Method 63a Determination of watersoluble potassium

Reagents.

• Potassium stock standard solution, 1000 pg K ml-1 - dry potassium dihydrogen orthophosphate (KH2PO4) for 2 h at 105°C, then allow to cool in a desiccator. Weigh 2.889 g into a 100-ml beaker, add sufficient water to dissolve, and transfer to a 1-l volumetric flask with beaker washings; make up to the mark with water and mix.

• Potassium working standard solutions, 0-400 pg K ml-1 - pipette 0, 5, 10, 20, 30 and 40 ml potassium stock standard solution into a series of 100-ml volumetric flasks and make up to the mark with water and mix. This will provide a series of standards containing 0, 50, 100, 200, 300, and 400 pg K ml-1.

Procedure. Weigh 5.000 g of the powdered sample into a 600-ml beaker, then add approximately 400 ml water, and cover with a watch glass. Place on a hotplate and bring to the boil, then continue boiling gently for 30 min. Allow to cool, then transfer, with washings, to a 1-l volumetric flask, make up to the mark and mix. Filter into a dry sample container, rejecting the first 50 ml filtrate. Pipette a 10-ml aliquot into a 100-ml volumetric flask, make up to the mark and mix. Analyse using a flame photometer, set up according to the manufacturer's instructions. Compare with a series of standards containing 0, 50, 100, 200, 300 and 400 pg K ml-1.

Calculation. As an example, 100 pg K ml-1 in the final sample solution would correspond to 1 g K (20% K) in the original 5 g sample. (Because of the final x10 dilution, 100 pg K ml-1 gives 10 x 100 x 1000 pg K in the 1 l of sample solution. Thus there is 1.000 g K in a solution of 5.000 g sample, which amounts to 20% K.) Therefore divide the pg K ml-1 by 5 to get the % K in the sample of fertilizer. (Pure potassium chloride will give a final sample solution of 262.2 pg K ml-1.) Multiply the % K by 1.2047 to get the equivalent value of % K2O.

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