Chromic acid, also known as chromium trioxide by its chemical name, has the chemical formula CrO3 and is a powerful oxidizing agent. It is a dark red color, odorless and hygroscopic solid which is highly soluble in water, ethanol, and acetone.
It is a highly toxic and carcinogenic compound. Exposure to it can cause skin and eye irritation, respiratory problems, and lung cancer. Its use has been banned in many countries, and safer alternatives have been developed for many of its applications.
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Chromic Acid Formula Structure
The chemical formula of chromic acid is H2CrO4, and it consists of two hydrogen atoms, one chromium atom, and four oxygen atoms. The structure of chromic acid is represented using the Lewis structure, which shows the arrangement of the atoms and electrons in the molecule.
In this, the double-bond oxygen atoms have two unshared pairs of electrons, and the single-bond oxygen atoms have one unshared pair of electrons.
Chromic Acid Preparation
Chromic acid can be prepared (made) in several ways, which are as follows.
Jones Oxidation
In this method, a solution of concentrated sulfuric acid (H2SO4) containing sodium dichromate (Na2Cr2O7) is used to oxidize alcohol to its corresponding aldehyde or ketone. The reaction usually occurs at room temperature, and chromic acid is formed as an intermediate.
R-CH2-OH + [O] → R-CHO + H2O
Na2Cr2O7 + 2H2SO4 → CrO3 + Na2SO4 + 2H2O
CrO3 + H2O → H2CrO4
Overall reaction:
R-CH2-OH + Na2Cr2O7 + 2H2SO4 → R-CHO + H2CrO4 + Na2SO4 + 2H2O
K2Cr2O7/H2SO4 Method
In this method, potassium dichromate (K2Cr2O7) is treated with concentrated sulfuric acid (H2SO4), and the mixture is heated. The resulting chromic acid is dissolved in this water to obtain a solution.
K2Cr2O7 + 4H2SO4 → 2CrO3 + K2SO4 + 4H2O
CrO3 + H2O → H2CrO4
Overall reaction:
K2Cr2O7 + 4H2SO4 → 2H2CrO4 + K2SO4 + 3H2O
Oxidation of Chromium (III) Compounds
Chromic acid is made by oxidizing chromium(III) compounds with oxidizing agents such as nitric acid or hydrogen peroxide. For example, chromium(III) oxide (Cr2O3) is oxidized with nitric acid (HNO3) to form chromic acid.
Cr2O3 + 3HNO3 → 2HCrO4 + 3NO2 + H2O
Overall reaction:
Cr2O3 + 3HNO3 → 2H2CrO4 + 3NO2
Chromic acid Uses
Chromic acid has many industrial and laboratory uses because of its strong oxidizing properties.
1. Chromic acid is commonly used for cleaning and etching metals, ceramics, and glass.
2. It is used as a wood preservative, particularly for outdoor wood structures such as decks and fences. It helps protect the wood from decay, rot, and insect damage.
3. It is used in electroplating to form a hard, corrosion-resistant coating on metal surfaces.
4. It is used as a powerful oxidizing agent. And this is also done to convert them into aldehydes and ketones.
5. It is used in some photographic processes to develop and fix images on film.
6. It is a catalyst in chemical processes.
7. It is used in leather tanning in the leather industry.
8. It is used in Pulp and paper production.
9. It is used in the Manufacturing of pigments and dyes.
10. It is in the Cleaning and maintenance of laboratory glassware.
Chromic acid must be handled with care as it is a highly corrosive and toxic substance that can cause severe burns and respiratory and cancer-like problems.
Chromic Acid Solution Preparation
Chromic acid is prepared by dissolving chromium trioxide (CrO3) in water.
Materials and Equipment
Chromium trioxide (CrO3)
1. Distilled water
2. Glass stirring rod
3. Glass container with a lid
4. Protective gloves and goggles
Procedure
Take out the required amount of chromium trioxide between 10-20 grams and pour it into a glass container.
Distilled water is taken in the ratio 2:1 (ie, 2 parts water to 1 part CrO3 by weight). Now slowly add distilled water to the container while stirring continuously with a glass rod to dissolve the chromium trioxide.
After the chromium trioxide is dissolved, cool the solution to room temperature before use. Heat is generated during this process as the solution dissolves, and if not carefully controlled, boils and splashes can occur.
Store the prepared chromic acid solution in a tightly sealed glass container and label it with concentration and hazard information.
