Contribution to the Molybdenum Blue Reaction and its Application in Soil Analysis

Abstract

This paper presents a study on the molybdenum blue reaction (MB) as a finishing detection step in soil analysis for quantification of plant available phosphorus. An ammonium acetate/calcium lactate reagent (pH= 4.2) was used for soil phosphorus extraction. The molybdenum blue color reaction using premixed Murphy-Riley reagent and ascorbic acid as a reductant was reinvestigated. UV-Vis characteristics of MB, optimal wavelength, reaction time and concentration of reductant were studied. The effect of ascorbic acid concentration and the reaction time on linearity, bias and uncertainty was discussed. The molybdenum blue reaction was found to obey Beer’s law in the targeted concentration range of 0.04 – 1.0 mg L-1 PO4 3--P. The linearity was proved by “lack-of-fit” test. The uncertainty budget was made and the uncertainty was estimated by modelling approach, as well as single laboratory and quality control approach. The recovery and the expanded uncertainty were found to be, respectively, (95.7 ± 8.7) % (P= 95%, n = 3) and 9.2 mg PO4 3--P /kg dry soil (k = 2). The results showed that the soil sample inhomogeneity and the repeatability of extraction process were the main factors which contribute to the uncertainty of measurement in soil analysis.