Discussion 710 Determination of total phosphorus in plant material and feeding stuffs

The official EC method may be downloaded from:

http://europa.eu.int/eur-lex/en/lif/dat/1971/en_371L0393.html This is document 371L0393 (which is presented in the Official Journal No. L279, 20.12.71, p.7), and is the Second Commission Directive 71/393/EEC of 18 November 1971. The determination of total phosphorus appears as Part III of the Annex. It is a molybdovanadate colorimetric procedure with the absorbance measured at 430 nm.

The AOAC alkalimetric ammonium molybdophosphate and photometric molybdovanadate methods for animal feed are described by Padmore (1990, pp. 87-88), and for plants by Isaac (1990, p. 56). A spectrophotometric molybdovanadate procedure is also described in MAFF/ADAS (1986, pp. 181-182). The official Bran+Luebbe AutoAnalyzer method for phosphate in soil, plant and fertilizer extracts is reproduced with permission in Appendix 6.

We will describe the autoanalysis method based on Faithfull (1971a) which uses the same Kjeldahl acid digest solution as that used for the nitrogen determination (see Discussion 7.7). The proper development of the yellow colour of the phosphovanadomolybdate complex is sensitive to the pH, which has an optimum value of 1.7. If spiky peaks shaped rather like an 'M', or an 'M' with an additional central peak, are obtained, it is probably caused by inaccurate adjustment of the sulphuric acid level to the 5-ml mark, or the diluted digest to the 10-ml mark in the digestion tube. The double peaks are caused by a difference in pH between the 50% sulphuric acid wash and the sample solutions.

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