Chemistry of (i) blue vitriol (ii) black oxide of copper (iii) red oxide of copper

Blue vitriol

Copper(II) sulfate is the chemical compoundwith the formula CuSO₄. This salt exists as a series of compounds that differ in their degree ofhydration. The anhydrous form is a pale green or gray-white powder, whereas the pentahydrate(CuSO₄·5H₂O), the most commonly encountered salt, is bright blue. The anhydrous form occurs as a rare mineral known as chalcocyanite. The hydrated copper sulfate occurs in nature aschalcanthite (pentahydrate), and two more rare ones: bonattite (trihydrate) and boothite (heptahydrate). Archaic names for copper(II) sulfate are "blue vitriol" and "bluestone".^[1]

Preparation

Preparation of copper(II) sulfate by electrolyzing sulfuric acid, using copper electrodes

Since it is available commercially, copper sulfate is usually purchased and not prepared in the laboratory. It can be made by the action of sulfuric acid on a variety of copper(II) compounds, for examplecopper(II) oxide; this oxide can be generated with the addition of hydrogen peroxide to the acid. It may also be prepared by electrolyzing sulfuric acid, using copper electrodes. It can also be prepared by electrolysis of magnesium sulfate [Epsom salts] solution at moderate voltage with a copper anode: this reaction produces hydrogen, copper sulfate solution, and magnesium hydroxide precipitate:

MgSO₄(aq) + 2 H₂O + Cu(s) → H₂(g) + Mg(OH)₂(s) + CuSO₄(aq)

Black oxide of copper

Copper(II) oxide or cupric oxide (CuO) is the higher oxideof copper. As a mineral, it is known as tenorite.

It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen. It can be formed by heating copper in air:

2 Cu + O₂ → 2 CuO

Here, it is formed along with copper(I) oxide as a side product; thus, it is better prepared by heating copper(II) nitrate, copper(II) hydroxide or copper(II) carbonate:

2 Cu(NO₃)₂ → 2 CuO + 4 NO₂ + O₂

Cu(OH)₂ (s) → CuO (s) + H₂O (l)

CuCO₃ → CuO + CO₂

Copper(II) oxide is a basic oxide, so it dissolves in mineral acids such as hydrochloric acid, sulfuric acid or nitric acidto give the corresponding copper(II) salts:

CuO + 2 HNO₃ → Cu(NO₃)₂ + H₂O

CuO + 2 HCl → CuCl₂ + H₂O

CuO + H₂SO₄ → CuSO₄ + H₂O

It reacts with concentrated alkali to form the corresponding cuprate salts:

2 XOH + CuO + H₂O → X₂[Cu(OH)₄]

It can also be reduced to copper metal using hydrogen orcarbon monoxide:

CuO + H₂ → Cu + H₂O

CuO + CO → Cu + CO₂

A laboratory method for preparing copper II oxide would be to electrolyze water containing sodium bicarbonate at a moderate voltage with a copper anode, collect the mixture of copper hydroxide, basic copper carbonate, and copper carbonate produced, and heat it.

Uses

Cupric oxide is used as a pigment in ceramics to produce blue, red, and green (and sometimes gray, pink, or black) glazes. It is also used to produce cuprammonium hydroxide solutions, used to makerayon. It is also occasionally used as a dietary supplement in animals, against copper deficiency.^[11]Copper(II) oxide has application as a p-type semiconductor, because it has a narrow band gap of 1.2 eV. It is an abrasive used to polish optical equipment. Cupric oxide can be used to produce dry cell batteries. It has also been used in wet cell batteries as the cathode, with lithium as an anode, and dioxalane mixed with lithium perchlorate as the electrolyte. Copper(II) oxide can be used to produce other copper salts. It is also used when welding with copper alloys.^[12]

Another use for cupric oxide is as a substitute for iron oxide in thermite. This can turn the thermite from an incendiary to a low explosive.

Red oxide of copper

Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu₂O. It is one of the principaloxides of copper. This red-coloured solid is a component of some antifouling paints. The compound can appear either yellow or red, depending on the size of the particles, but both forms degrade to copper(II) oxides in moist air.^[1]Copper(I) oxide is found as the reddish mineral cuprite.

Preparation

Copper(I) oxide may be produced by several methods.^[2]Most straightforwardly, it arises via the oxidation of copper metal:

4 Cu + O₂ → 2 Cu₂O

Additives such as water and acids affect the rate of this process as well as the further oxidation to copper(II) oxides. It is also produced commercially by reduction of copper(II) solutions with sulfur dioxide. Aqueous cuprous chloride solutions react with base to give the same material. In all cases, the color is highly sensitive to the procedural details.

Formation of copper(I) oxide is the basis of the Fehling's test and Benedict's test for reducing sugars. These sugars reduce an alkaline solution of a copper(II) salt, giving a bright red precipitate of Cu₂O.

It forms on silver-plated copper parts exposed to moisture when the silver layer is porous or damaged. This kind ofcorrosion is known as red plague.

Applications

Cuprous oxide is commonly used as a pigment, a fungicide, and an antifouling agent for marine paints.Rectifier diodes based on this material have been used industrially as early as 1924,long before siliconbecame the standard.