An ion sensor for sensing an ion concentration in a bulk solution comprises a reference electrode embedded in an reference electrolyte solution, and a first ion-selective electrode. The ion sensor moreover comprises a second electrode sensitive to the reference ions or to an ion different from the ion to be measured, whereby the second electrode is in direct contact with the bulk solution when the ion sensor is immersed therein. The potential difference between the first electrode and the reference electrode is a measure for the ion concentration in the bulk solution and is corrected with the potential difference between the second electrode and the reference electrode to compensate for the drift of the reference electrode.