1. 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 ) to the dialysis unit ( 205 ) for blood drawn from a patient ( 201 ) and a blood discharge ( 204 ) from the dialysis unit ( 205 ) for blood to be returned into the patient ( 201 ), a controlled citrate metering device ( 212 ) for supplying citrate at a citrate supply point ( 213 ) upstream of the dialysis unit ( 205 ), a controlled 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 ), which is connected to the at least one ion concentration measuring means ( 224 ) as well as to the citrate metering device ( 212 ) and to the substitution medium metering device ( 214 ), 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 ), characterized in that 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 ).