Determination of Nitrate in Estuarine Waters Automatic Determination Using a Brucine Method Lloyd Kahn and Francis T. Rrezenski Hudson-Champlain and Metropolitan Coastal Comprehensive Water Pollution Control Project, Federal Water Pollution Control Administration, Department of the Interior, Metuchen, N. J. 08840
Automation of the brucine method for nitrate determination described by Kahn and Brezenski on the Technicon AutoAnalyzer resulted in a twofold improvement of precision. The automated procedure is capable of analyzing 100 samples in 7 hours kith a standard deviation of less than 0.01 mg. of nitrate nitrogen per liter.
P
revious investigators (Britt, 1962; O’Brien and Fiore, 1962; Henriksen, 1965; Brewer, Chan, er al., 1966; Kamphake, Hannah, et nl., 1967) have adapted either the Mullin and Rile) (1955) o r the Morris and Riley (1963) procedure for the automatic determination of nitrates by the AutoAnalyzer. The Mullin and Riley procedure is based upon the reduction of nitrates to nitrite in alkaline solution by hydrazine in the presence of a copper salt solution. The nitrite is then utilized in the diazotization of sulfanilic acid or sulfanilamide and simultaneous coupling with the N-l-naphthyl-
ethylene diamine dihydrochloride to form a colored compound which is determined colorimetrically. The Morris and Riley procedure differs from the foregoing in that a column of cadmium amalgam, cadmium, o r zinc filings is substituted for the hydrazine in the reduction step. These methods, although extremely sensitive, are subject to interferences found in estuarine waters which prevent the stoichiometric reduction of nitrate to nitrite. This effect is attributed by the authors to the presence of compounds which are competing with the nitrate for the consumption of available hydrazine in the former case and tend to “poison” the reduction column in the latter procedure. Although each of the investigators appears to have solved the reduction step for his particular problem, difficulties may be encountered in applying the method to other environments. A recently developed determination, in which brucine sulfate is reacted under very acidic conditions with nitrate and the salt effect is eliminated by the addition of sodium chloride to the sample and standards, gives precise nitrate assays in estuarine and sea water (Jenkins and Medsker, 1964). The
F l o w Rats m l . / min.
M i x i n g Coils
95OC Bath
2 0 f t X 1.6 m m .
L-J \\
air
0.60
sample
0.60
coi I
acid-flex tubing
Time D e l a y Coil 20ft. X1.6mm.
T o waste-
T o waste
Recorder
Colorimeter 5 0 mm.cell 410mp Figure 1. Automatic nitrate analysis flow diagram
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Environinental Science and Technology
mixed reagent
0.16
sulfuric acid
0.64
Table I. Precision of the hitrate Method (Nitrate nitrogen, mg. per liter)
(1
Number of Replicates
Procedure
19 20 13 24" 25a
Automated Automated Automated Manual Manual
Mean
0 0 0 0 0
080 265 447 341 552
These results were obtained by the recommended procedure described bq
accuracy of the method was verified by the authors (Kahn and Brezenski, 1967), who modified the procedure to obtain more reproducible analytical calibration curves, which obey the Beer-Lambert relationship. This modified procedure was readily adapted to automation o n the Technicon AutoAnalyzer with gratifying results, analyzing from 0.05 to 0.6 mg. of N 0 3 - N per liter with a standard deviation of less than 0.01 mg. of N 0 3 - Nper liter-a twofold increase over the precision obtained by the nonautomated procedure. This method is preferred for the determination of nitrate because it eliminates the capricious reduction step which is an integral part of the other methods, and it more rapidly analyzes nitrate directly. Although the method when applied to filtered samples analyzed in this study gave accurate and precise results, it is expected t o be subject to interference by streams adsorbing in the 410-mp region and by turbidity. For such cases, a preliminary cleanup step would be required.
Standard Deviation
Range
0 0 0 0 0
065-0 250-0 327-0 317-0 522-0
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