Discussion 711 Determination of total potassium in plant material and feeding stuffs

The official EC method for potassium in feeding stuffs may be downloaded from:

http://europa.eu.int/eur-lex/en/lif/dat/1971/en_371L0250.html This is document 371L0250 (which is presented in the Official Journal No. L155, 12.07.71, pp. 13-37), and is the First Commission Directive 71/250/EEC of 15 June 1971. The determination of total potassium appears as Point 10 of the Annex. The sample ash is dissolved in HCl and the potassium content determined by flame photometry in the presence of caesium chloride and aluminium nitrate, which are included to largely eliminate interferences. The AOAC flame photometric method for plants is described by Isaac (1990, pp. 47-48). A flame photometric determination of potassium in plant solutions obtained by either dry or wet ashing is described in MAFF/ADAS (1986, pp. 191-192). This method says that there is no significant interference by other elements. We will describe (with Crown Copyright permission) the preparation of a sample solution of plant material by dry combustion as given in MAFF/ADAS (1986, pp. 8-9). The diluted Kjeldahl acid digest solution and standards from Method 7.7a may also be used for the flame photometric determination of potassium, but there are a couple of drawbacks to this convenience. First, because the sample solution is in 50% H2SO4, a total-consumption burner (where the sample capillary jet is situated in the burner nozzle) is unsuitable. This is because the atomized droplets of spray are incompletely vaporized by the flame, which causes a highly corrosive fall-out over the instrument and surrounding area (Faithfull, 1974). A premix burner with atomizer and spray chamber is satisfactory, however, the atomizer should be made of materials resistant to 50% sulphuric acid if nebulized directly from the sample cup.

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