Abstract
Low-molecular-weight organic acids play an important role in the mobilization of soil phosphorus (P). The molybdate blue colorimetric method, based on the formation of P molybdenum blue compound, is commonly used for analysis of phosphate in soil and environmental samples. However, some organic acids can act as a ligand to bond with molybdenum acid, which subsequently interfere with the colorimetric reaction. The recoveries of P were inhibited by the addition of oxalic (>2 mM) and citric acids (>3 mM) both in standard P solutions and soil extracts, but formic and maleic acids did not interfere with the P determination. The inhibition of oxalic and citric acids on P recovery remained even at higher level of P (up to 100 μg P 100 ml−1) though such interferences decreased by increasing P concentration. Comparison between oxalic and citric acids revealed the more pronounced interference by the addition of oxalic acid. The results suggested that the interference of organic acids with P determination is related to the types of organic acids and the ratio of organic acid ligands to P anions in the solutions. Thus, analysis of P using the molybdate blue colorimetric method should be undertaken cautiously in the presence of relative strong ligands like oxalic and citric acids.
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Acknowledgments
We would like to thank Ms Marijke Heenan for her assistance with some chemical analyses. Both CRC and ZHX received the Australian Research Council funding support.
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Wei, L.L., Chen, C.R. & Xu, Z.H. The effect of low-molecular-weight organic acids and inorganic phosphorus concentration on the determination of soil phosphorus by the molybdenum blue reaction. Biol Fertil Soils 45, 775–779 (2009). https://doi.org/10.1007/s00374-009-0381-z
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DOI: https://doi.org/10.1007/s00374-009-0381-z