Hydriodic Acid: A Powerful Reducing Agent in Modern Chemistry

Hydriodic acid (HI) is a strong, highly reactive acid composed of hydrogen and iodine. As one of the strongest hydrohalic acids, HI is not only a potent proton donor but also a powerful reducing agent, making it a key reagent in a wide range of chemical, pharmaceutical, and industrial applications.

Chemical Overview

IUPAC Name: Iodane (1−) hydrogen
Common Name: Hydriodic acid
Chemical Formula: HI
CAS Number: 10034-85-2 (aqueous solution)
Molar Mass: 127.91 g/mol
Appearance: Colorless to pale yellow solution
Odor: Sharp, acrid
Solubility: Completely miscible in water
Acidity (pKa): ~ −10 (very strong acid)

Physical and Chemical Properties

Property	Value/Description
State at Room Temp	Typically in aqueous solution
Concentration Range	~47–57% HI by weight in water
Boiling Point	127°C (concentrated aqueous solution)
Density	~1.7 g/cm³ (concentrated)
Reactivity	Strong reducing agent, highly corrosive

Hydriodic acid solutions can darken over time due to the formation of elemental iodine (I₂) via oxidation, especially in the presence of air or light.

Synthesis and Stability

Hydriodic acid is typically prepared by:

Direct synthesis: Reacting iodine (I₂) with hydrogen sulfide (H₂S) or red phosphorus and iodine in water.
Displacement reactions: Reacting sodium iodide (NaI) with concentrated phosphoric or sulfuric acid, although this method can lead to oxidation and iodine contamination.

To maintain its stability, HI should be:

Stored in dark, airtight bottles
Protected from air and light to prevent oxidation to I₂

Key Applications of Hydriodic Acid

1. Powerful Reducing Agent

HI is used in organic synthesis to reduce:

Alkyl halides to alkanes
Epoxides to alcohols
Nitro compounds to amines
Its reducing capability stems from the easy oxidation of iodide (I⁻) to iodine (I₂).

2. Pharmaceutical Intermediates

HI is used in the manufacture of key drug intermediates, particularly in the synthesis of:

Analgesics and antihistamines
Alkaloids
Methamphetamine (note: heavily regulated due to illicit use)

3. Laboratory Reagent

In analytical chemistry, HI is employed for:

Reductive dehalogenation
Testing and calibrating reducing strength
Reacting with aldehydes and ketones under acidic conditions

4. Preparation of Iodide Salts

HI is used to produce various iodide compounds, such as:

Sodium iodide (NaI)
Potassium iodide (KI)
These salts are important in nutritional supplements, pharmaceuticals, and as reagents.

Safety and Handling

Hydriodic acid is highly corrosive and toxic, requiring strict safety precautions.

Hazards:

Causes severe skin and eye burns
Harmful if inhaled or ingested
Can release toxic iodine vapors

Safety Guidelines:

Always wear gloves, goggles, and protective clothing
Work in a fume hood
Store in a cool, dark, well-ventilated area
Use non-metallic containers to avoid corrosion

First Aid Measures:

Skin/Eye Contact: Rinse with water for at least 15 minutes; seek immediate medical attention.
Inhalation: Move to fresh air; seek medical help if symptoms occur.
Ingestion: Do not induce vomiting; rinse mouth and get medical assistance immediately.

Environmental Impact and Regulation

Due to its role in the illegal synthesis of controlled substances, hydriodic acid is a regulated chemical in many countries. Usage and distribution often require:

Special licenses
Strict documentation and tracking
Compliance with environmental and safety regulations

Improper disposal can result in acidification of water sources and release of iodine, both harmful to ecosystems.

Conclusion

Hydriodic acid is a potent, multifunctional reagent used widely in organic synthesis, pharmaceutical production, and chemical analysis. Its dual nature as a strong acid and efficient reducing agent makes it a valuable, albeit tightly controlled, chemical. When handled responsibly and with proper safety measures, HI offers unmatched utility in advanced chemical transformations.

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