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Application Note

Measurement of Potassium in Unashed Plant Leaves

Mason, J.L, ‘Flame Photometric Determination of Potassium in Unashed Plant Leaves’, Anal. Chem., 35, (1963), p. 874-875.

This method uses a suspension of ground plant material to directly measure the potassium concentration. It eliminates the usual steps of ashing and subsequent dissolution of the resulting ash.

The method was compared to alternative methods such as chemical analysis and spectrophotometric methods.  Collaborative studies using a range of samples gave a mean of 1.74% potassium by this extraction technique as compared to 1.75% by alternative methods. The standard deviation of an analysis in the two different sample sets was ±0.0086 and ±0.0108.

Solution and Standard Preparation:

Deionised distilled water was used throughout to prepare the sample solutions. Glycerol-2-propanol Reagent was prepared by adding 200mls of 2-propanol to 125 grams of glycerol.

Sample Preparation Procedure:

  1. The air-dried plant material was ground in a mill fitted with a 60-mesh screen and the resulting powder was thoroughly mixed by tumbling end-over-end in a jar less than 50¼ full for 15 minutes.

  2. A 1-gram sample of the air-dried plant material was weighed into an aluminium weighing container and dried for 48 hours. The container was reweighed and the loss due to moisture is calculated.

  3. 25mg sample of the air-dried plant material was weighed into a 50ml Erlenmeyer flask. 10ml of Glycerol-2-propanol Reagent is pipetted onto the plant material and the mixture was gently swirled to wet the powder.

  4. The wetted plant material was quantitatively transferred to a 250 ml volumetric flask and made up to volume with deionised water. This is a 1:10,000 dilution.

  5. The suspensions can be analysed by directly introducing into the flame. Prior to measurement the suspension was thoroughly mixed and then allowed to stand for 5 minutes, to allow the particles to settle.

  6. The resulting reading should be corrected for moisture content and dilution factor. This is done by accounting for moisture loss during the drying process and also the 1;10,000 dilution to return you to the value of potassium in your original sample.

  7. Testing to factor in the difference in measured concentrations across a range of different parts of the plant that the tissue sample was acquired from should be accounted for into the results so that variances between samples can be accounted for when presenting data and sampling techniques.

Preparation of Standard Graph

To prepare the standard solutions for this analysis, 1000mg/l stock solution was used to form 1, 2.5, 5, 7.5 and 10mg/l standards in volumetric flasks by adding 1, 2.5, 5, 7.5 and 10ml of the 100mg/l stock solution respectively. Prior to bringing these solutions up to the mark 3.75ml of the glycerol-2-propanol reagent was added to each. A blank solution containing DI water and 3.75ml of the glycerol-2-propanol reagent was used. The flame photometer was turned on and allowed to stabilise over 30 minutes prior to calibration or analysis. To calibrate the flame photometer the samples were set to multipoint/single ion calibration mode found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium emission.

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