Validation of UV - VIS Spectrophotometry for Phosphorous Molybdenum Determination in Soils of Vietnam

Phosphorus existing under different forms and status in soils is a significantly important nutrient during the growth and development of trees. Spectrophotometry has been developed with many types of reagents to determine phosphorus in environmental samples. Molybdenum blue reaction with ascorbic acid as a reducing agent and antimony tartrate catalyst in strong acid environment gave maximum absorbance wavelength at 890 nm. Under the optimised conditions, this method gave very high recovery efficiency, accuracy and sensitivity (up to 0.06 mgP/L) in determination of total phosphorus (around 90%) and bio-available phosphorus (nearly 96%) in soils in Vietnam. The formed blue phosphomolybdenum complex was oxidized by the addition of hexavalent chromium and nitrite ions, leading to the decrease in intensity of blue color, negative error of 10% at 5 mg/L NO2- and 3 mg/L Cr6+, which was in contrast to the positive effect of silicate ions at high amount of over 5 mg/L SiO32-. -squared standard combined with two-way ANOVA analysis highlight the relationship between concentration, time and intensity signals, thus validating the molybdenum blue method before application in phosphorus determination for soil assessment. Olsen and Oniani extraction gave high recovery efficiency despite differences in quantified concentration of bio-available phosphorus. Although the studied soils low amount of total phosphorus (around 0.03 %P2O5), they were medium to rich in bio-available phosphorus with the level of 8 to 15 mg P2O5 / 100 g soil.