Cobalt(2+) hydrogen citrate

Endpoint summary

Additional information

Cobalt hydrogen citrate (C6H6O7Co) is an organic metal salt, which is well water soluble (27.65 g/L at 20 °C). In aqueous solution, the coordination ability of cobalt with citrate to form a cobalt citrate complex is low. The salt is nearly completely dissociated, therefore the salt hydrolysis may proceed, when it is transferred into the aquatic environment. As a result, the cobalt species (predominantly present as Co2+) and various citrate species will be formed depending on the pH and redox potential of the aquatic environment. Citric acid (AH3) has a pKa1 of about 3.13, a pKa2 of about 4.76 and a pKa3 of about 6.40. Depending on the prevailing pH value of the solution, different species are formed: Citric acid will be largely present in the form of H3cit (AH3) and H2cit (AH2-) at pH values in the range of < 2 – < 4. At pH > 5, H2cit (AH2-) will be formed predominantly, and it will be transferred to the citrate ion (A3-) completely when the pH level is raised to > 7. Factors that affect the speciation of cobalt in water and sediments include anions such as Cl−, OH−, CO32−, HCO3−, and SO42−, pH, and redox potential, as well as stable organic ligands such as humic acids. Under normal environmental conditions, the prevailing cobalt species in freshwater will be free Co2+, as compared to cobalt complexes (e.g. CoCO3 or CoSO4).

As evident, cobalt citrate (C12H10O14Co3) will form the species AH2- at a pH value of 5.5 (maximum of the curve). Thus, as released in the aquatic environment, cobalt citrate will be nearly completely present in the state of cobalt hydrogen citrate (C6H6O7Co). At very acidic conditions (e.g. present in the stomach), tri-citrate is available as uncharged citric acid (AH3) along with the cobalt cation. The same can be applied for cobalt citrate as well as cobalt hydrogen citrate in the pH range of 1-2. At the physiological pH of about 7.4, both compounds are predominantly present in the ionic form, resulting in the cobalt cation and the anion salt (A3-).

One study on the ready biodegradability is available for cobalt citrate (CAS 866-81-9). This study conducted according to OECD 301C and GLP resulted in 68% biodegradation after 28 days, hence the pass level of 60% was achieved and the test substance can be considered as readily biodegradable.

No bioaccumulation data are available for cobalt hydrogen citrate, however various reliable data exist for cobalt (measured as environmental concentrations) and different analogue cobalt substances.

In general, the bioaccumulation potential of cobalt in natural ecosystems is relatively low. First of all, low BAFs have been reported in eight laboratory (steady state) studies and four field studies; five BSAF-sediment values have been found to be well below 1; and four (out of four) average BSAF-soil values have been reported to be well below 1. In addition, results from six field investigations plus two laboratory studies indicate the absence of biomagnification of cobalt in natural food webs. Finally, cobalt is an essential micro-nutrient, the uptake of which is expected to be regulated to some extent by many organisms (Environment Canada, 2011). The data demonstrates that cobalt, like other essential elements, shows homeostatic control by organisms.

No adsorption/desorption data are available for cobalt hydrogen citrate, however various reliable data exist for cobalt (measured as environmental concentrations) and different analogue cobalt substances showing statistical or conservative partition coefficients for suspended matter, soil, STP, sediments in freshwater and in coastal waters. For cobalt, log Kd values for all types ranged from 0.41 to 5.83.