SOP: Determination of Potassium by the Volumetric Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
In acid solution potassium is precipitated as the yellow double salt sodium potassium cobaltinitrite. The precipitate is dissolved in hot dilute acid and titrated with standard potassium permanganate; since the end point is indeterminate, a standard quantity of sodium oxalate is added at the end of the titration to produce a sharp end point. One ml of 0.01 N KMnO₄ is equivalent to 0.07 mg of K approximately (the factor varies slightly with experimental conditions and should be determined with standard potassium solutions).
Apparatus required
- Centrifuge tube, 15 ml
- Centrifuge
- Burette
- Desiccator
Reagents
a. 40% sodium acetate [NaC₂H₃O₂·3H₂O] solution
b. Sodium cobaltinitrite solution: (i) dissolve 25 g cobalt nitrate [Co(NO₃)₂·6H₂O] in 50 ml water and add 12.5 ml glacial acetic acid; (ii) dissolve 140 g sodium nitrite [NaNO₂] in 210 ml water; slowly add solution (ii) to (i); bubble air through the solution for 3 h; filter and store in a refrigerator; aerate for 15 min and filter, if necessary, before use
c. 12% sulphuric acid (v/v): add acid to water slowly with constant stirring; cool and make up to volume
d. Acetone-water mixture: 300 ml water + 100 ml acetone
e. Dry acetone: add anhydrous sodium carbonate (Na₂CO₃) to acetone in the proportion of 10 g per litre and store in this condition
f. 0.01 N sodium oxalate (Na₂C₂O₄) solution: heat the sodium oxalate overnight at 105 °C and cool in a desiccator; dissolve 0.67 g in water and dilute to 1 litre
g. 0.1 N potassium permanganate stock solution: dissolve 3.16 g KMnO₄ in water and make to 1 litre; store in a dark-coloured bottle
h. 0.01 N potassium permanganate standard solution: dilute 10 ml of the stock solution to 100 ml with water; prepare fresh before use; standardize by titrating against sodium oxalate solution acidified with 15 ml of dilute H₂SO₄, heating to 80 °C before titrating
Note on reagent h: the printed standardization expression in the source is OCR-illegible; standardize the 0.01 N KMnO₄ against the 0.01 N sodium oxalate in hot acidified solution in the usual way before use.
Sample preparation
Destroy the organic matter of the sample by dry ashing and prepare the ash solution as described in SOP: Destruction of Organic Matter.
Procedure
Measure 1 ml of ash solution into a 15-ml centrifuge tube; add 3 ml of water, 1 ml of sodium acetate solution and 1 ml of sodium cobaltinitrite solution, the last added drop by drop
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Mix and allow to stand for 2 h at 5 °C
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Centrifuge at 1000 g for 15 min and decant the supernatant liquid
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Add 5 ml of acetone-water mixture, mix, centrifuge for 15 min and decant the wash solution; repeat the washing with acetone
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Evaporate the acetone by allowing the precipitate to stand for a few minutes
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Add a little standard potassium permanganate from a burette, then add 2 ml of dilute H₂SO₄; complete the reaction by adding permanganate from the burette with the tube set in a container of boiling water and with constant shaking, always maintaining an excess of permanganate
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When the precipitate is completely dissolved and a permanent pink colour is obtained, add 2 ml of sodium oxalate solution and titrate to an end point.
Calculation
True titration value = ml of 0.01 N KMnO₄ solution used − ml of 0.01 N Na₂C₂O₄ solution added
Potassium (mg) in the aliquot taken = True titration value × 0.07
Where,
- 0.07 = mg of K equivalent to 1 ml of 0.01 N KMnO₄ (approximate; determine the factor with standard potassium solutions under the experimental conditions)
Scale to the sample: Potassium, mg/100 g = (mg of K in the aliquot × Total volume of ash solution × 100) ÷ (ml of ash solution taken × Wt of sample taken for ashing).
References
- Kramer, B. & F.K. Tisdall, J. Biol. Chem., 46, 339 (1921).
- Ward, G.M. & F.B. Johnston, Chemical Methods of Plant Analysis, Canada Department of Agriculture, Publication 1064, p. 17 (1962).