Citric acid

Citric acid is a special kind of chemical found in nature. It has the formula C₆H₈O₇ and is a clear, weak acid. You can find it in many citrus fruits like oranges and lemons.

In living things, citric acid is very important. It is part of a process called the citric acid cycle, which helps all animals and plants that need oxygen to get energy from food.

People make more than two million tons of citric acid every year. It is used to add a sour taste to foods and drinks, keep them fresh, and help keep other ingredients mixed together.

Citric acid can change into other useful forms called citrates. These include salts like trisodium citrate and compounds like triethyl citrate, which are 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, making up to 8% of their dry weight. The amount of citric acid in citrus fruits can vary, with lemons and limes having more than oranges and grapefruits.

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. In 2018, over 2 million tons of citric acid were made worldwide, with China making more than half of it.

Chemical characteristics

Citric acid can be found as a monohydrate, which means it has water in its structure, but it can also be made without water by heating it. It mixes well with certain liquids and changes when heated to high temperatures.

Citric acid can control how acidic or basic 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 can connect 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 the 1953 Nobel Prize in Physiology or Medicine for discovering this.

Citrate also plays other important roles 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, which is useful when the body already has plenty of building blocks. Citrate is also important in bones, where it helps control the size of crystal-like structures.

Applications

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

It is also used in cosmetics and some medicines, and can help balance the pH in different products. Citric acid has many other uses too, 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 removing water from citric acid, we can get itaconic acid and its anhydride. Citraconic acid can be made by changing the structure of itaconic acid anhydride through 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 listed in important reference books used by scientists and doctors. These include the British Pharmacopoeia and the Japanese Pharmacopoeia. These books help make sure that medicines and other products are safe and work correctly.