CHROMIUM CHLORIDE (CRCL3): STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2024

The chromium chloride (CrCl ₃) is an inorganic salt composed of cations Cr ³⁺ and anions Cl ^- in a 1: 3; that is, for each Cr ³⁺ there are three Cl ^-. As will be seen later, their interactions are not ionic. This salt can appear in two forms: anhydrous and hexahydrate.

The anhydrous form is characterized by a reddish-violet color; while the hexahydrate, CrCl ₃.6H ₂ O, is dark green. The incorporation of the water molecules modifies the physical properties of said crystals; like their boiling and melting points, densities, etc.

Violet-reddish crystals of anhydrous chromium (III) chloride. Source: Ben Mills

Chromium (III) chloride (according to the stock nomenclature) decomposes at high temperatures, transforming into chromium (II) chloride, CrCl ₂. It is corrosive to metals, although it is used in chrome plating: a procedure in which metals are coated with a thin layer of chromium.

Cr ³⁺, from its respective chloride, has been used in the treatment of diabetes, especially in patients with total parenteral nutrition (TPN), who do not ingest the required amount of chromium. However, the results are much better (and more reliable) when supplied as picolinate.

Chromium chloride structure

Coordination octahedron for CrCl3 in its crystals. Source: Ben Mills

CrCl ₃ despite being a salt, the nature of its interactions are not purely ionic; they have a certain covalent character, product of the coordination between Cr ³⁺ and Cl ^-, which give rise to a deformed octahedron (upper image). Chromium is located in the center of the octahedron, and chlorines in its vertices.

The octahedron CrCl ₆ may, at first glance, contradict the formula CrCl ₃; however, this complete octahedron does not define the unit cell of the crystal, but rather a cube (also deformed), which cuts the green spheres or chlorine anions in half.

Anhydrous crystalline layers

Thus, the unit cell with this octahedron still maintains the 1: 3 ratio. Reproducing these deformed cubes in space, the CrCl ₃ crystal is obtained, which is represented in the upper image with a three-dimensional filling model, and a model of spheres and bars.

Layer of the crystalline structure of CrCl3 represented with spheres and bars model and three-dimensional filling. Source: Ben Mills

This crystalline layer is one of many that make up the flaky reddish-violet crystals of CrCl ₃ (do not confuse the true color of the crystal with that of the green spheres).

As can be seen, the Cl ^- anions occupy the surface, so their negative charges repel the other crystalline layers. Consequently, the crystals become flaky and brittle; but shiny, because of the chrome.

If these same layers are visualized from a lateral perspective, it will be observed, instead of octahedra, distorted tetrahedra:

Crystalline layers of CrCl3 seen from the side. Source: Ben Mills.

Here it is further facilitated the understanding of why the layers repel each other when Cl ^- anions on their surfaces bind.

Properties

Names

-Chromium (III) chloride

-Chromium (III) trichloride

-Anhydrous chromium (III) chloride.

Chemical formula

-CrCl ₃ (anhydrous).

-CrCl ₃.6H ₂ O (hexahydrate).

Molecular weight

-158.36 g / mol (anhydrous).

-266.43 g / mol (hexahydrate).

Physical description

-Reddish-violet solids and crystals (anhydrous).

-Dark green crystalline powder (hexahydrate, bottom image). In this hydrate you can see how water inhibits shine, a metallic characteristic of chromium.

Chromium chloride hexahydrate. Source: User: Walkerma

Melting point

-1,152 ° C (2,106 ° F, 1,425 K) (anhydrous)

-83 ° C (hexahydrate).

Boiling point

1300 ° C (2,370 ° F, 1,570) (anhydrous).

Water solubility

Aqueous solutions of chromium (III) chloride. Source: Leiem

-Slightly soluble (anhydrous).

-585 g / L (hexahydrate).

The image above shows a series of test tubes filled with an aqueous solution of CrCl ₃. Note that the more concentrated it is, the more intense is the color of the ³⁺ complex, responsible for the green color.

Solubility in organic solvents

Soluble in ethanol, but insoluble in ether (anhydrous).

