Common oxidation states of copper include the less stable copper(I) state, Cu+; and the more stable copper(II) state, Cu2+, which forms blue or blue-green salts and solutions. Under unusual conditions, a 3+ state and even an extremely rare 4+ state can be obtained. Using old nomenclature for the naming of salts, copper(I) is called cuprous, and copper(II) is cupric. In oxidation copper is mildly basic.
Copper(II) carbonate is green from which arises the unique appearance of copper-clad roofs or domes on some buildings. Copper(II) sulfate forms a blue crystalline pentahydrate which is perhaps the most familiar copper compound in the laboratory. It is used as a fungicide, known as Bordeaux mixture.
There are two stable copper oxides, copper(II) oxide (CuO) and copper(I) oxide (Cu2O). Copper oxides are used to make yttrium barium copper oxide (YBa2Cu3O7-δ) or YBCO which forms the basis of many unconventional superconductors.
- Copper(I) compounds: copper(I) chloride, copper(I) bromide, copper(I) iodide, copper(I) oxide.
- Copper(II) compounds: copper(II) acetate, copper(II) carbonate, copper(II) chloride, copper(II) hydroxide, copper(II) nitrate, copper(II) oxide, copper(II) sulfate, copper(II) sulfide, copper(II) tetrafluoroborate, copper(II) triflate.
- Copper(III) compounds, rare: potassium hexafluorocuprate (K3CuF6)
- Copper(IV) compounds, extremely rare: caesium hexafluorocuprate (Cs2CuF6)
Tests for copper(II) ion
Adding an aqueous solution of sodium hydroxide will form a blue precipitate of copper(II) hydroxide. The ionic equation is:
- Cu2+ (aq) + 2 OH− (aq) → Cu(OH)2 (s)
The full equation shows that the reaction is due to hydroxide ions deprotonating the hexaaquacopper(II) complex:
- [Cu(H2O)6]2+ (aq) + 2 OH−(aq) → Cu(H2O)4(OH)2 (s) + 2 H2O (l)
Adding ammonia solution (aqueous ammonia) causes the same precipitate to form. Upon adding excess ammonia, the precipitate dissolves, forming a deep blue ammonia complex, tetraamminecopper(II):
- Cu(H2O)4(OH)2 (s) + 4 NH3 (aq) → [Cu(H2O)2(NH3)4]2+ (aq) + 2 H2O (l) + 2 OH− (aq)
A more delicate test than ammonia is potassium ferrocyanide, which gives a brown precipitate with copper salts.
Biological role
Copper is essential in all plants and animals. The human body normally contains copper at a level of about 1.4 to 2.1 mg for each kg of body weight.[51] Copper is distributed widely in the body and occurs in liver, muscle and bone. Copper is transported in the bloodstream on a plasma protein called ceruloplasmin. When copper is first absorbed in the gut it is transported to the liver bound to albumin. Copper metabolism and excretion is controlled delivery of copper to the liver by ceruloplasmin, where it is excreted in bile.
Copper is found in a variety of enzymes, including the copper centers of cytochrome c oxidase and the enzyme superoxide dismutase (containing copper and zinc). In addition to its enzymatic roles, copper is used for biological electron transport. The blue copper proteins that participate in electron transport include azurin and plastocyanin. The name "blue copper" comes from their intense blue color arising from a ligand-to-metal charge transfer (LMCT) absorption band around 600 nm.
Most molluscs and some arthropods such as the horseshoe crab use the copper-containing pigment hemocyanin rather than iron-containing hemoglobin for oxygen transport, so their blood is blue when oxygenated rather than red.[52]
It is believed that zinc and copper compete for absorption in the digestive tract so that a diet that is excessive in one of these minerals may result in a deficiency in the other. The RDA for copper in normal healthy adults is 0.9 mg/day. On the other hand, professional research on the subject recommends 3.0 mg/day.[53] Because of its role in facilitating iron uptake, copper deficiency can often produce anemia-like symptoms. Conversely, an accumulation of copper in body tissues are believed to cause the symptoms of Wilson's disease in humans.
Chronic copper depletion leads to abnormalities in metabolism of fats, high triglycerides, non-alcoholic steatohepatitis (NASH), fatty liver disease and poor melanin and dopamine synthesis causing depression and sunburn. Food rich in copper should be eaten away from any milk or egg proteins as they block absorption.[clarification needed]
Toxicity
Toxicity can occur from eating acidic food that has been cooked with copper cookware. Cirrhosis of the liver in children (Indian Childhood Cirrhosis) has been linked to boiling milk in copper cookware. The Merck Manual states that recent studies suggest that a genetic defect is associated with this cirrhosis.[54] Since copper is actively excreted by the normal body, chronic copper toxicosis in humans without a genetic defect in copper handling has not been demonstrated.[51] However, large amounts (gram quantities) of copper salts taken in suicide attempts have produced acute copper toxicity in normal humans. Equivalent amounts of copper salts (30 mg/kg) are toxic in animals.[55]
Miscellaneous hazards
The metal, when powdered, is a fire hazard. At concentrations higher than 1 mg/L, copper can stain clothes and items washed in water.
Recycling
Copper is 100% recyclable without any loss of quality whether in a raw state or contained in a manufactured product. Copper is the third most recycled metal after iron and aluminium. It is estimated that 80% of the copper ever mined is still in use today.[56]
Insulated wire is also commonly recycled once the insulation is stripped off. High purity copper scrap is directly melted in a furnace and the molten copper is deoxidized and cast into billets, or ingots. Lower purity scrap is usually refined to attain the desired purity level by an electroplating process in which the copper scrap is dissolved into a bath of sulfuric acid and then electroplated out of the solution.[57]
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