 ·tHzO] in 250 ml of distilled water, warming if necessary.
Store in a glass bottle.
5. Composite reagent: Mix together 100 ml ammonium molybdate, 250 ml sulfuric acid,
100 ml ascorbic acid, and 50 ml potassium antimonyl-tartrate solutions. Prepare
daily for use and discard any excess.
6. Phosphate standard solution: Dissolve 0.2197 g of oven-dried (105°C, 24 h) potassium
dihydrogen phosphate (KH z P0 4 ) in distilled water and dilute to 1 liter. Store in
dark bottle with 1 ml of chloroform:
1.00ml = 50.0jlgP0 4 -P
Dilute with distilled water as necessary to make appropriate standards.
Procedures for concentrations greater than 10pg P0 4 -P/I
1. Obtain water samples of greater than 100 ml and warm, if necessary, to between 15
and 30°C.
2. Because certain waters are colored or turbid, measure thc absorbance (ODturb ) of a
subsample of the water to obtain a turbidity correction.
3. To 100 ml of the sample add 10 ± 0.5 ml of the composite reagent from a 25-ml
graduated cylinder; mix thoroughly at once.
4. After 10 min, but within 2 h, measure the extinction coefficient of the solution in a Ito 10-cm cell at a wavelength of885 nm. Adjust the photometer to zero reading using
distilled water before measuring the extinction coefficient of the sample.
5. Measure the absorbance of a reagent blank (OD b ) (steps 3 and 4 above using distilled
water in place of the sample).
6. Subtract the extinction value of the reagent blank and the turbidity-color correction
from the value for the sample extinction to obtain a corrected sample extinction:
ODcorr = ODs - (ODb + ODturb)