Nickel (II) hydroxide structure, properties, uses, risks

The nickel (II) hydroxide It is a green crystalline inorganic solid where the nickel metal has an oxidation number of 2+. Its chemical formula is Ni (OH)_two. It can be obtained by adding alkaline solutions of potassium hydroxide (KOH), sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH), dropwise to aqueous solutions of nickel (II) salts, such as nickel (II) chloride (NiCl_two), or nickel (II) nitrate (Ni (NO₃)_two).

Under such circumstances, it precipitates as a voluminous green gel that crystallizes after standing for a long time. Its crystals have the structure of brucite or magnesium hydroxide Mg (OH)_two.

In nature, Ni (OH)_two is found in the mineral theophrastite (from the English theophrastite), which was first reported in 1981 when found in northern Greece.

Ni (OH)_two crystallizes into two polymorphic phases, the α and β phase, which depends on the way it has crystallized.

It is soluble in acids and the shade of its greenish color depends on the starting nickel salt.

It has long been used as a cathode in rechargeable alkaline batteries. It has application in electrocatalysis, which makes it a very useful material in fuel cells and electrosynthesis, among several applications..

It presents health risks when inhaled, ingested or if it comes into contact with the skin or eyes. It is also considered a carcinogenic agent.

Article index

• 1 Crystal structure
• 2 Electronic configuration
• 3 Nomenclature
• 4 Properties
  • 4.1 Physical state
  • 4.2 Molecular weight
  • 4.3 Melting point
  • 4.4 Density
  • 4.5 Solubility
  • 4.6 Other properties
• 5 Uses
  • 5.1 In batteries
  • 5.2 In analytical applications
  • 5.3 In electrocatalysis of reactions
  • 5.4 In various uses
• 6 Risks
• 7 References

Crystal structure

Nickel (II) hydroxide can crystallize in two different ways: α-Ni (OH)_two and β-Ni (OH)_two.

Ni (OH) crystal_two has the hexagonal structure of brucite (Mg (OH)_two). The ideal form is NiO layers_two in a planar hexagonal arrangement of Ni cations in octahedral coordination with oxygen.

The α-Ni (OH) form_two it is characterized by being a rather amorphous disordered structure, with a variable interlaminar space, but which is greater than in the β phase. This is explained because it presents within its structure several species interspersed between the layers, such as H_twoO, OH^-, SW₄^two- and CO₃^two-, depending on the starting nickel salt anion.

Β-Ni (OH)_two It also has a layered structure, but much simpler, orderly and compact. The interlaminar space is 4.60 A. OH groups are "free", that is, they do not form hydrogen bonds.

Electronic configuration

In the Ni (OH)_two Nickel is in the 2+ oxidation state, which means that its outermost shell is missing 2 electrons. The electron configuration of Ni^two+ is: [Ar] 3d⁸, where [Ar] is the electron configuration of the noble gas argon.

In the Ni (OH)_two, electrons-d of the Ni atoms are located in the center of a small distorted octahedron of O. Each O atom takes one electron from an H and 1/3 of the Ni atoms, causing each Ni atom to lose 2 electrons-d.

A simple way to represent it is as follows:

H-O^- Neither^{two+ -}O-H

Nomenclature

- Nickel (II) hydroxide

- Nickel dihydroxide

- Nickel (II) oxide monohydrate

Properties

Physical state

Blue-green or yellowish-green crystalline solid.

Molecular weight

92.708 g / mol.

Melting point

230 ºC (melts with decomposition).

Density

4.1 g / cm³ at 20 ºC.

Solubility

Practically insoluble in water (0.00015 g / 100 g of H_twoOR). It is easily soluble in acids. It is also very soluble in ammonia solutions (NH₃), because with this form complexes of bluish violet color.

Other properties

It is not an amphoteric compound. This means that it cannot act both as an acid and as a base..

When the Ni (OH)_two is obtained from solutions of nickel chloride (NiCl_two) presents a bluish-green color, while if it precipitates from solutions of nickel nitrate (Ni (NO₃)_two) has a green-yellow color.

The alpha phase (α-Ni (OH)_two) has electrochemical properties greater than the beta phase. This is because in alpha there are a greater number of electrons available for each nickel atom..

The beta form (β-Ni (OH)_two) has presented characteristics of a semiconductor type-p.

Applications

In batteries

Longest-standing use of Ni (OH)_two it is in batteries. In 1904 Thomas Edison used it together with its oxide NiO (OH) as a material for the cathode of alkaline batteries..

