Method F-008A The Determination of Potassium in Plant Material

Equipment Required:

1. Sherwood Model 410 Flame Photometer
2. Balance weighing to +/-0.0005g
3. Water bath and oven
4. 10 Molar Hydrochloric acid
5. Potassium Standard (Sherwood part 00156621)

Method:

1. Completely oxidise at 500-525°C 2-3g of the ground oven dried sample.
2. Dissolve the ash in 10 molar Hydrochloric acid and allow it to digest on a steam bath for 20 minutes.
3. Evaporate the solution to dryness and heat the residue in an oven for 30 minutes at 105⁰C.
4. Allow to cool, return to the water bath and add 10ml 10 molar Hydrochloric acid. Warm to dissolve.
5. Transfer 2mls to a 100ml volumetric flask and dilute to the mark with distilled water.
6. Prepare a 5 ppm potassium standard by pipetting 5 ml Potassium Standard and dilute with a mixture containing 2ml conc. HCI per 100ml and make to 1 litre in volumetric flask.
7. Aspirate 5 ppm standard and set controls to display 5.0.
8. Prepare set standards by diluting 5 ppm standard with acid /water mixture as above
9. Without touching the controls aspirate into the Sherwood Flame Photometer in turn, and check that the display is linear
10. Aspirate the sample solution and read the potassium concentration directly from the display.
11. The amount of potassium in the plant material sample can be calculated taking into account the dilution factor.