The thermodynamics of protonation of the lanthanide(III) chelates of trans-1,2-diaminocyclohexanetetraacetic acid (LnDCTA−) was studied by potentiometric titration at the ionic strength of 1 (KCl). The protonation constants determined change monotonically
and the double-double effect in the log K against Z variation is manifested in all four segments of the lanthanide series.
The variations of the protonation enthalpy and entropy with Z are estimated to be also monotonical. The direction of the double-double
effect observed indicates that the coordination number in the protonated chelates is higher by 1 in the entire series than
in non-protonated chelates i. e. the protonation is accompanied by the coordination of the additional water molecule. The
coordination number 10 is expected for the HLnDCTA chelates at least at the beginning of the series. Presumably the present
conclusions are valid also for the LnEDTA− system.