Measurement of Na, K and Ca in Plant Extracts

The most important ions which naturally occur in plant tissues are sodium, potassium, calcium, magnesium, nitrate, sulphate and phosphate.

A sample of 5 g of oven-dried plant material contains on average, 25 mg calcium, 9 mg magnesium, 80 mg potassium and 3 mg sodium.

The recommended method for the measurement of calcium is to precipitate the calcium as an oxalate, the precipitate is then dissolved and measurement taken from the resulting solution.

Sodium and potassium may be measured directly following an ashing procedure and the preparation of the solution.

Procedure

5 g of oven-dried material is weighed into a wide silica basin and ashed at 4500°C for 1½ to 2 hours. The resulting ash is brought into solution by 10mL of 6M HCl and evaporated to dryness.

10 ml of 2M HCl is added to the residue and the basin is covered with a watch-glass and digested on a steam bath for 30 minutes. The surface of the watch glass is rinsed with deionised distilled water into the basin.

The contents of the basin are filtered through a Schleicher & Schűll filter paper number 589 into a 250 ml volumetric flask and after washing and made up to the mark with DI water. (Schleicher & Schűll number 589 is an ashless quantitative paper for medium fine precipitates.)

From this solution, 50 ml is taken for the direct determination of sodium and potassium. Procedure for Calcium Determination: The remainder of the above solution is used for the determination of calcium, as follows:

The solution is heated to 80°C and a few drops of methyl red indicator solution is added, followed by 15 ml of saturated ammonium oxalate solution. Ammonia (15%) is added until the colour changes and the precipitate has dissolved.

The solution is set aside overnight or put onto a steam bath for 2 hours.

The solution is filtered through a Schleicher & Schűll filter paper number 589.
The filter is washed with 15 ml of 3M HCl and finally with deionised water.

Once the precipitate has dissolved, the solution is transferred to a 250mL volumetric flask and made up to volume with deionised distilled water and is ready for the determination of calcium.

Preparation of Standard Graph

Set the flame photometer in accordance to MultiPoint/Single Ion Calibration found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium sodium and calcium emissions. Nebulise the working standard solutions and adjust the controls until a steady zero and maximum readings are obtained. Nebulise the intermediate working standard solutions and construct a graph relating raw emission data (known as RAW in BWB flame photometers) to concentration of all the standard solutions.