Colorimetric determination of nanomolar concentrations of ammonium in seawater using solvent extraction |
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| Institution: | 1. Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854-8066, USA;2. ALS Oil & Gas, 6510 Guhn Road, Houston, TX 77040, USA;1. Department of Agronomy Food Natural resources Animals and Environment (DAFNAE), University of Padova, Legnaro, PD, Italy;2. Interdepartmental Centre for Research in Viticulture and Enology (CIRVE), University of Padova, Conegliano, TV, Italy;3. Department of Microbiology, Faculty of Agriculture, University of Zagreb, Zagreb, Croatia |
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| Abstract: | A solvent extraction method for measuring nanomolar concentrations of ammonium in seawater is described. The procedure is based on formation of indophenol in alkaline solution by reaction of phenol, hypochlorite and ammonia using sodium aquopentacyanoferrate as a coupling agent. Indophenol is then concentrated by extraction into n-hexanol at low pH and re-extraction into aqueous alkaline buffer. The concentration of indophenol blue is determined colorimetrically. The molar absorbance is 2.08 × 105 absorbance units per molar NH4+ in seawater with a precision of ± 1.9 nM NH4+ (95% Cl) for concentrations ≤ 50 nanomolar. This represents a 12-fold improvement in sensitivity and a 26-fold improvement in precision over standard aqueous analyses. Calibration curves are linear to at least 2 μM NH4+. Sensitivity in seawater is 97% of that found in deionized distilled water due to a slight salt effect. |
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