Introducing "Insertive Stripping Voltammetry": Electrochemical Determination of Sodium Ions Using an Iron(III) Phosphate-Modified Electrode.

A new type of stripping voltammetry is introduced, in which the preconcentration step includes ion insertion into a solid phase followed by a quantification step in which the ion is expelled via linear sweep voltammetry. Specifically, sodium-ion concentrations in both aqueous solution and synthetic sweat are electrochemically determined using iron(III) phosphate-modified glassy carbon electrodes. The electrochemical method consists of a potentiostatic step, holding the potential of -0.5 V vs saturated calomel electrode (SCE) for 100 s, followed by linear sweep voltammetry. It is shown that a thermal and mechanical pretreatment at 800 °C and with a ball mill, respectively, improve the electrochemical response of the iron(III) phosphate toward Na+. The involved structural and morphological changes were assessed by thermogravimetric analysis, scanning electron microscopy, and powder X-ray diffraction. The sensor exhibits a good selectivity toward Li+ and K+ and shows a linear response between 0.025 and 0.2 M Na+. As a proof-of-principle, the sensor was used to determine the sodium level in synthetic sweat.