Sonbol
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Calcium Carbonate turns yellow
I tried making Calcium carbonate using calcium chloride and ammonia water as aqueous solution and bubbling CO2 into that. It was supposed to
precipitate a white powder but it turns yellow. What would be the product?
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Ubya
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if your calcium chloride is technical grade it could have some iron impurities maybe (precipitation of iron hydroxide)
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happyfooddance
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If you heat it to a red heat, what happens?
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fusso
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@hfd:
If organic: some will evaporate, some will char.
If inorganic: remain unchanged
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happyfooddance
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It's not a theoretical question, I am asking them what happens when they subject their compound to a red heat
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AJKOER
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Again, per my comments in another thread (see https://www.sciencemadness.org/whisper/viewthread.php?tid=15... for source links), assuming an iron presence, it may indeed be an iron impurity,
like [FeCl6](3-), which is a yellow complex!
First, the action of basic aqueous ammonia on CaCl2 creates NH4Cl:
CaCl2 + 2 NH3 + 2 H2O --> Ca(OH)2 + 2 NH4Cl
The Ca(OH)2 can dissolve with CO2 (aq) forming the bicarbonate.
The NH4Cl and NH4+ behave as follows in an equilibrium reaction:
NH4Cl = NH4+ + Cl-
NH4+ = H+ + NH3
As I argue below that the H+ is removed, driving the reaction to the right, I will also assume that the ammonia is stripped out of solution by pumped
in air, or by the action of CO2, or from the use of excess CaCl2.
The pumping of an oxygen source into a ferrous solution consumes H+ and forms Ferric via the electrochemical reaction:
4 Fe(ll) + O2 + 4 H+ --> 4 Fe(lll) + 2 H2O
The removal of H+ leaves excess Cl- which may find a home in the newly created ferric chloride creating the yellow [FeCl6](3-) complex.
[Edited on 21-7-2019 by AJKOER]
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Ubya
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in aqueous solution is wrong to talk about ammonium chloride, what are reacting are the ions
CaCl2=Ca2+ + 2Cl-
NH4OH=NH4+ + OH-
Ca+ + 2OH- = Ca(OH)2↓ Ksp= 5.5x10^-6
NH4+ and Cl- are spectators that remain in solution
CO2+H2O=H2CO3=HCO3- + H+ = CO3(2-) + 2H+ all equilibriums
Ca2+ + CO3(2-)=CaCO3↓ Ksp=3.8x10^-9 shifting the equilibriums to the right as it is less soluble
2H+ from H2CO3 react with 2OH- from Ca(OH)2 to from 2H2O
increasing CO2 concentration lowers the solubility of CaCO3 for the common ion effect, but increasing it too much shifts the equilibrium back to the
bicarbonate as the pH decreases.
the eventual excess of ammonia reacts with excess dissolved CO2 to form in solution ammonium carbonate/bicarbonate that can decompose at higher
temperatures into CO2, NH3 and water.
any presence of iron impurities are probably already oxidized to the +3 state, as at 20°C the dissolved oxygen in water is 8.32mg/L, 1mol of oxygen
can oxidize 4mols of iron(II), so 8.32mg of oxygen can oxidize 116.5mg of iron(II), much more of what i think is the amount of impurity.
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fusso
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Tests for Fe3+ (must be dissolved):
1. SCN-: turns red
2. C2O4 2-: turns green
3. Fe(CN)6 4-: turns blue
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teodor
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Most probably you see Fe, but the problem is that Fe contamination is most easily to see and if your CaCl2 is impure it can contain as well Mg and
many other metals which gives not so strong color or give no color at all. There are several methods of removing Fe, if you need I will post the info
here. But it will not help with Mg contamination etc.
So, I think you have 2 options:
- to buy a chemical grade CaCl2
- to test your CaCl2 or CaCO3 on impurities (there is an excellent book "E.Merck, Chemical Reagents, their purity and tests" (http://library.sciencemadness.org/library/books/chemical_rea...) and then search the most economical method to remove all impurities you don't
want to be present.
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