Determination of Potassium in Fruit Juices
This is a method for the determination of potassium levels in fruit juices such as blackcurrant, orange, lemon, grape, etc. This method is also suitable for the determination of potassium in wine.
Equipment Required
- pH/ION analyzer (recommended) or pH meter with millivolt scale
- Potassium combination ion selective electrode
Reagents
- Potassium chloride (analar)
- Sodium chloride (analar)
- 1M TEACL salt bridge solution
- Distilled/de-ionized water
Standards Preparation
- Potassium stock/standard solution 1000 mg/l. Dissolve 1.910g KCl (analar) in distilled water in a volumetric flask and dilute to 1000 ml.
- Prepare 100 ml of 10 mg/l, 10 mg/l and 1 mg/l standards from the stock solution by serial dilution.
Sample Preparation
Pipette 5 ml of fruit juice into a 50-ml volumetric flask. Dilute to the mark with distilled water.
Ionic Strength Adjuster Preparation
- Prepare 6 Molar sodium chloride solution by adding 35g NaCl to 100 ml distilled water.
- Add 1.0 ml of ISA to each 50 ml of sample or standard to keep a constant background strength.
Method
Take 50 ml of ISA to each 50 ml of 1 mg/l k+ standard and add 1 ml of ISA.
Immerse electrodes and measure the electrode potential (mV). Read the remaining standards in increasing concentrations in the same way, making sure to rinse electrodes before a different solution is measured.
Construct a standard curve of mV vs. concentration on semi-log paper.
Measure 50 ml of sample dilution with 1.0 ml ISA in the same manner as the standards.
Calculation
Interpolate from the graph the concentration of K+ present in the sample using its mV reading. This may be carried out directly in linear concentration units by use of the “activity” (or “ISE”) modes on pH/ION analyzers. This value should then be multiplied by 10 due to the dilution factor. The result is expressed in mg/l.