The Determination of Available Molybdenum in Soil by Rapid Catalytic Polarography

Available molybdenum in soil is an important proxy of ecological geochemistry evaluation. The classic method to determine available molybdenum in soil is catalytic polarography, which uses oxalic acid-ammonium oxalate (Tamm solution) as an extracting agent. The procedure to eliminate the iron ion, manganese ion, and oxalate and organics interferences is complicated and time consuming. Consequently, the interference mechanism is discussed in this paper. Since oxalate and organics are reductive, the metallic ions with variable valences changed to a reduction state during leaching. The oxidation-reduction reaction proceeds between the before mentioned materials and sodium chlorate in the measurement system, affecting the sensitivity of the catalytic wave. The method presented in this article is to add solid natrium hydroxide into oxalic acid-ammonium oxalate solution so that iron and manganese impurities can be removed through precipitation separation. Afterwards, HNO₃-H₂SO₄ was used to destroy the organics and oxalate in the leaching liquor. In the system of molybdenum-benzoglycolic acid-chlorate-sulfuric acid, available Mo in soil standard reference materials was determined by polarographic catalytic wave. Compared with the classic interference elimination method, this method effectively reduces reagent types, operating time and error sources in analysis. The detection limit of this method is 0.0015 μg/g, which is below the detection limit of the classic method (0.0068 μg/g). The precision (RSD, n=12) is less than 7% and relative error is less than 8%. Measured values of soil standard reference materials are in agreement with the certified values. Verified by thousands of soil samples, this method is suitable to measure available Mo in soil that pH range is from 3.6 to 10.5. The measuring range of available Mo is 0.005-2 mg/kg. The method has the advantage of simple operation, short process, is accurate and has stable analysis results. It could be widely used in large quantities for rapid analysis of available Mo in soil.