The method involves preventing the complex formation of ion, atom or molecule during the concentration determination by addition or withdrawal of a substance. The added substance is an acid, and the complex formation is prevented by change of pH value. The ion includes calcium or magnesium ions. The complex formation is prevented such that the citrate-addition in the blood circulation is interrupted in a transient manner. An independent claim is also included for a dialysis device with a hemodialyzer or a hemofilter.
