Preliminary speciation study of Co(II)-citrate system by ion pairing-reversed phase chromatography coupled to UV detector

This paper describes the ability of ion pairing-reversed phase chromatography coupled to UV detector to determine an experimental speciation of cobalt in the binary Co(II)-citrate system. A method has been developed to investigate aqueous Co(II)-citrate samples at physiological pH as a function of time, by varying metal-to-ligand ratios, concentration and temperature. Solid 1 : 1 dimeric and 1 : 2 monomeric Co(Il)-citrate complexes previously described have been synthesised and their analysis by IP-RPC-UV allowed the assignation of the solution state separated species. In solution state and at Co(II) concentration of 5 x 10(-2) M, 1 : 2 Co(II)-citrate monomeric species are predominant forms, while 1 : 1 mono and dimeric complexes are the minor species. At Co(II) concentration lower than 5 mM, only species of 1 : 1 stoichiometry are involved in the aqueous cobalt distribution. The monitoring of the formed species as a function of time allows highlighting the high inertness of this system. At low concentration ranges, electrospray ionisation-mass spectrometry experiments were carried out to support the chromatographic data. This study is the first one establishing a link among synthetic cobalt-citrate complexes and species existing in solution.