He chrome hydroxide is an inorganic compound product of the reaction of a base with a salt of chromium. Its chemical formula varies according to the oxidation state of chromium (+2 or +3, for this type of compound). Having thus Cr (OH) 2 for the hydroxide of chromium (II), and Cr (OH) 3 for the chromium (III) hydroxide.
For electronic reasons, the Cr 2+ it is more unstable than Cr 3+ , so the Cr (OH) 2 it is a reducing agent (it loses an electron to pass to +3). Thus, although both hydroxides can be obtained as precipitates, Cr (OH) 3 -also called chromic hydroxide- is the predominant compound.
Unlike those hydroxides obtained by the simple dissolution of metal oxides in water, Cr (OH) 3 it is not synthesized by this route due to the poor solubility of chromic oxide (Cr 2 OR 3 , upper image). However, Cr (OH) 3 It is considered as the Cr 2 OR 3 · XH 2 Or, used as an emerald green pigment (Guinet green).
In the laboratory part of the metallic chromium, which is dissolved in acid solution for the formation of the complex [Cr (OH 2 ) 6 ] 3+ . This aqueous complex then reacts with a base (NaOH or KOH) to form the corresponding chromic hydroxide.
If the previous steps are performed under conditions that ensure the absence of oxygen, the reaction originates Cr (OH) 2 (chromium hydroxide). Subsequently, a separation and dehydration of the precipitated solid is required. As a result,"true"(OH) is"born" 3 , a green powder with a polymeric structure and uncertain.
Index
- 1 Structure
- 1.1 The octahedron and polymerization
- 2 Physical and chemical properties
- 2.1 Anfoterismo
- 3 Synthesis of chromium hydroxide in the industrial field
- 4 Applications
- 5 References
Structure
The upper image is the simplest representation of Cr (OH) 3 in gaseous phase and isolated. Likewise and assuming the purely ionic character of their interactions, in the solid Cr cations can be visualized 3+ interacting with a tripled amount of OH anions - .
However, the nature of the Cr-OH bond is more covalent, due to the coordination chemistry of the Cr 3+ . For example, the complex [Cr (OH 2 ) 6 ] 3+ indicates that the metallic center of chromium is coordinated with six water molecules; Since these are neutral, the complex exhibits the positive charge of the original cation, Cr 3+ .
The octahedron and polymerization
In the upper image, the structure of the complex [Cr (OH 2 ) 6 ] 3+ . Cl ions - they can come, for example, from hydrochloric acid if it has been used for the dissolution of salt or chromic oxide.
When NaOH (or KOH) is added to the reaction medium, the OH ion - deprotonates a molecule of this complex, forming [Cr (OH 2 ) 5 (OH)] 2+ (Now there are five molecules of water because the sixth lost a proton). Consecutively, this new complex dehydrates another aqueous complex, creating dimers bound by hydroxide bridges:
(H 2 OR) 5 Cr-OH-Cr (OH 2 ) 5
As the basicity of the medium increases (the pH rises) the complex is formed [Cr (OH 2 ) 4 (OH) 2 ] + , and also increase the likelihood of new hydroxide bridges to create gelatinous polymers. In fact, this"gray-green jelly"refuses to rush orderly.
Finally, the Cr (OH 2 ) 3 (OH) 3 consists of an octahedron with the Cr 3+ in the center, and linked to three water molecules and three OH - that neutralize its positive charge; this without considering the polymerization.
When the Cr (OH 2 ) 3 (OH) 3 it dehydrates, the water coordinated with the Cr is eliminated 3+ , and since this cation is coordinated with six species (ligands), polymeric structures arise in which Cr-Cr bonds may be involved.
Also, when dehydrated, its structure can be considered Cr type 2 OR 3 · 3H 2 OR; in other words, the tri-hydrated chromic oxide. However, it is the physicochemical studies of the solid that can shed light on the true structure of Cr (OH) 3 in this point.
Physical and chemical properties
The Cr (OH) 3 It has the appearance of a blue-green powder, but when it comes into contact with the water it forms a gelatinous gray-green precipitate.
It is insoluble in water, but soluble in strong acids and bases. In addition, when heated, it decomposes, producing vapors of chromium oxide.
Anfoterismo
Why is chromium hydroxide soluble in acidic and basic solutions? The reason is due to its amphoteric character, which allows it to react with both acids and bases. This property is characteristic of Cr 3+ .
When reacting with acids, Cr (OH 2 ) 3 (OH) 3 it dissolves because the hydroxyl bridges break down, responsible for the gelatinous appearance of the precipitate.
On the other hand, when more base is added, the OH - they continue replacing the water molecules, forming the negative complex [Cr (OH 2 ) 2 (OH) 4 ] - . This complex makes the solution a light green color, which intensifies as the reaction proceeds.
When all the Cr (OH 2 ) 3 (OH) 3 Once it has reacted, a final complex is obtained as indicated by the chemical equation:
Cr (OH 2 ) 3 (OH) 3 + 3 OH - [Cr (OH) 6 ] 3- + 3 H 2 OR
This negative complex is associated with the surrounding cations (Na + , if the base is NaOH), and after the evaporation of the water the sodium chromite salt precipitates (NaCrO 2 , emerald green). Thus, both the acidic and the basic medium are capable of dissolving the chromium hydroxide.
Synthesis of chromium hydroxide in the industrial field
In industry it is produced by the precipitation of chromium sulphate with solutions of sodium hydroxide or ammonium hydroxide. Likewise, chromium hydroxide is produced by the schematized reaction:
CrO 7 2- + 3 SO 2 + 2H + => 2 Cr 3+ + 3 SO 4 2- + H 2 OR
Cr 3+ + 3OH - => Cr (OH) 3
As shown in the previous procedure, the reduction of chromium VI to chromium III has great ecological importance. Chromium III is relatively innocuous for biota, while chromium VI is toxic and carcinogenic, as well as very soluble, so it is important to eliminate it from the environment.
The wastewater and soil treatment technology includes a reduction of Cr (VI) to Cr (III).
Applications
- Formulation of makeups.
- Hair coloring agents.
- Nail polish.
- Skin care products.
- Cleaning products.
- In the finishing of metals, which represents 73% of its consumption in the industry.
- In the preservation of wood.
References
- Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 873, 874.
- PubChem. (2018). Chromic Hydroxide. Retrieved on April 18, 2018, from: pubchem.ncbi.nlm.nih.gov
- N4TR! UMbr. (June 22, 2015). Chromium (III) hydroxide. [Figure]. Retrieved on April 18, 2018, from: commons.wikimedia.org
- Martínez Troya, D., Martín-Pérez, J.J. Study for the experimental use of chromium oxides and hydroxides in the middle classes. BORAX nº 2 (1) -Review of Practical Chemistry for Secondary and Bachillerato-IES. Zaframagón-ISSN 2529-9581.
- Synthesis, characterization and stability of Cr (III) and Fe (III) hydroxides. (2014) Papassiopi, N., Vaxevanidou, K., Christou, C., Karagianni, E. and Antipas, G. J. Hazard Mater. 264: 490-497.
- PrebChem. (February 9, 2016). Preparation of chromium (III) hydroxide. Retrieved on April 18, 2018, from: prepchem.com
- Wikipedia. (2018). Chromium (III) hydroxide. Retrieved on April 18, 2018, from: en.wikipedia.org