The Electrochemical Capacity of Ni (OH) Cathodes_two it is directly related to the morphology and size of its particles. Ni (OH) nanoparticles_two Due to their small size, they have superior electrochemical behavior and a higher proton diffusion coefficient than larger particles..

It has been widely used as a cathode material in many rechargeable alkaline batteries such as nickel-cadmium, nickel-hydrogen, nickel-iron, among others. It has also been used in high performance supercapacitors.

The reaction in these devices involves the oxidation of Ni (OH)_two during the charging phase and the reduction of NiO (OH) during the discharge phase in the alkaline electrolyte:

Ni (OH)_two + Oh^- - and^- ⇔ NiO (OH) + H_twoOR

This equation is reversible and is called the redox transition..

In analytical applications

Α-Ni (OH)_two It has been used for the development of electrochemical sensors for the determination of Vitamin D₃, or cholecalciferol, a form of Vitamin D that can be obtained by exposing the skin to sunlight or through certain foods (egg yolk, cow's milk, fresh salmon, and cod liver oil).

The use of hybrid sensors containing α-Ni (OH)_two, together with graphene oxide and silica, allows for quantification of vitamin D₃ directly in biological matrices.

Furthermore, the disordered lamellar structure of α-Ni (OH)_two facilitates the entry and exit of ions in empty structural spaces, which favors the electrochemical reversibility of the sensor.

In electrocatalysis of reactions

The redox transition between Ni (OH)_two and NiO (OH) has also been used in the catalytic oxidation of many small organic compounds in alkaline electrolyte. The mechanism of this electrocatalytic oxidation is as follows:

Ni (OH)_two + Oh^- - and^- ⇔ NiO (OH) + H_twoOR

NiO (OH) + organic compound → Ni (OH) 2 + product

The organic compound can be, for example, glucose and the product glucolactone.

Electrocatalysis of oxidation reactions of small molecules has application in fuel cells, electroanalysis, electrosynthesis and electrodegradation.

In various uses

Its electrocatalytic properties have drawn attention for uses in photocatalysis, electrochromic devices, adsorbents and precursors of nanostructures..

Furthermore, it has a potential use as a pigment due to its high reflectance..

Risks

If heated to decomposition, it emits toxic gases. Exposure to Ni (OH)_two presents a number of risks. If inhaled, it is irritating to the mucous membrane of the upper respiratory tract, can cause asthma and can cause pulmonary fibrosis..

If it comes into contact with the eyes, it irritates the conjunctival membrane. In the skin causes sensitization, burning or itching and erythema, causing severe dermatitis and skin allergies.

It can also affect the kidneys, the gastrointestinal tract, the neurological system and can cause cardiovascular damage. May cause harm to the fetus of pregnant women.

Ni (OH)_two it is carcinogenic. It has been associated with the risk of developing nasal and lung cancer. Worker deaths from cancer have been reported at nickel-cadmium battery factories.

It has been classified as very toxic to aquatic life, with long-term damaging effects..

With regard to plants, there is a certain contradiction, because although nickel is toxic to plant life, it is also an essential micronutrient for its development. Required in extremely small amounts for optimal plant growth.

References

1. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
2. Andrade, T.M. et al. (2018). Effect of Precipitating Agents on the Structural, Morphological, and Colorimetric Characteristics of Nickel Hydroxide Particles. Colloid and Interface Science Communications. 23 (2019) 6-13. Recovered from sciencedirect.com.
3. Haoran Wang and Changjiang Song. (2019). Electronic and phonon structure of nickel hydroxide: first-principles calculation study. Eur. Phys. J. B (2019) 92:37. Recovered from link.springer.com.
4. National Library of Medicine. (2019). Nickel Hydroxyde. Recovered from: pubchem.ncbi.nlm.nih.gov.
5. Canevari, T.C. (2014). Synthesis and characterization of alpha-nickel (II) hydroxide particles on organic-inorganic matrix and its application in a sensitive electrochemical sensor for vitamin D determination. Electrochimica Acta 147 (2014) 688-695. Recovered from sciencedirect.com.
6. Miao, Y. et al. (2014). Electrocatalysis and electroanalysis of nickel, its oxides, hydroxides and oxyhydroxides toward small molecules. Biosensors and Bioelectronics. 53 (2014) 428-439. Recovered from sciencedirect.com.