Unfortunately, a voltmeter will not tell you the concentration of any particular electrolyte, only the total ability of the solution to carry voltage (basically it will just tell you total solute concentration, or total electrolyte concentration). In order to analyze separate components in a drink, urine, or blood, you will need some advanced analytical chemistry. You might inquire at a hospital or with the chemistry department at UCSB...also there are private laboratories which will conduct ion analysis.