In the present work, we assessed the suitability of sodium and potassium physiologically present in sweat, as internal reference allowing to re-calculate the corresponding volume of sweat collected on a PharmChek Patch. A method using capillary electrophoresis with indirect ultra-violet detection was developed for the determination of sodium and potassium in sweat. The concentrations determined in specimens collected from 12 females and 10 males, using a home-made system composed of polypropylene copolymer bag, were 1039+/-89 mg/L and 711+/-45 mg/L for sodium, and 489+/-293 mg/L and 474+/-196 mg/L for potassium, respectively. In parallel, for seven females and eight males, the comparison of the volume of sweat collected in the same way to the re-calculated volume of sweat accumulated in a patch using sodium as internal standard, gave an average agreement of 98.4+/-15.0%. Results demonstrated the usefulness of sodium as internal standard to determine the volume of sweat accumulated in a patch, and confirm the suitability of PharmChek patch for the collection and determination of cations in sweat.