ANALYST, MARCH zyxwvutsrqpon 1992, VOL. 117 zyxwvutsrq 379 z Determination of Ammonia Using Carbon Dioxide Laser P hotoacoust ic Spectroscopy Cornpared With Conventiona I Spectrophotometry* Aniko M . Solyomt and Gyorgy zyxwvut 2. Angelit Analtron Applied Research zyxwvut Co., PO5 63, H-131 zyxwvut I Budapest, Hungary Dane D. Bicanic laser Photoacoustic laboratory, Department of Agricultural Engineering and Physics, Agricultural University, Duivendaal 7, 670 I zyxwvutsrqp A P Wageningen, The Netherlands Marcel Lubbers Department of Soil Science and Geology, Agricultural University, Duivendaal 10, 6701 A R Wageningen, The Netherlands zyxwvutsr The potential of carbon dioxide laser photoacoustic spectroscopy and the traditional Indophenol Blue colorimetric method for detecting gaseous ammonia have been compared. The results obtained with the two independent techniques are comparable in the range of concentrations studied. Keywords: Infrared spectroscopy of ammonia; colorimetric Indophenol Blue reaction; carbon dioxide laser photoacoustic spectroscopy; windowless resonant photoacoustic cell with automatic resonance tracking system; atmospheric pollution Nitrogen plays an active part in the acidification of soils, where it is mainly present as organic nitrogen. The main organic forms are the ammonium and very mobile nitrate ions. Under aerobic conditions, ammonia is readily converted into nitrate owing to the activity of zyxwvutsr Nitrosomonas and Nitrobacter bacteria. Ammonia plays a significant role in the atmosphere, because it is the only highly soluble common base present. It can neutralize different acids; the ammonium ion, NH4+, is a frequently found constituent of atmospheric aerosols and fogs. Ammonia is produced by biological processes from organic nitrogen compounds and hence exists in both clean and polluted atmospheres. The Netherlands has a very intensive livestock farming industry and, therefore, the ammonia concentration in the air is the highest in Europe. There are several methods of determining ammonia and NH4+ in water, e.g., classical colorimetry, titration, ion- selective electrode methods and spectrofluorimetry. The most frequently encountered is the modified Indophenol Blue method, where ammonia is determined as an emerald green complex formed by the reaction of ammonia with salicylate and phenol-sodium hypochlorite solution, catalysed by sodium nitroprusside in a buffered alkaline medium of pH The Indophenol Blue complex displays a broad absorption feature between 600 and 700 nm, with a maximum at 650 nm. A practical detection limit of ~0.25 mmol m-3 in the range 0-9.0 mg I-' of NH4+ is obtained.* Photoacoustic (PA) spectroscopy is a sensitive method for the measurement of low concentrations of gases and as such appears to be a very attractive technique for studying atmospheric pollution .3-8 In recent years, numerous studies have demonstrated the feasibility of infrared laser PA spectroscopy for detecting ammonia in a variety of environ- mental and agricultural applications.9-15 Recently, a step-tunable carbon dioxide laser radiation source has been developed that, in conjunction with a windowless (open) resonant cell (with a high acoustic quality 12.8-1 3.' ~~~~ ~ ~ ~ ~ * Presented at the XXVII Colloquium Spectroscopicum Inter- t Present address: Department of Chemistry, University of nationale (CSI), Bergen, Norway, June 9-14, 1991. Arizona, Tucson. AZ 85721, USA. factor), has been used for trace analysis of ambient ammo- nia.16 The reason for undertaking this study, the results of which are described in this paper, was to investigate whether or not PA spectroscopy provides the same analytical results as the commonly recommended standard Indophenol Blue method. 17 Experimental Colorimetric Measurements The actual measurements were carried out at 660 nm using the Technicon Auto-Analyser I1 continuous flow system. *8 The blank, reagents and samples were transported to the colorimeter by means of a peristaltic pump and the colour development took place at 37 "C. The cuvette of the colorimeter was 50 mm long and 1.5 mm in diameter. The reagents for the colorimetric reaction were prepared following standard procedures. 19 Standard solutions of NH4+ for calibration were prepared from (NH4)2S04 (Merck, Darmstadt, Germany; pro analysi), sodium phosphate buffer (Merck, pro analysi) using a nitrogen-free NaOH solution (J. T. Baker, Phillipsburg, NJ, USA) and potassium sodium tartrate solution (Merck, pro analysi). The sodium salicylate- sodium nitroprusside solution (Merck, pro analysi) was kept in the dark and the sodium hypochlorite (Merck) solution was prepared fresh daily. Before the colorimetric measurement, the ammonia was preconcentrated by bubbling the gaseous mixture through a dilute H2SO4 solution in a midget impinger in order to form ammonium sulphate. The absorbing solution was 0.0025 mol dm-3 H2SO4 (Merck, pro analysi). Each midget impinger held 50 cm3 of the absorbing solution including the blank. The period of time for bubbling varied between 30 and 60 min depending on the ammonia concentration in the standard air sample. The volume of air sampled was recorded. After the collection of ammonia, the NH4+ was determined by col- orimetry using Indophenol Blue. Photoacoustic Measurements A schematic diagram of the experimental set-up is shown in Fig. 1. The source used was a laboratory-made, tuned grating infrared carbon dioxide waveguide laser (L) operating on a