An Acid‐Free Vanadate‐Molybdate Reagent for the Determination of Total Phosphorus in Soils

An acid‐free vanadate‐molybdate reagent was developed for the spectrophotometric determination of P in HClO 4 digests of soils. The optimum acidity in the analysis is derived from the residual acidity of the HClO 4 digest of soils. A comparative study of the proposed reagent with a commonly used molybdenum‐blue method on ten widely different soils indicated that the proposed method gave highly reproducible results and was not affected by high amounts of Fe 3+ which is invariably present in soils. In rare cases chromium interference can be serious because of the similarity of the color of the chromate ion to that of the molybdivanadophosphoric complex. For soils low in chromium, chromate interference can be removed by volatilization as chromyl chloride during the digestion. The modified reagent greatly simplifies the determination of total P and is admirably suited for automated analysis.