1. A device for the citrate anti-coagulated extracorporeal blood purification comprising: an extracorporeal blood purification system comprising an extracorporeal blood circuit, which itself comprises: a dialysis unit; a blood inflow in fluid communication with an input of the dialysis unit for transporting blood drawn from a patient into said dialysis unit, and a blood discharge in fluid communication with an output of the dialysis unit for returning blood to the patient; a controlled citrate metering device in fluid communication with the extracorporeal blood purification system for supplying citrate, the controlled citrate metering device being disposed at a citrate supply point upstream of the dialysis unit; a controlled substitution medium metering device in fluid communication with the dialysis unit for supplying substitution medium, the controlled substitution medium metering device being disposed at a substitution medium supply point downstream from the dialysis unit; at least one ion concentration measuring device for measuring bivalent cations being disposed upstream of the citrate supply point, wherein the ion concentration measuring device is adapted for the continuous generation of measuring values; and a controller in signal communication with the at least one ion concentration measuring device, the citrate metering device, and the substitution medium metering device, wherein the controller is adapted to regulate the metering of the substitution medium as a function of comparison between a setpoint value range and the ion concentration measured by the at least one ion concentration measuring device disposed upstream of the citrate supply point; and wherein the controller is adapted to continuously carry out a regulation of the metering of the citrate metering device and the substitution medium metering device in consideration of one of either a target value or target value range, which is predetermined in the controller by data collected downstream from the citrate supply point and upstream of the dialysis unit.