A device for the citrate anti-coagulated extracorporeal blood purification comprising: an extracorporeal blood purification system comprising an extracorporeal blood circuit (202, 202'), which comprises a dialysis unit (205, 205', 205''), a blood inflow to the dialysis unit (205, 205', 205'') for blood drawn from a patient and a blood discharge from the dialysis unit (205, 205', 205'') for blood to be returned into the patient, a controlled citrate metering device for supplying citrate at a citrate supply point upstream of the dialysis unit, a controlled substitution medium metering device for supplying a substitution medium at a substitution medium supply point downstream from the dialysis unit, at least one ion concentration measuring means (224, 224', 224e, 224'') for measuring bivalent cations and a controller (225, 225', 225''), which is connected to the at least one ion concentration measuring means (224, 224', 224e, 224'') as well as to the citrate metering device and to the substitution medium metering device, wherein the controller (225, 225', 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, 224', 224e, 224''), characterized in that the ion concentration measuring means (224, 224', 224e, 224'') is adapted for the continuous generation of measuring values and is arranged upstream of the citrate supply point and the controller (225, 225', 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, 225', 225'') downstream from the citrate supply point, wherein the target value or target value range, which can be predetermined downstream from the citrate supply point, represents the calcium ion concentration in the case of which a blood clotting in a filter (205a, 205a') assigned to the dialysis unit (205, 205', 205'') is effectively prevented, and wherein the ion concentration measuring means (224, 224', 224e, 224'')is introduced into the extracorporeal blood circuit. an extracorporeal blood purification system comprising an extracorporeal blood circuit (202, 202'), which comprises a dialysis unit (205, 205', 205''), a blood inflow to the dialysis unit (205, 205', 205'') for blood drawn from a patient and a blood discharge from the dialysis unit (205, 205', 205'') for blood to be returned into the patient, a controlled citrate metering device for supplying citrate at a citrate supply point upstream of the dialysis unit, a controlled substitution medium metering device for supplying a substitution medium at a substitution medium supply point downstream from the dialysis unit, at least one ion concentration measuring means (224, 224', 224e, 224'') for measuring bivalent cations and a controller (225, 225', 225''), which is connected to the at least one ion concentration measuring means (224, 224', 224e, 224'') as well as to the citrate metering device and to the substitution medium metering device, wherein the controller (225, 225', 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, 224', 224e, 224''), characterized in that the ion concentration measuring means (224, 224', 224e, 224'') is adapted for the continuous generation of measuring values and is arranged upstream of the citrate supply point and the controller (225, 225', 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, 225', 225'') downstream from the citrate supply point, wherein the target value or target value range, which can be predetermined downstream from the citrate supply point, represents the calcium ion concentration in the case of which a blood clotting in a filter (205a, 205a') assigned to the dialysis unit (205, 205', 205'') is effectively prevented, and wherein the ion concentration measuring means (224, 224', 224e, 224'')is introduced into the extracorporeal blood circuit.