SOP: Determination of Potassium by Flame Photometry
GIFSQ — Global Institute for Food Safety & Quality
Principle
Potassium in solution is atomized into an oxyhydrogen or oxyacetylene flame. The flame excites the potassium atoms, causing them to emit radiation at specific wavelengths; the amount of radiation emitted is measured on a spectrophotometer and is proportional to the potassium concentration in the solution under standard conditions.
Apparatus required
- Flame photometer with wavelength dial (set at 768 nm)
Reagents
a. Potassium chloride (KCl) stock solution: dissolve 1.909 g of analytical-grade potassium chloride in glass-distilled water and make up to 1 litre (1.0 mg K per ml, i.e. 1000 ppm)
b. Standard solution: measure 150 ml of the stock standard solution and 5 ml of HCl into a flask and make up to 1 litre; to compensate for interference by other ions, augment the standard with approximately equivalent concentrations of the ions occurring in highest proportion in the sample — a background roughly similar to an average plant extract is obtained with 150 ppm calcium, 75 ppm magnesium and 15 ppm phosphorus in the standard
c. 0.59% HCl for dilution of standard aliquots
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
Prepare the standard curve: dilute aliquots of the standard solution from 0 to 150 ml, making each aliquot to a volume of 150 ml with 0.59% HCl; atomize each as described below, setting the top standard at 100% transmittance; note the luminosity of the flame for each concentration; plot concentration on the abscissa against luminosity on the ordinate
↓
Dilute an aliquot of ash solution so that it contains less than 150 ppm potassium; add sufficient HCl so that the acid concentration is the same as in the standard solution
↓
Atomize the diluted extract in the calibrated flame photometer with the wavelength dial set at 768 nm and the transmittance set at 100% for the top standard solution of potassium
↓
Check the instrument periodically with the top standard solution; read the concentration from the standard curve.
Calculation
Potassium, mg/100 g = (ppm found from standard curve × Volume made up × Dilutions, if any × 100) ÷ (Wt of sample × 1000)
Where,
- ppm found from standard curve = potassium concentration read from the curve (mg/l)
- Volume made up = total volume of the diluted extract (ml)
- Dilutions, if any = further dilution factor applied
- Wt of sample = weight of sample taken (g)
References
- Ward, G.M. & F.B. Johnston, Chemical Methods of Plant Analysis, Canada Department of Agriculture, Publication 1064, p. 19 (1962).