The invention relates to a device for the citrate anti-coagulated extracorporeal blood purification comprising: an extracorporeal blood purification system comprising an extracorporeal blood circuit (202), which comprises a dialysis unit (205), a blood inflow (203) and a blood discharge (204), a citrate metering device (212) for supplying citrate at a citrate supply point (213) upstream of the dialysis unit (205), a substitution medium metering device (214) for supplying a substitution medium at a substitution medium supply point (215) downstream from the dialysis unit (205), at least one ion concentration measuring means (224) for measuring bivalent cations and a controller (225), wherein the controller (225) is adapted to regulate the metering of the substitution medium as a function of a comparison between a setpoint value range and the ion concentration measured by means of the ion concentration measuring means (224), wherein the ion concentration measuring means (224) is adapted for the continuous generation of measuring values and is arranged upstream of the citrate supply point (213) and the controller (225) is adapted to continuously carry out a regulation of the metering of the citrate and of the substitution medium in consideration of a target value or target value range, which can be predetermined in the controller (225) downstream from the citrate supply point (213). The invention further relates to a method for detecting the ion concentration.