Quantitative aqueous ammonium ion analysis by transmission infrared spectroscopy

An investigation was undertaken to determine whether ammonium ion could be quantitated in aqueous solution by using commercially available infrared filter instruments such as the Multispec M1 analyzer. Ammonium salt solutions were scanned using an infrared spectrophotometer modified specifically to facilitate the study of aqueous systems and were shown to have 2 main absorption bands at 3.49 microns (2865 cm-1) and 6.89 microns (1451 cm-1). The 3.49 microns band did not correspond to any individual band noted in the literature and was concluded to be a composite of fundamental bands and Fermi resonances due to hydrogen bonding affecting the ion in solution. The response of the 6.89 microns band to NH4+ concentration was approximately twice that of the 3.49 microns band, and quantitation of the ammonium ion was assessed by the dual wavelength method as employed in filter-based commercial infrared analyzers. Good quantitation was obtained using 5.56 microns as a reference wavelength, and the slope of the standard curve of ammonium sulfate was not significantly affected by pH in the pH 3-8 region. A 6.86 microns sample filter and a 5.56 microns reference filter were installed in a Multispec M1 analyzer and a linear response was obtained for concentrations of up to 0.6% ammonium sulfate. Accurately weighed samples of ammonium sulfate were converted to ammonia by using the micro-Kjeldahl procedure and then distilled into standard acid, diluted to volume, and assessed by titration and transmission infrared analysis. The infrared method was more accurate than the titration method in replicating the initial weights.(ABSTRACT TRUNCATED AT 250 WORDS)