Method F-025F The Determination of Sodium and Potassium in Fruit Juice
This method involves the dilution of the fruit juice sample with water
followed by filtration and finally aspiration directly into a Sherwood
Flame photometer.
Equipment Required:
Sherwood Flame photometer (industrial)
Filtration apparatus
Volumetric glassware
Reagents:
Sherwood 1000 ppm sodium and potassium standards
Deionised water
Method:
Prepare by dilution of the stock standards 20, 15, 10 and 5 ppm Na and K standards. Deionised water is the blank solution.
To 10 mls of the fruit juice add 50 mls deionised water.
Filter this solution through an ashless filter paper (eg. Whatman 540) into a litre volumetric flask. Ensure that the solid particles retained by the paper are washed thoroughly and washings directed into the same 1 litre flask. Dilute to the mark with delonised water, stopper the flask and mix by inversion.
Set up the flame photometer as outlined in its instruction manual for sodium.
Set blank to zero, ie. deionised water.
Aspirate the standards and record their stable readings.
Plot a graph of meter reading against standard concentration.
Aspirate the sample solution into the flame photometer.
Record the meter reading and from the graph read off its sodium concentration.
Adjust the filter position to select the potassium filter and repeat stages 5 to 9 for potassium.
Note that if the Na and K concentrations in the fruit juice are outside the range of standards the sample should be diluted accordingly.
Calculation:
Multiply the concentration of Na and K obtained from the graph by the dilution factor,
ie. x 100, to express the result in ppm or mg/l of Na or K in the original fruit juice.
