The cobalt hydroxide is the generic name for all compounds where cobalt cations and the OH anion participate-. All are inorganic in nature, and have the chemical formula Co (OH)n, where n is equal to the valence or positive charge of the cobalt metal center.
As cobalt is a transition metal with half-full atomic orbitals, by some electronic mechanism its hydroxides reflect intense colors due to Co-O interactions. These colors, as well as the structures, are highly dependent on their charge and on the anionic species that compete with the OH-.
Colors and structures are not the same for Co (OH)two, Co (OH)3 or for CoO (OH). The chemistry behind all these compounds is intended for the synthesis of materials applied to catalysis.
On the other hand, although they can be complex, the formation of a large part of them starts from a basic environment; as supplied by the strong base NaOH. Hence, different chemical conditions can oxidize cobalt or oxygen..
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What are the structures of cobalt hydroxide? Its general formula Co (OH)n is interpreted ionically as follows: in a crystal lattice occupied by a number of Con+, there will be n times that amount of OH anions- interacting with them electrostatically. Thus, for Co (OH)two there will be two OH- for each Co cationtwo+.
But this is not enough to predict which crystalline system these ions will adopt. By reasoning of coulombic forces, the Co3+ attracts OH more intensely- compared to Cotwo+.
This fact causes the distances or the Co-OH bond (even with its high ionic character) to be shortened. Also, because the interactions are stronger, the electrons in the outer shells of Co3+ they undergo an energetic change that forces them to absorb photons with different wavelengths (the solid darkens).
However, this approach is insufficient to clarify the phenomenon of color change depending on the structure..
The same is true for cobalt oxyhydroxide. Its formula CoO · OH is interpreted as a Co cation3+ interacting with an oxide anion, Otwo-, and an OH-. This compound represents the basis for synthesizing a mixed cobalt oxide: Co3OR4 [CoO · CotwoOR3].
Cobalt hydroxides can also be visualized, albeit less precisely, as individual molecules. Co (OH)two can then be drawn as a linear OH-Co-OH molecule, and the Co (OH)3 like a flat triangle.
With respect to CoO (OH), its molecule from this approach would be drawn as O = Co-OH. Anion Otwo- forms a double bond with the cobalt atom, and another single bond with the OH-.
However, the interactions between these molecules are not strong enough to "arm" the complex structures of these hydroxides. For example, Co (OH)two can form two polymeric structures: alpha and beta.
Both are laminar but with different orderings of the units, and they are also capable of intercalating small anions, such as CO3two-, between its layers; which is of great interest for the design of new materials from cobalt hydroxides.
Polymeric structures can be better explained by considering a coordination octahedron around the cobalt centers. For Co (OH)two, as it has two OH anions- interacting with Cotwo+, you need four water molecules (if aqueous NaOH was used) to complete the octahedron.
Thus, Co (OH)two is actually Co (HtwoOR)4(OH)two. For this octahedron to form polymers, it requires bonding through oxygen bridges: (OH) (HtwoOR)4Co-O-Co (HtwoOR)4(OH). The structural complexity increases for the case of CoO (OH), and even more for Co (OH)3.
-Formula: Co (OH)two.
-Molar mass: 92.948 g / mol.
-Appearance: pinkish-red powder or red powder. There is an unstable blue form of the formula α-Co (OH)two
-Density: 3.597 g / cm3.
-Solubility in water: 3.2 mg / l (slightly soluble).
-Soluble in acids and ammonia. Insoluble in dilute alkali.
-Melting point: 168º C.
-Sensitivity: sensitive to air.
-Stability: it is stable.
-Formula: Co (OH)3
-Molecular mass: 112.98 g / mol.
-Appearance: two ways. A stable black-brown shape and an unstable dark green shape with a tendency to darken.
The addition of potassium hydroxide to a solution of cobalt (II) nitrate, results in the appearance of a blue-violet precipitate that, when heated, becomes Co (OH)two, i.e. cobalt (II) hydroxide.
Co (OH)two precipitates when an alkali metal hydroxide is added to an aqueous solution of a Co salttwo+
Cotwo+ + 2 NaOH => Co (OH)two + 2 Na+
-It is used in the manufacture of catalysts for use in oil refining and in the petrochemical industry. In addition, Co (OH) is usedtwo in the preparation of cobalt salts.
-Cobalt (II) hydroxide is used in the manufacture of paint dryers and in the manufacture of battery electrodes.
-Cobalt hydroxides are the raw material for the synthesis of nanomaterials with novel structures. For example, from Co (OH)two nanocopes of this compound have been designed with a large surface area to participate as a catalyst in oxidative reactions. These nanocopes are impregnated on porous nickel or crystalline carbon electrodes.
-It has been sought to implement carbonate hydroxide nanorods with carbonate sandwiched in their layers. They take advantage of the oxidative reaction of Cotwo+ to Co3+, proving to be a material with potential electrochemical applications.
-Studies have synthesized and characterized, using microscopy techniques, nanodisks of mixed cobalt oxide and oxyhydroxide, from the oxidation of the corresponding hydroxides at low temperatures..
Bars, discs and flakes of cobalt hydroxide with structures at nanometric scales, open the doors to improvements within the world of catalysis and, also, of all applications concerning electrochemistry and the maximum use of electrical energy in modern devices.
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