Copper(II) carbonate
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Through Salt (chemistry), Copper and Hydroxide
This compound is rarely encountered because it is difficult to prepare and readily reacts with water moisture from the air. The terms "copper carbonate", "copper(II) carbonate", and "cupric carbonate" almost always refer (even in chemistry texts) to a basic copper carbonate (or copper(II) carbonate hydroxide), such as Cu
2(OH)2CO
3 (which occurs naturally as the mineral malachite) or Cu
3(OH)2(CO
3)2 (azurite). For this reason, the qualifier neutral may be used instead of "basic" to refer specifically to CuCO
3.
Preparation
Reactions that may be expected to yield CuCO
3, such as mixing solutions of copper(II) sulfate CuSO
4 and sodium carbonate in ambient conditions, yield instead a basic carbonate and CO
2, due to the great affinity of the Cu2+
ion for the hydroxide anion HO−
.
Thermal decomposition of the basic carbonate at atmospheric pressure yields copper(II) oxide rather than the carbonate.
In 1960, C. W. F. T. Pistorius claimed synthesis by heating basic copper carbonate at 180 °C in an atmosphere of carbon dioxide (450 atm) and water (50 atm) for 36 hours. The bulk of the products was well-crystallized malachite Cu
2CO
3(OH)2, but a small yield of a rhombohedral substance was also obtained, claimed to be CuCO
3. However, this synthesis was apparently not reproduced.
Reliable synthesis of true copper(II) carbonate was reported for the first time in 1973 by Hartmut Ehrhardt et al. The compound was obtained as a gray powder, by heating basic copper carbonate in an atmosphere of carbon dioxide (produced by the decomposition of silver oxalate Ag
2C
2O
4) at 500 °C and 2 GPa (20,000 atm). The compound was determined to have a monoclinic structure.
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Chemical and physical properties
The stability of dry CuCO
3 depends critically on the partial pressure of carbon dioxide (pCO2). It is stable for months in dry air, but decomposes slowly into CuO and CO
2 if pCO2 is less than 0.11 atm.
In the presence of water or moist air at 25 °C, CuCO
3 is stable only for pCO2 above 4.57 atmospheres and pH between about 4 and 8. Below that partial pressure, it reacts with water to form a basic carbonate (azurite, Cu
3(CO
3)2(OH)2).
- 3 CuCO
3 + H
2O → Cu
3(CO
3)
2(OH)
2 + CO
2
In highly basic solutions, the complex anion Cu(CO
3)2−
2 is formed instead.
The solubility product of the true copper(II) carbonate was measured by Reiterer and others as pKso = 11.45 ± 0.10 at 25 °C.
Structure
In the crystal structure of CuCO3, copper adopts a distorted square pyramidal coordination environment with coordination number 5. Each carbonate ion bonds to 5 copper centres.