Oxygen can combine with copper can combine in different ways to form two types of compound: copper(I) oxide, which is normally a reddish powder, and copper(II) oxide, which is usually a black powder. They occur naturally as minerals in crystal form. Both forms of copper oxide are used in the production of pigments, but they have a number of other, differing, uses.
There are two oxides of copper because it can bond with non-metals by donating either one or two electrons. Oxygen forms compounds with metals by gaining two electrons, so it can accept one electron from each of two copper atoms — forming copper(I) oxide — or two electrons from one atom — forming copper(II) oxide. The numbers “I” and “II” represent the number of electrons the metal has provided; this is known as its oxidation number. The chemical formulae of these compounds are Cu2O and CuO, respectively.
Copper(I) Oxide
Occurrence and Production
This compound, also known as cuprous oxide, occurs naturally in some parts of the world as the mineral cuprite. Most of the compound in industrial use, however, has been manufactured. It can be produced industrially by heating copper metal to a high temperature, by the electrolysis of salt solutions using copper electrodes, and by mixing some other copper compounds with a reducing agent. Some sugars, such as glucose, act as reducing agents and this reaction, in which Cu2O forms as a bright red powder, is a very sensitive laboratory test for these types of sugars.
Properties
Cu2O is a red powder or crystalline material that melts at 2250°F (1232°C). It tends to slowly oxidize to copper(II) oxide in moist air. Although it does not dissolve in water or any organic solvents, it reacts with strong acids such as hydrochloric, nitric, and sulfuric acid to form salts. It will also react with strong alkalis, such as sodium and potassium hydroxide, to form compounds known as cuprates.
Uses
This compound was one of the first semiconductors to be discovered. It also demonstrates the photoelectric effect, where exposure to light causes an electric current to flow. For this reason, it is used in photoelectric cells and light detectors. A ceramic material made from it acts as a superconductor at relatively high temperatures. Superconductors that do not require extreme cooling to operate are still an area of active research.
One of the biggest uses of copper(I) oxide is in agriculture. It is an ingredient in many fungicides used to protect a wide variety of crop plants from fungal diseases. The compound is also the active component in many antifouling paints, which are used to protect ships and undersea structures from becoming encrusted with marine plants and animals. It is also used as a pigment in the manufacture of some types of glass and in ceramic glazes, giving them a red color, and as a catalyst in some chemical processes.
Environmental Effects
The pesticide uses of Cu2O have led to concerns about its effects on the environment. Soluble copper compounds derived from these products are toxic to some forms of aquatic life. This is thought to be more of a problem in fresh water, where the compounds may accumulate from fungicides washed from plants. The presence of salt in the water tends to reduce the absorption of metals by marine life.
Copper(II) Oxide
Occurrence, Production and Properties
This compound — also known as cupric oxide (CuO) — occurs naturally as the black or grey mineral tenorite. Like Cu2O, it can be made by heating copper, but at a lower temperature. This method gives an impure form, however, and it is best prepared by heating some oxygen-containing copper compounds, such as the carbonate, hydroxide, or nitrate. It is a black solid that melts above 2192°F (1200°C). At this temperature, it loses some oxygen, leaving small amounts of copper(I) oxide. Like the other form, copper(II) oxide is insoluble in most solvents, but reacts with acids, forming salts.
Uses
CuO is sometimes added to clay glazes as a pigment. Several colors, including red, blue, and green, can be derived from it, depending on how it is used. It is a precursor in the production of cuprammonium hydroxide, which is used in the manufacture of rayon. Sometimes, the compound is added in small amounts to animal feed to protect against copper deficiency. It is also used as an abrasive for polishing lenses and other optical components.
Health Effects
Although copper is an essential element for mammals, many of its compounds, including both forms of copper oxide, are toxic in all but small doses. If inhaled, copper(I) oxide can cause shortness of breath, coughing, and damage to the respiratory tract. Swallowing this compound can cause irritation of the gastrointestinal tract, vomiting, stomach pain, and diarrhea. Copper(II) oxide can cause similar symptoms if swallowed, as well as vision problems and skin discoloration. Both compounds can cause metal fume fever, a condition that produces flu-like symptoms and is a hazard in occupations that involve strong heating of copper structures or wire.