The question is not well defined. One should specifically ask the oxidation state of the cobalt ion as plus three or plus two or even it could be some unusual oxidation states.Further the type of ligands ( the donor atoms and if uni-dentate or chelateed ) it is attached . The tetrahedral or octahedral or any other distorted state would dictate its possibility to show EPR signal in aqueous medium ( room temperature, frozen with the addition of some glycerine or similar stuff to create glassy state) Once all these information which are essential are known then only the answer may be provided.

Is your question about the identification of cobalt ion in a mixture of paramagnetic ions or non-magnetic ions such as sodium chloride, KCl, MgCl2...?

First, thank you for showing interest in my problem! I'm using EPR connected to a nitrogen deware to do the EPR detection at 100 Kelvins, and my cobalt ( 50 mg/L) is only dissolved in pure water; so its speciation must be Co (II). You have to know that the main problem is that cobalt in water have the possibility to be complexed by EDTA. This way Cobalt can't be retained by filters because EDTA change Cobalt properties. My "solution" is to create HO° radical by UV/H2O2 in order to degrade EDTA and, this way, cobalt will return in particle form and will be retained by filters. That is why, first, I’d like to obtain Cobalt signal alone in water with EPR.

I'm using Cobalt (II) nitrate hexahydrate

In pure water the Co(II) will be in hexa aqua form, I mean [Co(H2O)6] ++ form and Co(II) in such environment will not show any EPR even at 100K. EDTA is a robust molecule and degradation of this molecule by H2O2 including UV is not quantitatvely possible. To get Co(II) in particle form one can use high pH to precipitate Co(OH)2 and that can be readily be aged to hydrated oxide and more so using H2O2 where the oxidation state may change . But quantification is difficult. Anyways EPR is not a probe for [Co(H2O)6] ++ . A tetrahedral Co(II) is better for EPR but still requires lower temperature near to 4K but that requires special environment of donor ligand .