Citric acid

Citric acid is a natural chemical found in many fruits, especially oranges and lemons. Its chemical formula is C₆H₈O₇.

In living things, citric acid helps animals and plants get energy from food through a process called the citric acid cycle.

People produce large amounts of citric acid each year. It is used to give foods and drinks a sour taste, help them stay fresh, and keep ingredients mixed together.

Citric acid can change into other useful forms called citrates. These include salts like trisodium citrate and compounds like triethyl citrate, used in many products.

Natural occurrence and industrial production

Citric acid is found in many fruits and vegetables, especially in citrus fruits. Lemons and limes have a lot of it.

Citric acid was first discovered in 1784 by a scientist named Carl Wilhelm Scheele, who got it from lemon juice. Today, most citric acid is made by a special kind of mold called Aspergillus niger. This mold is fed sugar, and it produces citric acid. The acid is then cleaned and used in many products, like drinks, food, cleaning supplies, and cosmetics.

Chemical characteristics

Citric acid can be found with water in its structure, called a monohydrate. It can also be made without water by heating. It mixes well with some liquids and changes when heated to very high temperatures.

Citric acid can control how sour or bitter a solution is, which is important in many natural processes. For example, the taste of citrus fruits like oranges and lemons comes partly from citric acid. It also connects with other substances in ways that are useful in science and industry.

Biochemistry

Citrate is an important part of the citric acid cycle, also called the tricarboxylic acid (TCA) cycle or the Krebs cycle. This cycle is a key way that animals, plants, and bacteria get energy from food. In the cycle, citrate synthase helps combine oxaloacetate with acetyl CoA to make citrate. Citrate then changes into aconitic acid, and the cycle ends by making oxaloacetate again. These reactions give us much of the energy we need, in the form of Adenosine triphosphate (ATP). Hans Adolf Krebs won a prize for discovering this.

Citrate also has other important jobs in our bodies. It can move out of special parts of cells called mitochondria and help make materials needed for building fats. Citrate also helps control a key step in making these fats. When there is a lot of citrate, it can slow down a process that breaks down sugar for energy. Citrate is also important in bones, where it helps control the size of crystal-like structures.

Applications

Citric acid is used to give foods and drinks, like soft drinks and candies, a sour taste. It also helps keep these items fresh longer. In cleaning, citric acid can help remove limescale from kettles and make soaps work better in hard water by helping them create more foam.

It is also used in cosmetics and some medicines to balance the pH in different products. Citric acid has many other uses, like in making dyes for fabrics and in some photographic processes.

Synthesis of other organic compounds

Citric acid can be used to make many other organic compounds. By taking water away from citric acid, we can get itaconic acid and its anhydride. Citraconic acid can be made by changing the structure of itaconic acid anhydride with heat. We get the needed itaconic acid anhydride by heating citric acid without any air.

Aconitic acid is made by taking water away from citric acid using sulfuric acid. Acetonedicarboxylic acid can be made by removing a part of citric acid called a carboxyl group using a special kind of sulfuric acid called fuming sulfuric acid.

Safety

Citric acid is a weak acid, but pure forms can be harmful. Breathing it in might make you cough or hurt your throat. Drinking too much can cause stomach pain. If it gets on your skin or in your eyes, it can make them red and sore. Using it a lot can also wear down the protective coating on your teeth.

Compendial status

Citric acid is in special books used by scientists and doctors. These books are the British Pharmacopoeia and the Japanese Pharmacopoeia. They help make sure medicines and other products are safe and work well.