Method 63b Determination of ammonium oxalatesoluble potassium

Reagents.

• Potassium standard solutions - see 6.3a.

Procedure. Weigh 1 g of the powdered sample into a 600-ml beaker, then add 50 ml 4% (m/v) ammonium oxalate solution, 125 ml water and cover with a watch glass. Place on a hotplate and bring to the boil, then boil gently for 30 min and allow to cool. Transfer, with beaker washings, to a 500-ml volumetric flask and make up to the mark with water and mix. Filter into a dry sample container, rejecting the first 50 ml of filtrate. Pipette 25 ml of this solution into a 100-ml volumetric flask, make up to the mark with water 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. A concentration of 100 pg K ml-1 in the final sample solution corresponds to 0.2 g K (20% K) in the original 1 g sample. (Because of the final x 4 dilution, 100 pg K ml-1 gives 4 x 100 x 500 pg K in the 500 ml of sample solution. Thus there is 0.200 g K in a solution of 1.000 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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