An acid is a chemical compound that readily donates a proton, a positively charged hydrogen ion (\(\text{H}^+\)), when dissolved in water. Conversely, a base, also known as an alkali, readily accepts a proton or releases a hydroxide ion (\(\text{OH}^-\)) into a solution. The term “corrosive” describes the ability of a substance to cause irreversible, destructive changes to living tissue upon contact. Strong acids and bases are particularly notorious for causing severe chemical burns on human skin. Recognizing the distinct chemical properties of these substances is essential for responding effectively to accidental exposure.
Understanding Corrosivity: The pH Scale and Strength
A substance’s corrosivity is directly linked to its concentration of reactive ions, measured using the pH scale. This scale ranges from 0 to 14, where 7 is neutral (pure water). Solutions below pH 7 are acidic due to a high concentration of \(\text{H}^+\) ions, and those above 7 are basic, containing a high concentration of \(\text{OH}^-\) ions. The scale is logarithmic; a substance with a pH of 1 is ten times more acidic than one with a pH of 2.
Tissue damage depends on both concentration and strength. Strength refers to the substance’s degree of dissociation, or how completely it breaks apart into reactive ions. A strong acid, such as hydrochloric acid, fully dissociates, making it highly reactive even in dilute concentrations. A weak acid, like citric acid, only partially dissociates, requiring a higher concentration to achieve the same corrosivity.
How Acids Corrode: The Mechanism of Coagulation
When a strong acid contacts the skin, the primary mechanism of injury is coagulation necrosis. Hydrogen ions (\(\text{H}^+\)) are rapidly donated to proteins within the skin cells, quickly disrupting their three-dimensional structure in a process called denaturation. This causes the proteins to clump together, rapidly killing the affected tissue.
This denaturation results in the formation of a hard, dense, leather-like layer called an eschar or coagulum. The eschar acts as a physical barrier, slowing and often preventing the acid from penetrating into deeper tissues beneath the epidermis. Common strong acids like sulfuric acid or nitric acid cause this injury, which is usually immediately painful and appears as a white, gray, or blackened lesion.
The self-limiting nature of coagulation necrosis means acid burns often present with a defined boundary and are typically shallower than burns from strong bases. Highly concentrated acids can still cause full-thickness injury, however. An exception is hydrofluoric acid, which penetrates deeply before releasing its damaging ions, acting more like a base in its destructive pattern.
How Bases Corrode: The Mechanism of Liquefaction
Strong bases, or alkalis, cause a distinct and often more severe injury known as liquefaction necrosis. This mechanism involves two powerful chemical reactions that allow the substance to penetrate deep into the skin, unlike the coagulation caused by acids. The first action is the solubilization and hydrolysis of proteins, dissolving the structural components of the cells.
The second action is saponification, the breakdown of fats and oils. The hydroxide ions (\(\text{OH}^-\)) in the base react with the lipids in cell membranes to form a soapy, water-soluble solution. This process rapidly destroys the integrity of the cell walls. Since the tissue is liquefied and dissolved, no protective barrier is formed, allowing the base to continuously penetrate and destroy deeper layers of tissue, including subcutaneous fat and sometimes muscle.
This deep penetration means alkaline burns continue to progress long after the initial contact, sometimes for hours. Common bases, such as sodium hydroxide (lye) found in drain cleaners, are dangerous because the absence of an immediate eschar allows the alkali to track along fascial planes, leading to extensive tissue damage that is often worse than the initial surface appearance suggests.
Safe Handling and Emergency Response
Managing corrosive substances requires strict adherence to safety protocols, including the consistent use of personal protective equipment (PPE). When working with strong acids or bases, gloves, chemical splash goggles, and protective clothing are necessary to prevent direct skin or eye contact. Adequate ventilation is also important, as many corrosives emit harmful, irritating vapors.
In the event of skin exposure, immediate first aid is necessary to minimize tissue damage. Contaminated clothing and jewelry must be removed immediately, as they hold the chemical against the skin. If the substance is a dry chemical powder, it should be gently brushed off the skin before water is applied.
The exposed area must then be flushed with a copious amount of cool running water for a minimum of 15 to 20 minutes, or until emergency medical services arrive. This immediate and prolonged irrigation dilutes the chemical and washes it away from the tissue. Never attempt to neutralize the chemical with a weak acid or base, as the reaction can generate significant heat, adding a thermal burn to the existing injury. Seeking professional medical attention is necessary after any chemical burn, regardless of the apparent severity.