Method 711c Determination of potassium in plant material by flame photometry Kjeldahl acid digest

Procedure. Switch on and set up the flame photometer according to the manufacturer's instructions. The standards (including a 50% sulphuric acid blank) and sample solutions in 50% sulphuric acid obtained from the Kjeldahl digest procedure 7.7a. may be aspirated directly into the flame photometer, but note the provisos in Discussion 7.10.

After a sufficient warm-up time, aspirate the 0 and 400 pg K ml-1 standards, and adjust the zero and bring the maximum reading on-scale. If the sensitivity of the instrument is too great, then dilute the standards and samples appropriately. Measure the other standards and construct a graph relating emission reading to concentration; check that it is a straight line or smooth slight curve. Analyse the sample solutions in batches, and repeat the analysis of standards approximately every 10 min. If the output is on a chartrecorder, use a chart reader to assist the reading of the peaks (see Chapter 1, 'Chart reader'). Re-draw the standard curve if it changes during the analysis.

Calculation. Divide the pg K ml-1 by 100 to obtain the % K in the sample, or by 10 to obtain the g K kg-1 in the sample. If the sample was not oven dried, use the separate moisture determination to correct the result to K in DM.

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