How Chromic Acid Is Made and Its Key Chemical Reactions
Chromic acid is a really strong oxidizing agent that is brought into use for oxidising diverse classes of organic compounds. The most common of these compounds are alcohols. Also, chromic acid is used to designate a mixture that is prepared by mixing dichromate and concentrated sulphuric acid. This type of acid is used as a cleaning mixture for washing glass. It can also refer to the molecular class, H2CrO4. This molecular acid has lots in common with sulphuric acid, H2SO4. Chromic acid is also known by the name of Tetraoxo Chromic acid or Chromic(VI) acid.
Chromic Acid Formula
Chromic acid is the conjugate acid of hydrogen chromate and it possesses chromium in an oxidation number of +6 (or VI). The chromic acid formula is H2CrO4:
IUPAC Name: Chromic Acid.
Chronic Acid Formula: H2CrO4.
The Simplified Molecular-Input Line-Entry System (SMILES): O[Cr](=O)(=O)O
Density: 1.201 g cm.
Melting Point: 197 °C.
Chromic Acid Boiling Point: 250 °C,
Chromic Acid Molecular Weight: 118.01 g/mol
Chromic Acid Preparation
Since chromic acid is known to be highly toxic, it is advisable not to make large portions of it and then store it for a larger period of time. Also, when you are working with any reagent, it is always better to wear proper equipment, for example, gloves, coat, etc. Besides, make sure that you ventilate the room where the acid is to be prepared properly.
Step 1: Get a clean litre flask or a beaker. Fill the beaker with 20 grams Sodium dichromate (Na2Cr2O7) or potassium dichromate (K2Cr2O7).
Step 2: Add some water to the beaker or the flask and then stir it using a glass stirring rod in order to make a paste.
Step 3: when you are stirring, pour 300 ml of concentrated sulfuric acid (H2SO4 (aq)) to the beaker.
Step 4: Pour this prepared chromic acid to a glass container having a stopper. Make sure that you put a label to this beaker and also put the date when it was made.
Chromic Acid Structure
The chromic acid structure is as follows:
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The chromic acid structure includes chromium having an oxidation state of +6 (or VI). Also, the chromic acid splits as:
H2CrO4 ⇌ [HCrO4]- + H+
Different Uses of Chromic Acid
Chromic acid is used as an intermediate for the chromium plating process.
It is also used for coloured glasses and ceramic glazes.
Chromic acid is also used for cleaning laboratory glassware.
It was also used as a hair dye in the year 1940.
It can be used for brightening raw brass and it is thus also used in the instrument repair field, especially the musical instrument repair industry.
It is used as a bleach in photographic reversal procedure.
The said chromic acid can oxidise different forms of organic compounds. Thus, you can find diverse variants being created for this particular reagent. It is also referred to as the Jones reagent in different aqueous sulfuric acid as well as acetone. This oxidises primary as well as secondary alcohols into ketones and carboxylic acids.
Health Hazards of Chromic Acid
Chromic acid which comes under hexavalent chromium compounds is highly toxic as well as carcinogenic. Thus, chromic acid oxidation is used mainly in the aerospace field and not in any other industry. This acid is a strong oxidiser and can react violently if you combine it with some easily oxidisable substance and can even cause some explosion.
Chromic Acid Reactions
Chromic acid can oxidise different types of organic compounds and thus diverse reagent variations can be developed:
Chromic acid in acetone and sulfuric acid is called Jones reagent.
Collins reagent is basically an adduct of pyridine and chromium trioxide which is used for different oxidations.
Chromyl chloride or CrO2Cl2 is also a molecular compound that can be generated from chromic acid.
FAQs on Chromic Acid: Formula, Structure, and Uses
1. What is chromic acid, and what is its chemical formula?
Chromic acid is a strong oxoacid of chromium, typically referring to a mixture made by adding concentrated sulfuric acid to a dichromate solution. The molecular formula for the parent chromic acid is H₂CrO₄. However, the term is also commonly used to refer to its anhydrous form, chromium trioxide (CrO₃), which is the solid precursor used to prepare chromic acid solutions.
