CHROMIC ACID: STRUCTURE, PROPERTIES, PRODUCTION, USES - CHEMISTRY - 2025

The chromic acid or H ₂ CrO ₄ is theoretically the acid associated with chromium oxide (VI) or chromia CrO ₃. This name is due to the fact that in acidic aqueous solutions of chromic oxide the species H ₂ CrO ₄ is present together with other species of chromium (VI).

Chromic oxide CrO ₃ is also called anhydrous chromic acid. CrO ₃ is a reddish-brown or purple solid that is obtained by treating solutions of potassium dichromate K ₂ Cr ₂ O ₇ with sulfuric acid H ₂ SO ₄.

Chromic oxide CrO ₃ crystals in a crucible. Rando Tuvikene. Source: Wikipedia Commons.

Aqueous chromic oxide solutions experience an equilibrium of certain chemical species whose concentration depends on the pH of the solution. At basic pH the chromate ions CrO ₄ ^2- predominate, while at acid pH the ions HCrO ₄ ^- and dichromate Cr ₂ O ₇ ^2- predominate. It is estimated that at acid pH chromic acid H ₂ CrO ₄ is also present.

Due to their great oxidizing power, chromic acid solutions are used in organic chemistry to carry out oxidation reactions. They are also used in electrochemical processes to treat metals so that they acquire resistance to corrosion and wear.

Certain polymeric materials are also treated with chromic acid to improve their adhesion to metals, paints, and other substances.

Chromic acid solutions are highly dangerous for humans, most animals and the environment. For this reason, liquid or solid waste from processes where chromic acid is used are treated to remove traces of chromium (VI) or to recover all the chromium present and regenerate the chromic acid for reuse.

Structure

The molecule of chromic acid H ₂ CrO ₄ is formed by a chromate ion CrO ₄ ^2- and two hydrogen ions H ⁺ attached to it. In the chromate ion the element Chromium is in an oxidation state of +6.

The spatial structure of the chromate ion is tetrahedral, where chromium is in the center and oxygen occupies the four vertices of the tetrahedron.

In chromic acid the hydrogen atoms are each together with an oxygen. Of the four bonds of chromium with the oxygen atoms, two are double and two are simple, since these have the hydrogens attached to them.

Structure of chromic acid H ₂ CrO ₄ where the tetrahedral form of chromate and its double bonds are observed. NEUROtiker. Source: Wikipedia Commons.

On the other hand, chromic oxide CrO ₃ has a chromium atom in the +6 oxidation state surrounded by only three oxygen atoms.

Nomenclature

- Chromic acid H ₂ CrO ₄

- Tetraoxochromic acid H ₂ CrO ₄

- Chromic oxide (anhydrous chromic acid) CrO ₃

- Chromium trioxide (anhydrous chromic acid) CrO ₃

Properties

Physical state

Anhydrous Chromic Acid or Chromic Oxide is a purple to red crystalline solid

Molecular weight

CrO ₃: 118.01 g / mol

Melting point

CrO ₃: 196 ºC

Above its melting point it is thermally unstable, it loses oxygen (is reduced) to give chromium (III) oxide Cr ₂ O ₃. It decomposes at approximately 250 ° C.

Density

CrO ₃: 1.67-2.82 g / cm ³

Solubility

CrO ₃ is very soluble in water: 169 g / 100 g of water at 25 ºC.

It is soluble in mineral acids such as sulfuric and nitric. Soluble in alcohol.

Other properties

CrO ₃ is very hygroscopic, its crystals are deliquescent.

When CrO ₃ dissolves in water, it forms strongly acidic solutions.

It is a very powerful oxidant. Vigorously oxidizes organic matter in almost all its forms. Attacks fabric, leather, and some plastics. Also attacks most metals.

It is strongly poisonous and very irritating due to its high oxidizing potential.

Chemistry of aqueous solutions where chromic acid is present

Chromic oxide CrO ₃ dissolves rapidly in water. In aqueous solution, chromium (VI) can exist under different ionic forms.

At pH> 6.5 or in alkaline solution, chromium (VI) acquires the chromate ion form CrO ₄ ^{2 -} yellow in color.

