Since 1969, the nutrient chemistry group within the Ocean Chemistry Division (OCD) at the National Oceanic and Atmospheric Administration's Atlantic Oceanographic and Meteorological Laboratory (NOAA/AOML) has been performing shipboard and laboratory analyses of seawater nutrient concentrations. Our analysts have used Technicon models I and II, and AlpKem model RFA/2 autoanalyzers to routinely measure dissolved silicate, ammonia, nitrate, nitrite, and orthophosphate in coastal and oceanic water samples. We have made numerous modifications and improvements in existing methodologies to provide the precision and accuracy required by the marine scientific community. The data resulting from our analyses has been published in NOAA technical reports and has been incorporated into nutrient distribution studies in the North Atlantic.
EPA approached OCD and asked us to utilize this experience to develop and document standardized methods to be employed in the future by their investigators.
Silicate reacts with molybdate in an acidic solution to form silicomolybdate. Silicomolybdate is then reduced with ascorbic acid to molybdenum blue. The absorbance of the molybdenum blue measured at 660 nm is linearly proportional to the concentration of silicate in the sample.
Ammonium in the sample reacts with phenol and alkaline hypochlorite to form a indophenol blue. The blue color is intensified with sodium nitroferricyanide. The absorbance of measured at 640 nm is linearly proportional to the concentration of ammonia in the sample.
Nitrate and Nitrite:
Nitrite is diazotized with sulfanilamide and coupled with N-(1-naphthyl)ethylenediamine dihydrochloride to form an azo dye. The absorbance of the azo dye, measured at 540 nm is linearly proportional to the concentration of nitrite in the sample. In a second analysis, nitrate is quantitatively reduced to nitrite by passing the sample through a copperized cadmium coil. Then nitrate + nitrite, (that is reduced nitrate plus nitrite already in the sample), is measured as noted above. Nitrate concentrations are obtained by subtracting nitrite from nitrate + nitrite.
Orthophosphate reacts with molybdenum (VI) and antimony (III) in an acidic medium to form an antimonyphosphomolybdate complex. This complex is subsequently reduced with ascorbic acid to form a blue complex and the absorbance is measured at 660 nm.
Jia-Zhong Zhang, Ortner, Peter, Fischer, Charles and Moore, Lloyd. Determination of Ammonia in Estuarine and Coastal Waters by Gas Segmented Continuous Flow Colorimeteric Analysis. Submitted to the EPA's manual entitled " Methods for the determination of Chemical Substances in Marine and Estuarine Environmental Samples ".
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