A dialyzer having a hemo-dialyzer and/or a hemo-filter and an extra-corporal blood circulation with which a device for the adding of citrate to the blood is connected upstream and a device for the adding of a substitution solution containing ions to the blood is connected downstream of the hemo-dialyzer and/or of the hemo-filter. Ion concentration in the dialyzate downstream of the hemo-dialyzer and/or of the hemo-filter is detected with respect to the direction of flow of the dialyzate. The concentration of the ion, atom or molecule to be determined is reliably ascertained by preventing the complexing of the ion, atom or molecule, at least during the determination of the concentration, by the addition or withdrawal of a substance.