What Is Chromic Acid?
Chromic acid is a compound represented by H2CrO4.
Chromic acid is also used to refer collectively to substances in which Chromium Oxide (Ⅵ) dissolves in water to form Chromic acid salts. It has a molecular weight of 118.0 g/mol, a melting point of 197°C, a density of 1.201 g/cm3, and a CAS number of 7738-94-5. The molecule is equal to one molecule of water added to Chromium trioxide.
Uses of Chromic Acid
1. Oxidizing Agent
Chromic acid, other chromates, and compounds containing hexavalent chromium such as chromium trioxide are powerful oxidants and are often used in organic synthesis. They are often used primarily to obtain carboxylic acids by acting on primary alcohols or to obtain ketones by acting on secondary alcohols. It is named Jones oxidation, Sallet oxidation, or Collins oxidation, depending on the reagent and solvent conditions. Since chromic acid is a very strong oxidizing agent, side reactions may occur, such as further oxidation of the target compound to yield an unexpected compound and great care must be taken. PCC, a salt formed from chlorochromic acid ion and pyridinium ion, in which the hydrogen atom of the chromic acid ion is replaced by a chlorine atom, is a mild oxidant and can oxidize primary alcohols to aldehydes. In these oxidation reactions, chromic acid is reduced to trivalent Chromium.
2. Surface Treatment Agents
It is used as a photographic, plating, pigment, leather tanning, dye, and dyeing agent. It is also useful for chromic acid treatment, one of the metal passivation methods. By immersion or cathodic electrolysis in the treatment solution, a corrosion-resistant coating can be imparted. This chromic acid treatment is effective in improving corrosion resistance, paint adhesion, and preventing discoloration. Care must be taken when using chromic acid and Dichromic Acid (hexavalent chromium), as they are highly oxidizing and strongly corrode skin and mucous membranes, causing dermatitis and chrome ulcers.
Properties of Chromic Acid
Chromic acid is a red solid at normal temperature and pressure, with a decomposition point of 250°C. A normal concentration of chromic acid solution is yellowish red, but at higher concentrations, it turns red or dark red and becomes blackish.
In an aqueous solution, two molecules of chromic acid and two molecules of Oxonium Ion combine to form one molecule of Dichromic Acid and three molecules of water molecules. In strongly acidic or basic conditions below pH 1, the equilibrium is tilted in the direction of chromic acid formation.
Other Information on Chromic Acid
1. Hazards of Hexavalent Chromium
Hexavalent Chromium, including chromic acid, is notorious for being extremely toxic. There are two types of Chromium ions: trivalent and hexavalent. While trivalent Chromium is widely found in nature in a stable form, hexavalent Chromium is found only in some ores. Hexavalent Chromium can cause dermatitis when it adheres to human skin and can cause vomiting if ingested into the body. Furthermore, it is carcinogenic because of its ability to damage DNA and can cause lung cancer if inhaled, while long-term ingestion has the potential to cause suspension bridges and stomach cancer. Therefore, when handling hexavalent chromium in powder form, it is necessary to prevent it from scattering into the surroundings and to strictly control it so that it does not get into the eyes, nose, or mouth, and also so that it does not remain on the skin or clothing.
2. Various Chromic Acid Salts
Chromium ions with an oxidation number of 6 are called hexavalent chromium, and the ions are present in aqueous solution in the form of CrO42-, HCrO4-, Cr2O72-, etc. These ions also polymerize to produce the polyacid ions Cr3O102- and Cr4O132-. These ions are usually used in the form of sodium or potassium salts (such as K2Cr2O7 and Na2Cr2O7) and are usually characterized by a yellow or red color.