Determination of Exchangeable and Easily Reducible Manganese in Soil by ICP-OES with Ammonium Chloride Extraction

Manganese is an essential and important mineral element for plants. The content of available manganese in soil is very beneficial to plant growth. Exchangeable manganese (ExMn) and easily reducible manganese (RMn) are the two most important forms of available manganese in soil. Establishing an accurate and rapid method for detecting ExMn and RMn in soil is of great significance for the evaluation of manganese availability in soil. In this work, ammonium chloride was used to extract ExMn from soil, and ammonium chloride with hydroquinone–ascorbic acid (1∶2, w/w) was used to extract RMn. Ammonium chloride was used instead of ammonium acetate in the extraction process, which not only inhibited the precipitation of carbonate-bound manganese in calcareous soil but also effectively extracted ExMn and RMn from soil. Ultrasonic extraction was used instead of shaking extraction, reducing the extraction time from 30min to 5min. The content of ExMn in the solution was directly determined by inductively coupled plasma-optical emission spectrometry (ICP-OES), and the content of RMn was obtained by the subtraction method. During the detection process, the instrument conditions were optimized, and the spectral line of 257.61nm was selected, with a detection limit of 0.13mg/kg. For ExMn, the relative standard deviation (RSD) of the ICP-OES method was 0.4%–3.3%, compared with 2.1%–6.9% for flame atomic absorption spectrometry (FAAS) and 3.6%–9.8% for colorimetry. For RMn, the RSDs were 0.7%–3.6%, 0.9%–5.3% and 1.1%–9.2%, respectively. These results demonstrate that the proposed ICP-OES method can be used to simultaneously determine ExMn and RMn in soil with high precision.