Aldehydes, Ketones and Carboxylic Acids

Citric Acid

The chemical formula of citric acid is \(C_{6}H_{8}O_{7}\). It is a weak organic acid. Citric acid is an intermediate in the acid cycle, generally occurring in the metabolism of all aerobic organisms. Aspergillus species produce a large number of metabolites, citric acid is one of the metabolites. Citric acid is a family of carboxylic acids. It is naturally found in various fruits and vegetables. For example, Oranges, tomatoes, and Lemon contain the highest amount of citric acid.

It is useful in consumer goods, food, beverage sectors, and in various industries. Citric acid occurs in two forms; Monohydrate and Anhydrous. In both forms, the chemical name and structure of citric acid remain the same. It is a try basic acid, odourless, appears as a white crystalline solid and sour in taste. In 1784, Carl Wilhelm Scheele isolated this organic acid for the first time.

Citric acid

                                                                                                      Citric acid

Citric Acid Structure

  • Molecular Formula = \(C_{6}H_{8}O_{7}\)
  • IUPAC name = 2-hydroxypropane-1,2,3-tricarboxylic acid
  • The Simplified molecular-input line-entry system=  \(OC\left ( = O \right )CC\left ( O \right )\left ( CC\left ( O \right )=O \right )C\left ( O \right )= O\)

Preparation of Citric Acid

Step 1. 450 ML of lemon juice is required to extract a good amount of crystallized citric acid. Use a pH strip to test the lemon juice. It should be around two or 3 pH scale.

Step 2. Add a little bit of a drop containing 10% strength sodium hydroxide. Then test the solution again.

Step 3. Using a coffee filter transfer the solution into another glass. Double-check for any solid particles in the solution of the new flask.

Step 4. In 70 ML of distilled water put 28 g of calcium chloride.

Step 5. Combine the solutions in one beaker and apply heat to it.

Step 6. Use a filter to extract the calcium citrate. Mix the calcium citrate with heavily diluted sulphuric acid and then stir it.

Step 7.  Before storing the citric acid in a beaker, filter the solution.

Step 8. Evaporate the water by heating the solution on medium heat.

Step 9. Hence filter out the citric acid and let it cool down in a bowl.

Uses of Citric Acid

  1. As a flavouring agent and preservative.
  2. In soft drinks, food, citric acid is useful to make candies due to its sourness.
  3. In the hard water treatment process.
  4. It is also in use in the industrial area for the manufacturing of soap, detergent, electroplating, and leather tanning.
  5. Preservative for storing blood in the pharmaceutical industry.
  6. Citric acid is useful in cosmetics to remove dead skin, it also diminishes wrinkles and improves skin tone.
  7. Variety of beauty care products such as nail clean face chemicals, shampoos, cleanser, body wash.
  8. It has uses as in emulsifier by some ice companies, to refrain the fat globules.
  9. From the evaporator and boilers, the Limescale is removed with the help of citric acid.
  10. In caramel for sucrose crystallization.

Citric acid has sweet and sour sensory notes, whereas succinic has a salty – bitter taste. As the soil and climatic condition changes, the amount of citric acid also varies in vegetables and fruits.

Citric Acid Characteristics

With help of lemon juice isolation of citric acid occurs with crystallization. It exists in monohydrate or an anhydrous form. From cold water, the monohydrate is formed whereas from hot water the anhydrous is formed. It dissolves in absolute ethanol but decomposes with a loss of carbon dioxide.

The metallic cations form complexes with citrate ion, hear chelate effect takes place. The complexes are even formed with alkali metal cations. Formation of Chelate complex along with all three carboxylate groups gives 7 and 8 members of chelate rings. As a consequence, deprotonation of a hydroxyl group and formation of a more stable five-membered ring, as an ammonium ferric citrate, \(\left ( NH_{4} \right )_{5}Fe\left ( C_{6}H_{4}O_{7} \right )_{2}\cdot 2H_{2}O\).

The molecular weight/molar mass of citric acid is 192.124 \(\frac{g}{mole}\), and its density is 1.66 \(\frac{g}{cm^{3}}\). One of the important properties is the density because it defines how much of it can mix in any particular substance. The boiling point of citric acid is \(310^{\circ}C\), and the melting point is \(153^{\circ}C\).

Citric Acid Benefits

It forms many metallic salts which include complexes with copper, iron, magnesium, calcium, and manganese. Because of the salts, their uses are as a sequestering agent in industrial processes and as an anti-coagulant blood preservative. By chelating traces of metals such as iron, it reduces metal-catalyzed oxidation because of its antioxidant properties in fats and oils.

A process to remove sulphur dioxide from flue gases is developed, in which citric acid is used as a scrubber. It forms a complexion that reacts with \(H_{2}S\) to give elemental sulphur regenerating citrate. Citric acid esters such as triethyl, butyl, and acetal tributyl ester, are used in plasticizers and plastic films and monostearyl citrate is used in place of citric acid as an antioxidant in oils and fats.

Citric Acid Cycle reactions

Oxaloacetic acid and Acetyl CoA condensed to form Citric acid. The transfer of acetyl group \(CH_{3}COO\) from CoA to oxaloacetic acid at the ketone carbon takes place. The net effect is to hitch a 4 carbon piece and 2 carbon piece to form acid which is six carbons, called the synthesis of acid.

There are two steps to Isomerise the position of the -OH group on citric acid. To make alkene dehydration of alcohol takes place. For the subsequent steps, the cis-aconitic acid remains sure of the enzyme aconitase. Markovnikov’s rule is not followed by hydration. Furthermore, The net effect of the reaction is to move the – 0H group from C-3 to C-2 and it is isocitric acid. The conversion of alcohol to Ketone occurs, that is the oxidation reaction. It places the – 0H group since it must be adjacent to a carboxylic acid group and forms Malic acid. Then oxidation of alcohol to a key tone, which is the final reaction citric acid cycle takes place to make oxaloacetic acid.

FAQs about Citric Acid

Q1. What are the risks regarding citric acid?

Answer: Under the Food and Drug Administration (FDA), companies produce citric acid with chemical reactions. Currently, no study suggests that artificially consuming citric acid harms health or the body(when consumed in large amounts for extended periods). There are several cases of people falling sick and allergic reactions to the additive. Some reports show joint pain with swelling and stiffness as well as muscular and stomach pain in some people. These people were also suffering shortness of breath. On the contrary, there is no problem in consuming citric acid from fruits and vegetables such as lemons and limes.

Q2. Citric acid is a disinfectant?

Answer: It helps for general disinfection and cleaning because it kills bacteria, mould, and mildew. Lime, rust, calcium deposits, soap scum, hard water strains are also effective at removing. It also serves as a preservative in many cleaning products.

Q3. Is citric acid and ascorbic acid the same?

Answer: Both ascorbic acid and citric acid are types of acid for different purpose of food preservation. They are not identical and set a has it has greater acidity than ascorbic acid. Their chemical structures are scientifically slightly different, which leads to different functionality.

Q4. For the production of citric acid, which microbe is useful?

Answer:  The fungus Aspergillus niger is useful in the production of citric acid. It is also widely used in the production of another organic acid like gluconic acid. In adverse conditions, it causes black mould disease and certain vegetables and fruits.

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