I am preparing sterile FeCl2 and FeCl3 stocks for supplementing into bacterial media Tryptic Soy broth (TSB). However, i am not too sure what is the best way to prepare these stocks, if it is by filtering using a 0.2uM filter, or by autoclaving. By filtering, the colour is light red, and if autoclaved, the colour of the stock turns dark red. What happens when we autoclave FeCl2 and FeCl3? Some literature seems to suggest that autoclaving leads to precipitation of iron and this could be hematite, and could potentially affects downstream experiments when using these bacteria cultures. How should i prepare these stocks?
https://www.sciencedirect.com/science/article/abs/pii/S0304386X97000030
https://www.sciencedirect.com/topics/medicine-and-dentistry/ferrous-chloride
I suggest to try - in agreement with your first paper - the autoclaving in concentrated (at least 1 M) hydrochloric acid. That should prevent the precipitation reaction or minimize the extent of precipitation. If the precipitation still occurs, the hematite dissolves as FeCl3. However, according to another reference, FeCl2 in itself promotes the reductive dissolution of hematite. After autoclaving, I would filter the samples to remove all the insoluble residues (if any). And using inert atmosphere (nitrogen or argon) for preparing and filtering FeCl2 stock solutions is a must.
Much depending on temperature, the hydrolysis of aqueous ferric chloride in hydrothermal conditions can possibly originate the precipitation/crystallization of ferric oxyhydroxide(s), namely, of the goethite polymorph (α-FeOOH).
On purifying ferrous chloride sol. in aq. dil. HCl from ferric iron impurities; cf. my post at: https://www.researchgate.net/post/How_can_i_dissolve_the_FeCl2_salt_in_the_aqueous_solution_while_the_ultimate_pH_be_about_31
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