Nickel Hydroxide, Theophrasite
|Molar mass||92.724 g/mol (anhydrous)
110.72 g/mol (monohydrate)
|Melting point||230 °C (446 °F; 503 K) (anhydrous, decomposes)|
|0.013 g/100 mL|
|Solubility||soluble in dilute acid, ammonia (monohydrate)|
Std enthalpy of
LD50 (Median lethal dose)
|1515 mg/kg (oral, rat)|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Nickel(II) hydroxide Ni(OH)2 is an insoluble compound with strong redox properties and widespread industrial and laboratory applications. It most commonly is used in rechargeable battery electrodes, by oxidation to nickel(III) oxide-hydroxide.
Nickel(II) hydroxide has two well-characterized polymorphs, its α and β forms. The α structure consists of Ni(OH)2 layers with intercalated anions or water molecules occupying the space between layers. The β form is a hexagonal closest-packed structure of Ni2+ and OH− ions, without other intercalated ions. In the presence of water, the α polymorph typically decays to the β form due to dissolution and recrystalization. In addition to the α and β polymorphs, several γ nickel hydroxides have been described, distinguished by crystal structures with much larger inter-sheet distances.
The mineral form of Ni(OH)2, theophrastite, was first identified in the Vermion region of northern Greece, in 1980. It is found naturally as a translucent emerald-green crystal formed in thin sheets near the boundaries of idocrase or chlorite crystals. A nickel-magnesium variant of the mineral, (Ni,Mg)(OH)2 had been previously discovered at Hagdale on the island of Unst in Scotland.
Ni(OH)2 readily undergoes oxidation to nickel oxyhydroxide, NiOOH, in combination with an reduction reaction, often of a metal hydride (reaction 1 and 2).
Reaction 1 Ni(OH)2 + OH− → NiOOH + H2O + e−
Reaction 2 M + H2O + e− → MH + OH−
Net Reaction (in H2O) Ni(OH)2 + M → NiOOH + MH
Due to reactivity in redox processes nickel (II) hydroxide is frequently used in electrochemical cells. In particular, as a good capacitor, it is frequently used for the storage of electrochemical energy. For example, it has been proposed as a useful electrode for use in electrical car batteries.
Of the two isomers, α-Ni(OH)2 has a higher theoretical capacity and thus is generally considered to be preferable in electrochemical applications. However, it transforms to β-Ni(OH)2 in alkaline solutions, leading to many investigations into the possibility of stabilized α-Ni(OH)2 electrodes for industrial applications.
Nickel hydroxides have also been proposed as materials for a variety of other purposes. These applications include a chromosomal DNA quantification assay.
The earliest reported synthesis of nickel (II) hydroxide was described by Glemser and Einerhand, who used the oxidation of nickel nitrate with K2S2O8 and NaOH.
A variety of alternative methods for the synthesis of nickel(II) hydroxide have been proposed to optimize its usefulness in a variety of applications. For example, to prevent the decay of the α to β polymorphy, Jeevandam et al. proposed a sonochemical synthesis technique that produced Ni(OH)2 ¬layers with substituted aluminum ions. Fu et al. used an approach in which nickel foil was washed in acetone, dilute NaOH and HNO3 to form aqueous Ni(NO3)2•6H2O that was then collected through electrodeposition on a nickel foil electrode.
The Ni2+ ion is a known carcinogen in both humans and mice, possibly by entry into cells via phagocytosis . In the CHO cell line, Ni(OH)2, the LC50 dose has been shown to be 3.6 μg/ml. This high level of toxicity relative to other Ni2+ containing compounds is hypothesized to be due to the insoluble nature of the compound, and concentration in the nucleus. Toxicity and related safety concerns have driven research into increasing the energy density of Ni(OH)2 electrodes, such as the addition of calcium or cobalt hydroxides.
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