84
Exercise 7
ammonium levels; S-cm path length for the range 0 to 200,ug NH4-N/I; 10-cm for
< 20,ug NH4-N/I).
7. Prepare a standrad curve of 0.0. versus concentration of your corrected standrads.
Sample concentrations then can be read directly from the graph. If the standard
curve were linear, as it should be in this case, sample concentrations would be
calculated directly, where
Then
ODb = absorbance of distilled water plus reagents (blank)
ODs = absorbance of standards and samples plus reagents
Concentration of standard
F _
(,ug NH4-N/I)
_ . .
- 00 f
d d 0
- extmctlOn factor
s 0 stan ar - Db
Note: Such standard curves can be generated easily on a hand calculator or
computer, as well as pertinent statistics such as a correlation coefficient (r 2 ).
Nitrate and Nitrite Nitrogen
By far the best method for analysis of N0 3 -N in water is to reduce the nitrate in
alkaline-buffered solution to nitrite by passing the sample through a column of
cop peri zed cadmium metal filings (Wood et aI., 1967). The nitrite is then measured by a
sensitive diazotization method that results in a stable pink azo dye whose absorbance
obeys Beer's Law up to about SOO,ug N0 3 -N or N0 2 -N/I. Nitrite concentrations ofthe
original sample can be determined very accurately (S.D. ± ca. O.S,ug NOrN/I) prior to
reduction of N0 3 to N0 2 by the same diazotization technique.
The cadmium-reduction method requires special glass columns for efficient analysis.
Once the equipment is assembled and operational, however, this method permits
analysis of about eight samples per hour per column. Four columns easily can be
operated simultaneously.
Sample Storage. Samples should be filtered through comb us ted (SOO°C) glass fiber
filters (Reeve Angel 984 H, O.S-,um pore size or Whatman GF/F, 0.6-0.7-,um pore size).
The filtrate is assumed to contain only dissolved materials. The filtrate can be stored at
4 to SoC but should be analyzed within 24 h.
Apparatus and Reagents
I. Reduction column. Columns constructed of borosilicate glass of the approximate
dimensions illustrated in Fig. 7.2 are recommended. The bore of the portion
retaining the copporized cadmium filings should not be less than 8 mm. Very fine
"wool" turnings of copper are recommended (Strickland and Parsons, 1968) in place
of glass wool at the bottom of the column; if copper turnings are used, however, they
must be very fine.
2. Cadmium-copper filings: Filings of cadmium I> 99.9/,0 purity} should be sieved to
retain particles of a size between O.S and 2 mm. A void ingesting or inhaling the toxic
cadmium particles! Perform the sieving of filings and the filling of columns in a hood.
Approximately 40 g per column of the Cd filings are washed with 2N HCI (8S ml of
concentrated HCI diluted to SOO ml with distilled water) in a separatory funnel and
Exercise 7
ammonium levels; S-cm path length for the range 0 to 200,ug NH4-N/I; 10-cm for
< 20,ug NH4-N/I).
7. Prepare a standrad curve of 0.0. versus concentration of your corrected standrads.
Sample concentrations then can be read directly from the graph. If the standard
curve were linear, as it should be in this case, sample concentrations would be
calculated directly, where
Then
ODb = absorbance of distilled water plus reagents (blank)
ODs = absorbance of standards and samples plus reagents
Concentration of standard
F _
(,ug NH4-N/I)
_ . .
- 00 f
d d 0
- extmctlOn factor
s 0 stan ar - Db
Note: Such standard curves can be generated easily on a hand calculator or
computer, as well as pertinent statistics such as a correlation coefficient (r 2 ).
Nitrate and Nitrite Nitrogen
By far the best method for analysis of N0 3 -N in water is to reduce the nitrate in
alkaline-buffered solution to nitrite by passing the sample through a column of
cop peri zed cadmium metal filings (Wood et aI., 1967). The nitrite is then measured by a
sensitive diazotization method that results in a stable pink azo dye whose absorbance
obeys Beer's Law up to about SOO,ug N0 3 -N or N0 2 -N/I. Nitrite concentrations ofthe
original sample can be determined very accurately (S.D. ± ca. O.S,ug NOrN/I) prior to
reduction of N0 3 to N0 2 by the same diazotization technique.
The cadmium-reduction method requires special glass columns for efficient analysis.
Once the equipment is assembled and operational, however, this method permits
analysis of about eight samples per hour per column. Four columns easily can be
operated simultaneously.
Sample Storage. Samples should be filtered through comb us ted (SOO°C) glass fiber
filters (Reeve Angel 984 H, O.S-,um pore size or Whatman GF/F, 0.6-0.7-,um pore size).
The filtrate is assumed to contain only dissolved materials. The filtrate can be stored at
4 to SoC but should be analyzed within 24 h.
Apparatus and Reagents
I. Reduction column. Columns constructed of borosilicate glass of the approximate
dimensions illustrated in Fig. 7.2 are recommended. The bore of the portion
retaining the copporized cadmium filings should not be less than 8 mm. Very fine
"wool" turnings of copper are recommended (Strickland and Parsons, 1968) in place
of glass wool at the bottom of the column; if copper turnings are used, however, they
must be very fine.
2. Cadmium-copper filings: Filings of cadmium I> 99.9/,0 purity} should be sieved to
retain particles of a size between O.S and 2 mm. A void ingesting or inhaling the toxic
cadmium particles! Perform the sieving of filings and the filling of columns in a hood.
Approximately 40 g per column of the Cd filings are washed with 2N HCI (8S ml of
concentrated HCI diluted to SOO ml with distilled water) in a separatory funnel and