If the pH is lowered (1 <pH <6.5), chromium (VI) mainly forms the HCrO ₄ ^- ion, which can dimerize to the dichromate ion Cr ₂ O ₇ ^2-, and the solution turns orange. At pH between 2.5 and 5.5 the predominant species are HCrO ₄ ^- and Cr ₂ O ₇ ^2-.

Structure of the dichromate ion Cr ₂ O ₇ ^2- which is found together with two sodium Na ⁺ ions. Capaccio. Source: Wikipedia Commons.

The balances that occur in these solutions as the pH decreases are the following:

CrO ₄ ^2- (chromate ion) + H ⁺ ⇔ HCrO ₄ ^-

HCrO ₄ ^- + H ⁺ ⇔ H ₂ CrO ₄ (chromic acid)

2HCrO ₄ ^- ⇔ Cr ₂ O ₇ ^2- (dichromate ion) + H ₂ O

These balances occur only if the acid that is added to lower the pH is HNO ₃ or HClO ₄, because with other acids different compounds are formed.

Acidic dichromate solutions are very strong oxidizing agents. But in alkaline solutions the chromate ion is much less oxidizing.

Obtaining

According to the sources consulted, one of the ways to obtain chromic oxide CrO ₃ consists of adding sulfuric acid to an aqueous solution of sodium or potassium dichromate, forming a red-orange precipitate.

Chromic oxide hydrate or chromic acid. Himstakan. Source: Wikipedia Commons.

Chromic acid H ₂ CrO ₄ is found in aqueous solutions of chromic oxide in an acid medium.

Chromic acid uses

In the oxidation of chemical compounds

Due to its strongly oxidizing ability, chromic acid has long been used successfully to oxidize organic and inorganic compounds.

Among innumerable examples are the following: it allows to oxidize primary alcohols to aldehydes and these to carboxylic acids, secondary alcohols to ketones, toluene to benzoic acid, ethylbenzene to acetophenone, triphenylmethane to triphenylcarbinol, formic acid to CO ₂, oxalic acid to CO ₂, lactic acid to acetaldehyde and CO ₂, ferrous ion Fe ²⁺ to ferric ion Fe ³⁺, iodide ion to iodine, etc.

It allows the conversion of nitroso-compounds to nitro-compounds, sulfides to sulfones. It is involved in the synthesis of ketones starting from alkenes, as it oxidizes hydroborated alkenes to ketones.

Compounds highly resistant to the usual oxidants, such as oxygen O ₂ or hydrogen peroxide H ₂ O ₂, are oxidized by chromic acid. This is the case for certain heterocyclic borans.

In metal anodizing processes

Chromic acid anodization is an electrochemical treatment applied to aluminum to protect it for many years from oxidation, corrosion and wear.

The anodizing process involves the electrochemical formation of a layer of aluminum oxide or alumina on the metal. This layer is then sealed in hot water, with which the conversion to aluminum oxide trihydrate is achieved.

The sealed oxide layer is thick, but structurally weak and not very satisfactory for subsequent adhesive bonding. However, adding a small amount of chromic acid to the sealing water develops a surface that can form good bonds.

The chromic acid in the sealing water dissolves some of the coarse cell-like structure and leaves a thin, strong, firmly attached layer of aluminum oxide, to which the adhesives adhere and form strong and durable bonds.

Chromic acid anodization also applies to titanium and its alloys.

In chemical conversion treatments

Chromic acid is used in metal coating processes by chemical conversion.

During this process, metals are immersed in solutions of chromic acid. This reacts and partially dissolves the surface while depositing a thin layer of complex chromium compounds that interact with the base metal.

This process is called chromate conversion coating or conversion chrome plating.

The metals that are generally subjected to conversion chrome plating are various types of steel, such as carbon steel, stainless steel, and zinc-coated steel, and various non-ferrous metals, such as magnesium alloys, tin alloys, aluminum alloys, copper., cadmium, manganese and silver.

This treatment provides resistance to corrosion and shine to the metal. The higher the pH of the process, the greater the resistance to corrosion. Temperature accelerates the acid reaction.

