A blood treatment apparatus having an extracorporeal circuit has a dialyzate circuit with a dialyzer, having a blood-side chamber, and a dialyzate-side chamber separated from the blood-side chamber by a membrane. The blood-side chamber is in fluid communication with the extracorporeal circuit and the dialyzate-side chamber is in fluid communication with the dialyzate circuit, and the apparatus has a preparation unit for online preparation of a solution. The apparatus has a first means for carrying out a priming mode and a second means for carrying out an initial treatment mode. The first means is configured to control the preparation unit to prepare a flushing solution having a pH of ≥7.3, with the extracorporeal circuit being filled with the flushing solution. The second means is configured to control the preparation unit to prepare the dialyzate having a pH of ≥7.3, with the dialyzate circuit being filled with the dialyzate.