Density

-2.87 g / cm ³ (anhydrous).

-2.76 g / cm ³ (hexahydrate).

Storage temperature

Decomposition

When heated to decomposition, chromium (III) chloride emits toxic fumes of chlorine-containing compounds. These compounds are also released when chromium (III) chloride comes into contact with strong acids.

Corrosion

It is highly corrosive and can attack certain steels.

Reactions

It is incompatible with strong oxidants. It also reacts strongly with lithium and nitrogen.

When heated in the presence of hydrogen, it reduces to chromium (II) chloride, with the formation of hydrogen chloride.

2 CrCl ₃ + H ₂ => 2 CrCl ₂ + 2 HCl

pH

In an aqueous solution, and with a concentration of 0.2 M: 2.4.

Synthesis

Chromium (III) chloride hexahydrate is produced by reacting chromium hydroxide with hydrochloric acid and water.

Cr (OH) ₃ + 3 HCl + 3 H ₂ O => CrCl ₃.6H ₂ O

Then, to obtain the anhydrous salt, the CrCl ₃.6H ₂ O is heated in the presence of thionyl chloride, SOCl ₂, hydrochloric acid, and heat:

Cl ₃ + 6SOCl ₂ + ∆ → CrCl ₃ + 12 HCl + 6SO ₂

Alternatively, CrCl ₃ is obtained by passing chlorine gas over a mixture of chromium and carbon oxide.

Cr ₂ O ₃ + 3 C + Cl ₂ => 2 CrCl ₃ + 3 CO

And finally, being the most used method, it is to heat its oxide with a halogenating agent, such as carbon tetrachloride:

Cr ₂ O ₃ + 3CCl ₄ + ∆ → 2CrCl ₃ + 3COCl ₂

Applications

Industrial

Chromium chloride is involved in the in situ preparation of chromium (II) chloride; reagent involved in the reduction of alkyl halides, and in the synthesis of (E) -alkenyl halides.

-It is used in the chrome plating technique. This consists of depositing, by means of electroplating, a thin layer of chromium on metal objects or other material with a decorative objective, thus increasing the resistance to corrosion and also the surface hardness.

-It is used as a textile mordant, serving as a link between the dyeing material and the fabrics to be dyed. In addition, it is used as a catalyst for the production of olefins and waterproofing agents.

Therapeutics

The use of USP chromium chloride supplement is recommended in patients who only receive intravenous solutions, administered for total parenteral nutrition (TPN). Therefore, only when these patients do not receive all their nutritional requirements.

Chromium (III) is part of the glucose tolerance factor, an activator of insulin-promoting reactions. Chromium (III) is thought to activate glucose, protein, and lipid metabolism, facilitating the action of insulin in humans and animals.

Chromium is present in many foods. But its concentration does not exceed 2 per serving, broccoli being the food with the highest contribution (11 µg). Additionally, the intestinal absorption of chromium is low, with a value of 0.4 to 2.5% of the amount ingested.

This makes it difficult to establish a diet for the supply of chromium. In 1989, The National Academy of Sciences recommended 50 to 200 µg / day as an adequate intake of chromium.

Risks

Among the possible risks of consuming this salt as a chromium supplement are:

-Strong stomach pains.

-Abnormal bleeding, which can range from difficulties for a wound to heal, redder bruises, or a darkening of stool due to internal bleeding.

-Irritations in the digestive system, causing ulcers in the stomach or intestines.

-Dermatitis

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2019). Chromium (III) chloride. Recovered from: en.wikipedia.org
3. Chromium (III) Chloride. Recovered from: alpha.chem.umb.edu
4. PubChem. (2019). Chromium chloride hexahydrate. Recovered from: pubchem.ncbi.nlm.nih.gov
5. National Institutes of Health. (September 21, 2018). Chromium: Dietary Supplement Fact Sheet. Recovered from: ods.od.nih.gov
6. Tomlinson Carole A. (2019). Chromium Chloride Side Effects. Leaf Group Ltd. Recovered from: healthfully.com