Sciencemadness Discussion Board - Carbonates/ hydroxides from carbonates?

Carbonates/ hydroxides from carbonates?

RU_KLO - 29-11-2023 at 09:00

I want to understand why hydroxides are formed when Sodium carbonate is added to a Fe / Cr chlorides mixture (stainless steel + HCl)?

The question arises from SM wiki chromates (potassium/sodium).

"The chromium(III) hydroxide can be easily made by dissolving stainless steel in hydrochloric acid, followed by precipitation with sodium carbonate.[4]"
"Sodium carbonate (not sodium hydroxide) is added to the solution to neutralize all remaining acids and precipitate a mixture of iron and chromium hydroxides"

Also I started to check precipitation of carbonates by adding sodium cabonate to metal sulfates/chlorides to check how it precipitates.

When adding Na2CO3 sol. to an old FeCl2 (probably with FeCl3) sol., obtained blue / green, also some white - turning light green - precipitates.
On air standing, all became brown (in one night).

so here are the questions:

1) Why NaOH cannot be used to neutralize Fe/Cr chlorides to get Fe/Cr hydroxides? (in chromate syntesis) Common sense is : use Hydroxide to get hydroxide and use carbonate to get carbonate.

2) cCan some one explain or share the FeCl2/FeCl3 + Na2CO3 to get Hydroxide? (my asumption is : FeCl2(aq) + Na2CO3(aq) -> FeCO3(s) + 2NaCl(aq) - no hydroxide. I read that iron (III) carbonate does not exist.

Thanks

DraconicAcid - 29-11-2023 at 09:14

These transition metal ions are hydrated in aqueous solution. Because of the high charge, the water molecules are easily deprotonated by random bases.

[Fe(H2O)6](3+) + 3 B --> 3 HB+ + Fe(OH)3(s) + 3 H2O

For iron(III), carbonate, ammonia, and hydroxide will all precipitate iron(III) hydroxide. For chromium, excess hydroxide will result in the formation of soluble [Cr(OH)4]-

Maurice VD 37 - 1-12-2023 at 07:57

RU KLO's first question was about the reaction of Cr^3+ and carbonate. He wanted to know why it produces an hydroxide.
The equation of the reaction should be :
2 Cr3+ + 3 CO3^2- --> Cr2(CO3)3
But this rection does not happen. Or if it happens, Cr2(CO3)3 is immediately hydrolyzed according to :
Cr2(CO3)3 + 3 H2O --> 2 Cr(OH)3 + 3 CO2
This hydrolysis reaction is produced when any triply-charged ion, like Cr3+ or Al3+ or Fe3+, is in contact with a solution containing the carbonate ion. These metallic ions do not produce carbonates.

chornedsnorkack - 1-12-2023 at 08:44

Quote: Originally posted by Maurice VD 37

This hydrolysis reaction is produced when any triply-charged ion, like Cr3+ or Al3+ or Fe3+, is in contact with a solution containing the carbonate ion. These metallic ions do not produce carbonates.

Not true.
Cr³⁺, Al³⁺ and Fe³⁺ indeed do not produce carbonates. But it is not applicable to "any triply-charged ion" because triply charged ions include lanthanides - and these do form carbonates.

Lanthanides seem to form carbonate precipitates. Th and U form soluble carbonate complexes (besides solid carbonates).
Picking a good base to produce hydroxide depends strongly on the metals. Some dissolve in excess of hydroxide, so you could like a weaker base... but carbonate may or may not go to the precipitates. Some metals are easily precipitated by ammonia, others are dissolved by it... or blown up.