Method F-017G The Determination of Potassium in Mixtures Containing Plant Derived Resins

The sample is firstly converted into an inorganic residue by ashing, the residue dissolved in hydrochloric acid is diluted with distilled water and potassium is determined using a Sherwood Flame photometer.

Equipment Required:

1. Sherwood flame photometer
2. Small crucible (nickel or porcelain)
3. Bunsen burner, tripod, etc
4. Accurate balance weighing to +/-0.0005g

Reagents:

1. Potassium dihydrogen ortho phosphate (analar)
2. 10 molar hydrochloric acid

Sample Preparation:

1. Weigh accurately 1 g of the resin sample into a small clean crucible.
2. Heat the sample using a bunsen burner in a fume cupboard. ignite the vapours and ensure the sample burns freely. Re-ignition may be necessary before only a char remains.
3. Transfer the crucible to a muffle furnace pre-heated to 500-525°C and maintain this temperature until all the carbon is burned off.
4. Dissolve the ash in hydrochloric acid and allow it to digest on a steam bath for about 20 minutes.
5. Evaporate the solution to dryness and heat the residue in an oven for 30 minutes at 105°C.
6. Return to the water bath and add 20ml of 10 molar HCI, warm to dissolve.
7. Transfer 2ml of the solution to a 1 litre volumetric flask containing approximately 200ml distilled water. Dilute to the mark with distilled water. The sample is now ready for analysis.

Standard Preparation:

This is the 1000mg/l potassium standard.

Prepare from the stock standard 10,6,4 and 2mg/l potassium standards ensuring that each standard contains the equivalent of 2mls 10 molar HCI per litre.

Method:

1. With the potassium interference filter selected aspirate the standards into the flame photometer.
2. Plot a graph of meter reading vs concentration of potassium.
3. Aspirate the sample solution into the flame and record the meter reading.
4. From the graph read off the sample potassium concentration.

Calculation: