List of therms» Citric acid

Citric acid is a weak organic acid found in citrus fruits. It is a good, natural preservative and is also used to add an acidic (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 almost all living things. It also serves as an environmentally benign cleaning agent and acts as an antioxidant. Citric acid exists in a variety of fruits and vegetables, but it is most concentrated in lemons and limes, where it can comprise as much as 8% of the dry weight of the fruit.

The physical properties of citric acid are summarized in the table at right. The acidity of citric acid results from the three carboxyl groups COOH each of which can lose a proton in solution. If this happens, the resulting ion is the citrate ion. Citrates make excellent buffers for controlling the pH of acidic solutions.
In lithography citric acid is used in counteretching process (couteretching solution contains 1 spoon of citric acid disolved in 300 ml of cold water).

Citrate ions form salts called citrates with many metal ions. An important one is calcium citrate or "sour salt", which is commonly used in the preservation and flavoring of food. Additionally, citrates can chelate metal ions, which gives them use as preservatives and water softeners. At room temperature, citric acid is a white crystalline powder. It can exist either in an anhydrous (water-free) form, or as a monohydrate that contains one water molecule for every molecule of citric acid. The anhydrous form crystallizes from hot water, while the monohydrate forms when citric acid is crystallized from cold water. The monohydrate can be converted to the anhydrous form by heating it above 74 °C. Chemically, citric acid shares the properties of other carboxylic acids. When heated above 175°C, it decomposes through the loss of carbon dioxide and water.