Abstract
Numerous works have demonstrated that ion exchange membranes can be used for accurate determination of the availability of nitrate and other key nutrients in soils. After letting the ion exchange membrane interact with the soil for a known period of time, the membrane is typically immersed in a strong reagent to desorb the ions and the resulting solution is analyzed by standard chemical methods. The present study shows that mid-infrared photoacoustic spectroscopy can be used to estimate directly the amount of nitrate sorbed onto such membranes and could replace advantageously the standard chemical analysis. The study was conducted with two commercially available membranes, and in both cases the average determination error achieved with a simple partial least squares model was approximately 1.6–1.8 μeq, which under the specific experimental conditions corresponds to approximately 4.5–5.0 mg[N]/kg[dry soil]. Such errors are about 30% larger than those reported in a previous study in which the membrane was analyzed by transmittance spectroscopy. However, the present method is suitable for a much wider range of membranes, such as those commonly used for water treatment and that are too thick for transmittance measurements. For such membranes, photoacoustic spectroscopy is a very cheap and rapid alternative to standard chemical analysis.
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