AN-I-035
2025-04
Sodium content in food using an ion-selective electrode
Fast, accurate, and selective determination in table salt, potato chips, and baby food according to AOAC 976.25
Summary
World Health Organization (WHO) guidelines recommend a maximum daily sodium intake of 2 g for adults, which is equivalent to 5 g of dietary salt (sodium chloride, NaCl). Diets high in sodium can have negative health consequences as the risk of developing high blood pressure (hypertension) increases. High blood pressure is one of the main risk factors for cardiovascular disease. Therefore, increased sodium intake indirectly increases this risk.
Sodium in our food is mainly present in the form of sodium chloride salt and should be measured accordingly. Ion-selective electrodes (ISEs) are an accurate, selective, and rapid way to measure sodium in food. Compared to chromatographic or spectroscopic methods, ISEs are cost effective, space saving, and easy to use. This Application Note describes sodium analysis in different matrices like table salt, potato chips, and baby formula (milk powder) using the separate sodium-selective electrode (Na-ISE).
Both direct measurement and standard addition are demonstrated. The method is based on the standard AOAC 976.25.
Introduction
This application utilizes a Na-ISE. The ion-selective polymer membrane of the electrode is made of polyvinyl chloride (PVC), plasticizer, and ionophore including additive. It is not necessary to condition the PVC membrane – the polymeric membrane is immediately ready for use. The ionophore within the polymer membrane is responsible for the selectivity, i.e., it is able to selectively take up Na+ ions.
Direct measurement is recommended for unproblematic samples and in the case of low-level sodium measurements (mg/L or µg/L range). The ion activity in the sample is interpolated from a calibration curve. Plot the calibration curve using standard solutions. The expected ion activity in the sample should be in the middle of the concentration range of the standard solutions.
Standard addition (STDADD) is recommended for undefined or complex sample matrices. In the standard addition method, a defined amount of the ion of interest is added to a known volume of sample (in several steps). The unknown concentration can be calculated from the resulting potential differences between the sample and the sample with added standard solution. This calculation is performed automatically by modern ion meters or software such as OMNIS.
Sample and Sample Preparation
The analysis is demonstrated on table salt (NaCl), potato chips, and baby food (milk powder).
For all samples, solutions were prepared in advance by dissolving an appropriate amount of sample in deionized water.
Experimental
This analysis is performed on an OMNIS Advanced Titrator and an OMNIS Dosing Module equipped with a separate sodium-selective electrode (Figure 1).
To a reasonable amount of sample, either ISA solution consisting of c(CaCl2) = 1 mol/L is automatically added and the potential is measured, or the standard addition is carried out with the sodium standard solution β(Na+) = 2000 mg/L.
Results
Both direct measurement and standard addition provide accurate and reproducible results for different sodium levels with SD(rel) < 2.0% as shown in Table 1 and Table 2.
The calibration for direct measurement is shown in Figure 2 and an example of standard addition is shown in Figure 3.
| Sample (n = 6) | Mean value in % | SD(abs) in % | SD(rel) in % |
|---|---|---|---|
| Table salt | 39.34 | 0.00 | 0.0 |
| Sample (n = 6) | Mean value | SD(abs) | SD(rel) in % |
|---|---|---|---|
| Potato chips | 3912 mg/kg | 42.6 mg/kg | 1.1 |
| Baby food | 2606 mg/kg | 3.58 mg/kg | 0.1 |
Conclusion
Ion-selective electrodes can determine sodium content in food with fast, reproducible, and accurate results.
Handling the Na-ISE is user-friendly. There is no need to condition the electrode, which means it is immediately ready for use. Furthermore, this method conforms to AOAC 976.25.
The system used in this Application Note offers users flexibility combined with high-end software. The measuring range of the separate polymer Na-ISE lies between 5 × 10–6 mol/L and 1 mol/L Na+ (corresponds to approximately 0.11 mg/L Na+) and is suitable for a wide range of different samples, from foodstuffs like fast food or table salt with reduced sodium content to pharmaceutical products and cosmetics.
Metrohm helps you to measure sodium content in food with confidence. As well as improving the precision and speed of measurements, OMNIS delivers results that are equal to or better than other established titration systems.
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