Method 63c Determination of potassium in the acid digest from Methods 61a or 61b

Reagents.

• Potassium standard solutions - see Method 6.3a.

Procedure. Using a pipette filler, pipette 25 ml of the acid digest solution from Method 6.1a (in 10% v/v H2SO4) or Method 6.1b (in 50% v/v H2SO4) into a 50-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 made up either in 5% (v/v) H2SO4 or 25% (v/v) H2SO4 as appropriate, containing 0, 50, 100, 200, 300, and 400 pg K ml-1.

Calculation. A concentration of 100 pg K ml-1 in the final diluted digest solution corresponds to 0.025 g K (10% K) in the original 0.25 g sample for the sample digest solution from Method 6.1a, and corresponds to 0.005 g K (10% K) in the 0.05 g sample for the sample digest solution from Method 6.1b. Therefore, divide the pg K ml-1 by 10 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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