Determination of Sodium in Food using Ion Selective Electrodes

The measurement of sodium in food is important due to the link between high blood pressure (hypertension) and sodium concentration. The sodium level is found by the technique of known addition.

Equipment Required

1. Ion Meter, pH/mV Analyzer, or pH Meter with millivolt scale
2. Sodium Combination Ion Selective Electrode (Glass or PVC)
3. Double Junction Reference Electrode (only required if a Mono electrode is being used)

Reagents

1. Sodium stock solution 2000 ppm
2. Prepare 100 ppm, 1000 ppm and 2000 ppm sodium standard solutions by serial dilution of the stock solution
3. Sentek Sodium ISAB

SAMPLE Preparation

Blend contents of sample including any water. Weigh a 10 g sample and dilute to 100 ml with DI ionised water. Mix and transfer to a clean, dry 150 ml beaker. Add 2ml of ISAB per 50ml of sample or standards. Stir all samples and standards during the measuring process.

Method

Take 100 ml of sample, immerse electrode and record the electrode potential mV. Pipette 5 ml of standard (see below table), stir thoroughly, allow for stabilisation and record the electrode potential mV.

Table

Calculation

Where:

Cu= Concentration of unknown ion
Cs= Concentration of Standard addition
Vs = Volume of Standard addition
V¬U= Volume of sample
ΔE = Change in electrode potential in mV
S = Slope of electrode in mV

The difference in the measurements mV2-mV1=ΔE. An accurate value of the electrode slope will be required, this can be found by producing a calibration curve using the 100 and 1000 ppm standards. To determine the mg per cent (mg/100 g of food), multiply the answer obtained from the equation for known addition by 10.

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