Acid

An acid (often represented by the generic formula HA [H⁺A⁻]) is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0. That approximates the modern definition of Johannes Nicolaus Brønsted and Martin Lowry, who independently defined an acid as a compound which donates a hydrogen ion (H⁺) to another compound (called a base). Common examples include acetic acid (in vinegar) and sulfuric acid (used in car batteries). Acid/base systems are different from redox reactions in that there is no change in oxidation state.

Citric acid is a weak organic acid, and it is a natural preservative and is also used to add an acidic, or sour, taste to foods and soft drinks. In biochemistry, it is important as an intermediate in the citric acid cycle and therefore occurs in the metabolism of virtually all living things. It can also be used as an environmentally benign cleaning agent and acts as an antioxidant and a lubricant.

Citric acid has been used as an additive to soft drinks, beer, and seltzer, and occurs naturally in many juices. This causes a problem in measurement because the standard measuring technique for sugar is refractive index. The refractive index of sugar and citric acid is almost identical. For soft drinks and orange juice the best measure of sweetness is the sugar/acid ratio. Recently, the use of infrared sensors has allowed measurement of both Brix (sugar content) and acidity by detecting sugars and citric acid through their characteristic molecular vibrations; this gives an accurate assessment of a drink's sweetness.

Sulfuric (or sulphuric) acid, H₂SO₄, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion. Principal uses include lead-acid batteries for cars and other vehicles, ore processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis.