Abstract: Electrochemical sensing device (S) for measuring the content of ions in biological fluid samples (D) comprising two membrane half cells, a salt bridge (3) connecting them, means for bringing a biological fluid sample (D) in contact with the measuring cell, wherein the first and second membranes (11, 21) of the half-cells are selective to the same ions, the first volume (13) and second volume (23) adjacent to the membranes are filled with known concentrations (C1, C2) of the ions to which the membranes (11, 21) are selective, these known concentrations being different such that a voltage can be measured between the first electrode (12) and the second electrode (22) that allows calibrating the sensing device (S) and then measuring the ion-content of the sample. The invention also refers to a method using such sensing devices. The sensing devices are useful especially in the area of so-called home monitoring.