For electronic reasons, Cr 2+ is more unstable than Cr 3+ , so Cr (OH) 2 is a reducing agent (it loses an electron to go 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 simply dissolving metal oxides in water, Cr (OH) 3 is not synthesized by this route due to the poor solubility of chromic oxide (Cr 2 O 3 , top image). However, Cr (OH) 3 is considered as Cr 2 O 3 · xH 2 O, used as an emerald green pigment (Guinet green).
In the laboratory, the starting point is metallic chromium, which is dissolved in an acid solution to form the [Cr (OH 2 ) 6 ] 3+ complex . This aqueous complex then reacts with a base (NaOH or KOH) to form the corresponding chromic hydroxide.
If the previous steps are carried out under conditions that ensure the absence of oxygen, the reaction originates Cr (OH) 2 (chromous hydroxide). Subsequently, a separation and dehydration of the precipitated solid is required. As a result, the true Cr (OH) 3 is “born” , a green powder with an uncertain polymeric structure.
Chromium hydroxide structure
The upper image is the simplest representation of Cr (OH) 3 in gas phase and isolated. Likewise, and assuming the purely ionic nature of their interactions, Cr 3+ cations can be visualized in the solid, interacting with a tripled amount of OH – anions .
However, the nature of the Cr-OH bond is more of a covalent type, due to the coordination chemistry of Cr 3+ .
For example, the [Cr (OH 2 ) 6 ] 3+ complex indicates that the chromium metal center is coordinated with six water molecules; since these are neutral, the complex exhibits the positive charge of the original cation, Cr 3+ .
The upper image shows the structure of the [Cr (OH 2 ) 6 ] 3+ complex . Cl – ions can come, for example, from hydrochloric acid if it has been used to dissolve the salt or chromic oxide.
When NaOH (or KOH) is added to the reaction medium, the OH – deprotonates a molecule of this complex, forming [Cr (OH 2 ) 5 (OH)] 2+ (now there are five water molecules because the sixth one lost a proton ).
Consecutively, this new complex dehydrates another aqueous complex, creating dimers linked by hydroxide bridges:
(H 2 O) 5 Cr – OH – Cr (OH 2 ) 5
As the basicity of the medium increases (the pH rises) the [Cr (OH 2 ) 4 (OH) 2 ] + complex is formed , and the chances of new hydroxide bridges to create gelatinous polymers also increase. In fact, this “gray-green jelly” refuses to precipitate in an orderly fashion.
Finally, Cr (OH 2 ) 3 (OH) 3 consists of an octahedron with Cr 3+ in the center, and linked to three water molecules and three OH – which neutralize its positive charge; this without considering polymerization.
When Cr (OH 2 ) 3 (OH) 3 is dehydrated, the water coordinated with Cr 3+ is eliminated , and as this cation coordinates with six species (ligands), polymeric structures arise in which the bonds are possibly involved. Cr – Cr.
Likewise, when dehydrated, its structure can be considered of the Cr 2 O 3 · 3H 2 O type; in other words, that of 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 at this point.
Chromium hydroxide physical and chemical properties
Cr (OH) 3 has the appearance of a blue-green powder, but when it comes into contact with water it forms a grayish-green gelatinous precipitate.
It is insoluble in water, but soluble in strong acids and bases. In addition, when heated it decomposes, producing chromium oxide fumes.
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 dissolves due to the breaking of the hydroxyl bridges, responsible for the gelatinous appearance of the precipitate.
On the other hand, when more base is added, the OH – continue to replace the water molecules, forming the negative complex [Cr (OH 2 ) 2 (OH) 4 ] – . This complex turns the solution a light green color, which intensifies as the reaction continues.
When all the Cr (OH 2 ) 3 (OH) 3 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 O
This negative complex is associated with the surrounding cations (Na + , if the base is NaOH), and after evaporation of the water the sodium chromite salt (NaCrO 2 , emerald green color) precipitates . Thus, both acidic and basic environments are capable of dissolving chromium hydroxide.
Chromium hydroxide synthesis in the industrial field
In industry it is produced by the precipitation of chromium sulfate with sodium hydroxide or ammonium hydroxide solutions. Likewise, chromium hydroxide is produced by the reaction outlined:
CrO 7 2– + 3 SO 2 + 2H + => 2 Cr 3+ + 3 SO 4 2– + H 2 O
Cr 3+ + 3OH – => Cr (OH) 3
As shown in the above procedure, the reduction of chromium VI to chromium III is of great ecological importance.
Chromium III is relatively harmless to biota, while chromium VI is toxic and carcinogenic, as well as very soluble, so it is important to eliminate it from the environment.
The sewage and soil treatment technology includes a reduction from Cr (VI) to Cr (III).
Chromium hydroxide has several applications:
- Makeup formulation.
- Hair coloring agents.
- Nail polish.
- Skin care products.
- Cleaning products.
- In metal finishing, which represents 73% of its consumption in the industry.
- In the preservation of wood.