Citric acid has a molecular composition of C6H8O7. This means that one molecule of citric acid has six carbon atoms, eight hydrogen atoms and seven oxygen atoms. The way that these atoms are arranged in the molecular structure gives citric acid its chemical properties. In particular, citric acid has three carboxylic acid (-COOH) groups. The -COOH group is an organic acid group, and it is the presence of three such groups that makes citric acid acidic.
The atoms present in one molecule of citric acid are arranged as shown in Figure 1. The backbone of the structure is the chain of three carbon atoms. Carbon atoms need to form four bonds to stabilize their electron distribution, and it was already stated that one molecule of citric acid has three -COOH groups. Each of these -COOH groups is present, with each C belonging to the backbone. The remaining bond requirements are satisfied by the presence of the hydrogen atoms.
Citric acid can chelate metals. Chelation is the process of one or more molecules of citric acid binding to metal ions and sequestering them in a complex form. By chelating the metal ion from the solution (e.g. hard water), citric acid makes the metal ion inactive. This is why it is used as a water softening agent and also in laundry detergents. Because citric acid is naturally occurring and has the properties of a chelating agent, it can be used as an enviromentally benign cleaning product.
Citric acid plays an important role in metabolism. It participates in the tricarboxylic acid cycle (Krebs cycle) and leads to the eventual oxidation of the carbohydrates and fats into carbon dioxide and water. Citrate is one of the metabolic intermediates in the body after carbohydrates undergo glycolysis. The last step of glycolysis results in the production of pyruvate. Pyruvate enters the tricarboxylic acid cycle, which is a series of 10 reactions to generate more energy (either ATP or NADH+) in the cell.
Citric acid has many uses. It is used in the food industry as an additive to impart sour taste to candies, sodas and other foods. Because of its weakly acidic nature, it is an excellent preservative and is often added to preserve vegetables. It is also used in personal care products such as soaps and shampoos.
It is also used as an additive to laundry detergents and as a water softening agent. Its ability to chelate metals makes it suitable to serve these functions.