Hi, all. I am making some ferrous hydroxide by adding ferrous chloride solution (with HCl adding into prevent Fe2+ oxidation) into the excess sodium hydroxide solution (excess NaOH in solution with weight percentage of 5%). The sodium hydroxide solution and ferrous chloride solution were bubbled with N2 for 9 hours (flow rate of 70ml/min), separately. Then, ferrous chloride solution is injected into the NaOH solution by syringe under N2 purging. While injection, you saw the dark green flocculent precipitates, and the precipitates turned into white-green within 30 minutes, then green-yellow within an hour. The two solutions were bubbled for 9 hours. I guess they were oxidized by dissolved O2. Do you think the bubbling time is still not enough? or is there any other things I should pay attention to? I am actually very confused about making the ferrous hydroxide. Welcome any advice and experience. Thanks a lot!
I. The ferrous chloride salt (possibly hydrated) used to prepare its aq. sol. can be generally expected to be partially oxidized, at least to trace level. When (freshly) dissolved in aq. dil. HCl, you may consider to reduce contaminant ferric ions to the ferrous state by adding electrolytic iron powder (a few wt% of the weighted salt). Then the sol. should be heated (not boiled) to facilitate ferric to ferrous reduction and the dissolution of any possibly excess of ferrous chloride; cooled to room temperature; being then either vacuum filtered while using nitrogen as blanketing gas, or pressure filtered by using nitrogen as pressurizing gas. Hydrogen can possibly be released during the purification process, owing to the concomitant reaction of metallic iron with HCl: Fe + 2HCl → FeCl2 + H2. Water should be either distilled or both deionized and N2-purged or boiled. The purified ferrous chloride solution may then be redox-titrated, if required.
II. By adding NaOH to the ferrous chloride solution, ferrous hydroxide precipitates (white to greyish green). This precipitate oxidises readily to ferric hydroxide (yellowish brown to brownish red) if its mother solution is left exposed to air.
III. Also to be noticed, is that NaOH is prone to carbonation if left exposed to atmospheric CO2 and humidity. If extensive enough, such carbonation may possibly induce FeCO3 coprecipitation, which coprecipitate should also readily oxidize to ferric hydroxide upon such air exposure.
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