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I'm wondering how chromium (as a metal or as a salt) behaves in a concentrated carbonate solution. Will it form any, one or several carbonate specie(s)? Will the metal solubilize? How does it affect chromium salts and their behavior? Does it matter which carbonate salt the solution is made from (Ca/Mg/Na/K/Mn/Fe/Zn/Cu carbonate)?

The question is a bit broad but it only reflects my ignorance of its behavior. I'd welcome more understanding of any of the aforementioned aspects of its behavior.

In some ways I think my question amounts to finding out how it will behave at high pH but I'd rather be asking specifically in case I might be missing some aspect of what is going on.

Mo is a transition metal and seems to form an insoluble carbonate. But I haven't been able to find evidence showing chromium (and Tungsten for that matter) do so as well (and I don't know if extrapolations can be that easily made amongst the behaviour of elements of the same group), nor have I find any numerical data as to their solubility.

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    $\begingroup$ Solids precipitating from heavy metal salt solutions in alkaline environment, with or without carbonates, is always tough subject to evaluate. I expect a mixture of hydroxide, hydrated oxide and basic carbonate, all in not well defined composition. $\endgroup$
    – Poutnik
    Commented Oct 6, 2020 at 8:25
  • $\begingroup$ Thanks. So there not a simple answer? But chromium carbonate does (in part) form? If so, would you know the solubility (in g/100ml) of chromium carbonate(s)? $\endgroup$
    – Hans
    Commented Oct 6, 2020 at 8:28
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    $\begingroup$ So at e.g. pH=11, [Cr^3+]=6.3x10^-22 mol/L, so literally few ions per litre. This does not obviously address colloid particles. $\endgroup$
    – Poutnik
    Commented Oct 6, 2020 at 8:47
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    $\begingroup$ What you get will also depend on the oxidation state. I would expect Cr2+ and Cr3+ to behave somewhat differently $\endgroup$
    – Ian Bush
    Commented Oct 6, 2020 at 10:09
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    $\begingroup$ Chromium metal is unaffected. In simple terms that is why it is added to make alloys corrosion resistant . Alloys like Vitallium ( Cr + Co ) are immune to almost any solution. $\endgroup$
    – blacksmith37
    Commented Oct 6, 2020 at 15:09

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Chromium(III) ions are like Fe(III) and Al(III) ions. Exactly like aluminum and ferric carbonates, chromium(III) carbonate does not exist in aqueous solution. In principle it could be obtained according to $$\ce{2 Cr^{3+} + 3 CO3^{2-} -> Cr2(CO3)3}$$ But like with the aluminum and ferric carbonates, the chromium carbonate is immediately destroyed according to : $$\ce{Cr2(CO3)3 + 3 H2O -> 3 CO2 + 2 Cr(OH)3}$$ As a consequence, chromium carbonate does not exist in aqueous solutions and, if carbonate ions are added to the solution, the ions of Cr(III) are transformed into chromium hydroxyde $\ce{Cr(OH)3}$ precipitate. And some bubbles of $\ce{CO2}$ may appear in the solution.

Furthermore, like $\ce{Al(OH)3}$ and $\ce{Fe(OH)3}$, $\ce{Cr(OH)3}$ cannot be redissolved in concentrated solutions of carbonates. A pH higher than $12$ is needed to redissolve $\ce{Cr(OH)3}$ according to : $$\ce{Cr(OH)3 + OH^- -> Cr(OH)4^-}$$ And such a high pH value cannot be obtained with concentrated solutions of carbonates. , ,

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    $\begingroup$ $\ce{CO2}$ would not be generated in carbonate presence. It would be rather $\ce{Cr(H2O)6]^3+ + n CO3^2- -> [Cr(H2O)(6-n)(OH)n]^{3-n} + n HCO3-}$, particularly $\ce{Cr(H2O)6]^3+ + 3 CO3^2- -> [Cr(H2O)3(OH)3] + 3 HCO3-}$. $\endgroup$
    – Poutnik
    Commented Oct 6, 2020 at 12:21
  • $\begingroup$ Thanks for your answer. If I understand well throwing in either Cr/Al/Fe2(CO3)3 in water would fizz and precipitate 100% of it as the corresponding hydroxide? $\endgroup$
    – Hans
    Commented Oct 6, 2020 at 12:22
  • $\begingroup$ What about the other oxidation states of Cr/Al/Fe; and about the respective metals, do they form stable carbonates or also turn and precipitate into corresponding hydroxides? $\endgroup$
    – Hans
    Commented Oct 6, 2020 at 12:24
  • $\begingroup$ And what about Mo and W. Do they behave entirely differently even though they are in the same group as Cr? $\endgroup$
    – Hans
    Commented Oct 6, 2020 at 12:26
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    $\begingroup$ @ Hans. 1) It is better to say that Cr, Fe and Al carbonates do not exist. They will not fizz in water : they don't exist, even in the dry state. 2) Fe(II) makes a carbonate FeCO3 that is easily oxidized in humid air to from Fe(III) hydroxide. 3) I don't know the situation of Molybdenum and Tungsten. 4) Cr(II) hydroxyde is unstable. It absorbs the tiniest traces of O2 to produce Cr(OH)3. And if dry it get decomposed according to the extremely strange equation : $\ce{2 Cr(OH)2 -> Cr2O3 + H2O + H2}$ $\endgroup$
    – Maurice
    Commented Oct 6, 2020 at 12:58

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