An extracorporeal blood treatment apparatus is provided comprising a filtration unit (2) connected to a blood circuit (17) and to a dialysate circuit (32), a preparation device (9) for preparing and regulating the composition of the dialysis fluid; a control unit (12) is configured for receiving a conductivity or sodium concentration set point for the dialysis fluid and for calculating a mass transport of a substance at an instant t of a treatment session based on said set value of the parameter for the dialysis fluid in the dialysis supply line (8).