Characterization of the low-cost electrodes Calibration curves were prepared using both low-cost syringe sensors and standard ISE electrodes with cut membranes assembled using commercial bodies. To compare this two types of electrodes potassium-selective membranes were used with valinomycin or dibenzo-18-crown-6 as ionophores. Valinomycin shows excellent selectivity towards potassium, when compared to sodium ions, whereas the crown ether is less selective but more lipophilic, resulting in longer electrode lifetime . Selectivity coefficients were calculated based on SSM. The results are summarized in Table . The sensitivity and linear range for both types of sensors with the membranes based on the same ionophore are comparable, with differences in the range of measured experimental error. All electrodes presented satisfactory repeatability (n=3). Selectivity coefficients from sensors present the same trend for sensors of both types of architecture, with membranes based on valinomycin. As expected a decrease in selectivity was observed in relation to sodium ions for the sensors with dibenzo(18crown-6) as an ionophore as compared with valinomycin. The syringe electrodes with a valinomycin-based membrane were applied to measure potassium ion in different types of samples, including pharmaceutical supplements (potassium supplement, mixed electrolytes supplement), bottled mineral water from different brands (3 samples), tomato juices from different brands (7 samples), banana juice, dried fruits (dates), tomato sauce (passata), and a beetroot soup concentrate. Measurements results were in good agreement with the concentration calculated from the information given on the package for mineral water samples, pharmaceutical supplements and banana juice (Fig. ). Higher deviation was observed for dates sample (average of 26 %), tomato sauce (average of 17 %), all the tomato juices (average of 32 %), with the highest deviation observed for beetroot soup reaching 75 % of the expected value. Some deviation is expected as the value on the package is given as average and some lot to lot variation is possible, however such error should not exceed few percent.