Nitric acid (\(HNO_3\)) is a highly important compound used across numerous industrial and laboratory settings, playing a role in everything from fertilizer production to metal etching. Understanding the properties of nitric acid requires examining its fundamental chemical classification and the serious hazards it presents in practical use.
Defining Nitric Acid as a Strong Acid
The strength of an acid is defined by its capacity to donate a proton (\(H^+\)) when dissolved in water. Nitric acid is classified as a strong acid because it undergoes virtually complete dissociation, or ionization, in an aqueous solution. This means almost every molecule breaks apart to release its proton, which immediately bonds with a water molecule (\(H_2O\)) to form a hydronium ion (\(H_3O^+\)).
The high concentration of \(H_3O^+\) ions gives the solution its high acidity and powerful corrosive properties. This near-total ionization is reflected in its low pKa value, typically reported as less than \(-1\) or around \(-1.5\). A very low pKa indicates a strong tendency to release a proton, establishing nitric acid firmly in the category of strong mineral acids.
The Dual Threat: Corrosiveness and Oxidizing Power
Nitric acid’s danger stems from a unique combination of two destructive properties: it is both a highly corrosive strong acid and a potent oxidizing agent. While other strong acids, like hydrochloric acid, are corrosive, nitric acid’s additional oxidizing capability allows it to attack a far wider range of materials.
When nitric acid contacts organic materials, such as human skin, its acidity causes hydrolysis of proteins and fats, resulting in severe chemical burns. Simultaneously, its oxidizing action reacts with the protein keratin in the skin. This reaction with amino acids (tyrosine, tryptophan, and phenylalanine) is known as the xanthoproteic reaction, which leaves a characteristic bright yellow stain.
The oxidizing power is particularly significant when the acid reacts with metals. Unlike many other acids, which react with metals to produce hydrogen gas, nitric acid’s strong oxidizing nature prevents this by immediately oxidizing the hydrogen to water. It is capable of dissolving most metals, with the exception of noble metals like gold and platinum, by oxidizing the metal atoms. The nitrogen atom in nitric acid is in its highest possible oxidation state (\(+5\)), meaning it readily accepts electrons from other substances, making it a powerful electron-stripping agent. This is why it is used in the production of explosives and dyes, where its ability to add a nitro group (\(-\text{NO}_2\)) to organic molecules is crucial.
How Concentration Influences Nitric Acid’s Strength
The practical strength and hazard profile of nitric acid are heavily dependent on its concentration. Commercially available concentrated nitric acid is typically sold at a concentration of about 68% by mass. This concentration represents the azeotrope, a mixture where the liquid and vapor phases have the same composition, allowing it to be distilled without changing concentration.
When the concentration rises above 86%, the acid is referred to as “fuming nitric acid,” which presents a significantly increased hazard. Fuming nitric acid is highly unstable and releases copious amounts of toxic, reddish-brown nitrogen dioxide (\(NO_2\)) gas upon exposure to air. This gas is a serious inhalation hazard, causing delayed pulmonary edema.
Fuming nitric acid is further categorized into white fuming nitric acid (WFNA), which is above 95% concentration, and red fuming nitric acid (RFNA), which contains substantial dissolved nitrogen dioxide. Highly concentrated nitric acid is a stronger oxidizer than its dilute counterpart, making it more reactive with organic compounds and capable of igniting combustible materials upon contact.
Safe Handling and Emergency Procedures
Given nitric acid’s extreme corrosiveness and oxidizing power, strict safety protocols are necessary to prevent severe injury.
- Use appropriate personal protective equipment (PPE), which includes chemical-resistant gloves, a lab coat, and eye protection such as safety goggles or a full face shield.
- Work with nitric acid, especially in concentrated or fuming forms, inside a chemical fume hood to prevent the inhalation of corrosive vapors and toxic nitrogen oxides.
- In the event of skin or eye contact, immediately flush the affected area with large amounts of water for a minimum of 15 minutes.
- Contaminated clothing must be removed promptly while flushing continues.
- Seek immediate medical attention, even if the injury appears minor, because the full extent of corrosive burns may be delayed.
- Segregate the acid from incompatible substances, particularly organic materials, bases, and flammable liquids, to prevent violent or explosive reactions.
- Store containers tightly closed in a cool, well-ventilated area to minimize decomposition and the buildup of hazardous fumes.