Coatings of various colors can be applied, such as blue, black, gold, yellow and clear. It also provides better adhesion of the metal surface to paints and adhesives.

In eroded or pitted surfaces

Chromic acid solutions are used in the preparation of the surface of objects made of thermoplastic material, thermoset polymers and elastomers for subsequent coating with paints or adhesives.

H ₂ CrO _{4 has} an effect on the chemistry of the surface and its structure, as it helps to increase its roughness. The combination of pitting and oxidation increases the penetration of the adhesives and can even cause changes in the properties of the polymer.

It has been used to erode branched low-density polyethylene, linear high-density polyethylene and polypropylene.

It is widely used in the electroplating or electroplating industry to facilitate metal-polymer adhesion.

In various uses

Chromic acid is used as a wood preservative, also in magnetic materials and for catalysis of chemical reactions.

Chromic acid recovery

There are many processes that use chromic acid and generate streams or residues that contain chromium (III) which cannot be disposed of because they have chromium (VI) ions that are very toxic, nor can they be reused because the concentration of chromate ions is very low.

Their disposal requires the chemical reduction of chromates to chromium (III), followed by precipitation of the hydroxide and filtration, which generates additional costs.

For this reason, various methods to remove and recover chromates have been studied. Here are some of these.

By using resins

Ion exchange resins have been used for many years for the treatment of water contaminated with chromates. This is one of the treatments approved by the US Environmental Protection Agency, or EPA (Environmental Protection Agency).

This method allows the recovery of concentrated chromic acid as it is regenerated again from the resin.

The resins can be strong or weak based. In strongly basic resins the chromate can be removed since the ions HCrO ₄ ^- and Cr ₂ O ₇ ^2- are exchanged with the ions OH ^- and Cl ^-. In weakly basic resins, for example those of sulfate, the ions are exchanged with SO ₄ ^{2 -}.

In the case of the strongly basic R- (OH) resins, the overall reactions are as follows:

2ROH + HCrO ₄ ^- + H ⁺ ⇔ R ₂ CrO ₄ + 2H ₂ O

R ₂ CrO ₄ + 2HCrO ₄ ^- ⇔ 2RHCrO ₄ + CrO ₄ ^2-

R ₂ CrO ₄ + HCrO ₄ ^- + H ⁺ ⇔ R ₂ Cr ₂ O ₇ + H ₂ O

For every mole of R ₂ CrO ₄ converted, one mole of Cr (VI) is removed from the solution, which makes this method very attractive.

After removing the chromates, the resin is treated with a strongly alkaline solution to regenerate them in a safe place. The chromates are then converted to concentrated chromic acid to be reused.

Through electrochemical regeneration

Another method is the electrochemical regeneration of chromic acid, which is also a very convenient alternative. Chromium (III) is anodically oxidized to chromium (VI) by this procedure. The anode material in these cases is preferably lead dioxide.

Use of microorganisms to clean effluents with traces of chromic acid

A method that has been investigated and is still under study is the use of microorganisms naturally present in certain effluents contaminated with hexavalent chromium ions, which are those contained in chromic acid solutions.

Effluents harmful to the environment. Author: OpenClipart-Vectors. Source: Pixabay.

Such is the case of certain bacteria present in leather tanning wastewater. These microbes have been studied and it has been determined that they are resistant to chromates and are also capable of reducing chromium (VI) to chromium (III) which is much less harmful to the environment and living beings.

For this reason, it is estimated that they can be used as an environmentally friendly method for the remediation and detoxification of effluents contaminated with traces of chromic acid.

Chromic Acid and Chromic Oxide Hazards

CrO ₃ is not combustible but it can intensify the combustion of other substances. Many of their reactions can cause fire or explosion.

CrO ₃ and chromic acid solutions are potent irritants to the skin (can cause dermatitis), eyes (can burn) and mucous membranes (can cause bronchoasma) and can cause so-called "chromium holes" in the respiratory system..

Chromium (VI) compounds such as chromic acid and chromic oxide are severely toxic, mutagenic, and carcinogenic to most living things.

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