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Most of the effects of sulfuric acid result from its strong acidity and its great affinity for water. Corrosion of metals by sulfuric acid is caused by its acidity. The effects of sulfuric acid on organic materials, including human tissue, are largely the result of its dehydrating properties. Materials such as wood, paper and cotton cloth are rapidly charred on contact with the acid.
The effects of sulfuric acid on metals are typical of a strong acid: it will react with those metals that are more reactive than hydrogen to form a metal sulfate salt and release hydrogen gas. It will react in this way with many common metals, including iron, zinc and aluminum. The reaction is more vigorous with the dilute acid than with the concentrated acid. This limits the materials that can be used to store the acid, although in concentrated form it can be stored in stainless steel tanks. The release of hydrogen gas poses a potential explosion risk in the event of a spillage or leak, if the acid comes into contact with metals.
Sulfuric acid is a very powerful dehydrating agent and a great deal of heat is released when the concentrated acid comes into contact with water. If water is added to an excess of the acid, the heat produced causes the water to boil immediately, which may result in the acid being sprayed over a wide area. For this reason, concentrated sulfuric acid should always be diluted by adding it — slowly — to water; the process should never be reversed.
The dehydrating effects of sulfuric acid explain its reactions with many common organic materials. It will remove hydrogen and oxygen from molecules that contain these elements in the 2:1 ratio found in the water molecule (H2O) — for example, carbohydrates — which include sugars, starch and cellulose. Sulfuric acid will react with carbohydrates to remove the hydrogen and oxygen, leaving behind carbon. A well known laboratory demonstration illustrates this; concentrated sulfuric acid is added to sucrose table sugar in a beaker and quickly converts it to a mass of charcoal, with a good deal of heat produced. This is the reason that sulfuric acid chars wood and paper — substances which consist mainly of carbohydrates.
The effects of sulfuric acid on skin are well documented, and are again due to the acid’s dehydrating properties rather than its acidity. Skin contact with the concentrated acid results in pain and swelling of tissue within a few seconds. If contact is sufficiently prolonged, deep burns can result and there may be some charring, resulting in a brown discoloration. Due to the swelling caused, sulfuric acid burns often result in permanent scarring.
When using sulfuric acid, suitable protective gloves and safety goggles should always be worn; a face shield and protective apron are also recommended. Small spillages onto unprotected skin can be dealt with by prompt washing with plenty of water. Larger spillages pose the risk of spraying of acid if water is applied immediately — it is better to quickly wipe away as much of the acid as possible before washing.
Sulfuric acid is not volatile at room temperature and so does not normally pose an inhalation hazard; however, in the event this acid or its industrial precursor, sulfur trioxide comes into contact with water, the violence of the reaction may produce a fine mist of sulfuric acid droplets. This can damage the eyes, respiratory tract and lungs if inhaled. Chronic exposure to sulfuric acid mist — for example, in a sulfuric acid plant — can have long-term health effects and may pose a cancer risk for workers.
Reactions of sulfuric acid with other materials may result in hazardous products. It will, for example, release toxic and corrosive vapors on contact with halides, such as chlorides, fluorides and bromides. Contact with chlorates and permanganates produces strongly oxidizing compounds that pose a fire or explosion risk.