A reverse iontophoresis device 2 comprises a chamber 7 containing a liquid electrolyte 8 that provides conductivity between a working electrode 15 and the skin 4 of a subject. Sensor electrodes 22,24 in the chamber 7 measure the concentration of a target analyte that accumulates in the liquid 8. The volume of liquid 8 in the chamber 7 is critical as the measured concentration depends on it. A method of determining the volume of liquid 8 in the chamber 7 comprises performing reverse iontophoresis to transport a test analyte from the skin 4 of the subject into the liquid 8 in the chamber 7； measuring the concentration of the test analyte in the liquid 8； and determining the actual volume of liquid 8 in the chamber 7 by comparing the measured concentration of the test analyte with the expected concentration of the test analyte when the chamber 7 is full of the liquid 8. The test analyte may comprise sodium ions.
