One objective of dialysis treatment is to normalize the blood plasma electrolytes and remove waste products such as urea and creatinine from blood. However, due to a shift in plasma osmolarity, a rapid or excessive change of the electrolytes can lead to complications like cardiovascular instability, overhydrating of cells, disequilibrium syndrome and cardiac arrhythmias. Especially for critical ill patients in intensive care unit with sepsis or multi-organ failure, any additional stress has to be avoided. Since the exchange velocity of the electrolytes mainly depends on the concentration gradients across the dialysis membrane between blood and dialysate, it can be controlled by an individualized composition of dialysate concentrations. In order to obtain a precise concentration gradient with the individualized dialysate, it is necessary to continuously monitor the plasma concentrations. However, with in-line sensors, the required hemocompatibility is often difficult to achieve. In this work, we present a concept for continuous in-line monitoring of electrolyte concentrations using ion-selective electrodes separated from the blood flow by a dialysis membrane, and therefore meeting the fluidic requirements for hemocompatibility. First investigations of hemocompatibility with reconfigured human blood show no increased hemolysis caused by the measuring system. With this concept, it is possible to continuously measure the plasma concentrations with a relative error of less than 0.5 %.