2. What are the principal uses of chromic acid in various industries?
Chromic acid is a versatile compound with several important industrial applications. Its primary uses include:
- Metal Finishing: It is a key ingredient in the chromium plating process, which provides a durable, corrosion-resistant, and decorative finish on metal parts.
- Organic Synthesis: As a powerful oxidizing agent, it is used in organic chemistry (as Jones reagent) to oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones.
- Wood Preservation: It is used to treat wood to protect it from fungi, insects, and decay, especially in outdoor applications.
- Cleaning: Historically, it was widely used to create a 'chromic acid bath' for cleaning laboratory glassware due to its ability to remove stubborn organic residues.
3. Why is chromic acid considered a powerful oxidizing agent?
Chromic acid's strength as an oxidizing agent comes from the high oxidation state of the chromium atom, which is +6. This is the highest possible oxidation state for chromium, making it highly unstable and electron-deficient. To achieve a more stable, lower energy state (typically the +3 oxidation state), the Cr(VI) ion has a strong tendency to accept electrons from other substances. This process of accepting electrons is the definition of reduction, and in doing so, it oxidizes the other substance.
4. How does the chromic acid test (Jones test) distinguish between different types of alcohols?
The chromic acid test is a classic qualitative test in organic chemistry used to differentiate alcohols. The outcome depends on the alcohol's structure:
- Primary Alcohols: Are oxidized first to an aldehyde and then further to a carboxylic acid.
- Secondary Alcohols: Are oxidized to a ketone.
- Tertiary Alcohols: Do not have a hydrogen atom attached to the carbinol carbon and therefore do not react with chromic acid.
5. What is the structural difference between chromic acid (H₂CrO₄) and chromium trioxide (CrO₃)?
The primary difference is that chromium trioxide (CrO₃) is the anhydrous form, meaning it contains no water. It is a solid, dark red crystalline compound. Chromic acid (H₂CrO₄) is the hydrated form, which exists in aqueous solution when CrO₃ reacts with water. The chromate ion (CrO₄²⁻) in the acid has a tetrahedral geometry with the chromium atom at the centre.
6. What safety precautions are essential when working with chromic acid?
Handling chromic acid requires strict safety measures due to its high toxicity and corrosivity. Key precautions include:
- Using personal protective equipment (PPE) such as chemical-resistant gloves, safety goggles, and a lab coat.
- Working in a well-ventilated area, preferably under a fume hood, to avoid inhaling its toxic fumes.
- Avoiding contact with skin and eyes, as it can cause severe burns and permanent damage.
- Keeping it away from flammable or organic materials, as it is a strong oxidizer and can cause fires or explosions.
7. How does chromic acid (H₂CrO₄) relate to dichromic acid (H₂Cr₂O₇)?
Chromic acid and dichromic acid exist in an equilibrium that is dependent on the pH of the solution. In a neutral or slightly acidic solution, the yellow chromate ion (CrO₄²⁻) is the predominant species. As the solution becomes more acidic, two chromate ions condense (lose a water molecule) to form the orange-red dichromate ion (Cr₂O₇²⁻), which is the ion of dichromic acid. This equilibrium is reversible.
8. Why is the use of chromic acid for cleaning glassware discouraged in modern laboratories?
Although it is a very effective cleaning agent, its use is now heavily discouraged for two main reasons:
- Extreme Toxicity: Chromic acid is highly toxic, corrosive, and a confirmed human carcinogen, posing significant health risks to laboratory personnel.
- Environmental Concerns: The waste produced contains hexavalent chromium (Cr⁶⁺), which is a persistent and hazardous environmental pollutant. Its disposal is difficult, expensive, and strictly regulated, making safer, alternative cleaning solutions (like Nochromix or piranha solution, used with caution) preferable.
