Basic copper carbonate is a generic term for a series of compounds, not just a composition of Cu2(OH)2CO3 (chemical formula).
For example, the main component of natural malachite is basic copper carbonate. The chemical formula can be written as Cu2(OH)2CO3 or CuCO3·Cu(OH)2. The color is green, but there is also a natural ore called azurite. The main chemical formula can be Writing Cu3(OH)2(CO3)2 or 2CuCO3.Cu(OH)2, still known as basic copper carbonate, is blue in color. In fact, in the sodium carbonate solution, CO3 2- and OH- are simultaneously present in the solution due to the hydrolysis of the carbonate ions. After the addition of the copper sulfate solution, depending on the concentration of the reactants, a base having a different composition may be formed. Precipitation of copper carbonate, the basic formula of basic copper carbonate precipitation can be expressed as xCuCO3·yCu(OH)2, and can even be expressed as xCuCO3·yCu(OH)2·zH2O, which often has a very complicated structure, which can be interpreted as The hydroxide ion acts as a complex multinuclear complex of oxygen bridge. After a period of “aging” of the newly precipitated basic copper carbonate, the composition may also undergo some changes.
In general (presumably), the thicker the sodium carbonate solution, the higher the concentration of carbonate ions, the more likely it is to form a xCuCO3·yCu(OH)2 precipitate with a larger x value, and the color of the precipitate is more blue, so preparation In the case of basic copper carbonate Cu2(OH)2CO3, the method of adding sodium carbonate solution to copper sulfate solution should be adopted. It should not be reversed. When CO2 bubbles are emitted, slowly add sodium carbonate solution and stir well. , control the final solution pH should not be too high (pH = 8), and leave the precipitate for a period of time (can be placed for 24h), after the precipitation is fully aged, and then filtered and so on.
High concentration of carbonate ions may also form soluble complex ions with Cu2+. A small amount of copper sulfate solution is added dropwise to a larger amount (usually more than 20ml) of saturated sodium carbonate solution, and stirred, the precipitate will be redissolved to obtain bright blue or even The indigo solution is believed to be a soluble [Cu(CO3)2]2- complex ion. It is also mentioned in the literature that after mixing a concentrated CuSO4 solution with an excess of concentrated K2CO3 solution, in addition to the precipitation of basic copper carbonate xCuCO3·yCu(OH)2, K2[Cu(CO3)2] crystals may be precipitated. It is also possible to use sodium bicarbonate solution instead of sodium carbonate solution, so that it is easy to obtain a precipitate having a composition close to that of Cu2(OH)2CO3, but the reaction is more intense, and a large amount of CO2 bubbles or even foam is generated, and the feed rate must be controlled to prevent the reaction from being uncontrolled. material.
If the preparation of copper hydroxide precipitate is reversed, the method of adding copper sulfate solution to sodium hydroxide solution should be adopted, and the pH of the solution should be controlled to be high, so that blue Cu(OH)2 precipitate can be obtained. If it is reversed, there is a high concentration of SO4 2- in the solution, which is easy to precipitate blue-green basic copper sulfate xCuSO4·yCu(OH)2 precipitate. Some data describe the chemical formula of this basic copper sulfate precipitation as Cu2 (OH). ) 2SO4.