Ascorbic acid (Vitamin C) is a white or light yellowish crystalline solid chemical compound that can easily dissolve in water to give a mildly acidic solution (pKa = 4.10). However, citric acid is a pure white crystalline (monoclinic crystals) solid, that occurs naturally in citrus fruits. It is a weak organic acid (pKa = 3.10 to 6.4).
Ascorbic acid and citric acid are weak organic acids used as preservatives, buffers, and vitamin supplements. They are found in nature and made synthetically as well.
The chemical formula of ascorbic acid is C6H8O6, while citric acid is C6H8O7. Both acids have the same numbers of carbon and hydrogen but they differ in the numbers of oxygen. There are six oxygen atoms in ascorbic acid and seven in citric acid.
Ascorbic acid vs Citric Acid
| Ascorbic Acid | Citric Acid |
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Chemical formula is C6H8O6 |
Chemical formula is C6H8O7 |
| It is vitamin C | It is not vitamin C |
| It is antioxidant | It is not antioxidant |
| It is not a chelating agent | It is a chelating agent |
| It is not a cleaning agent | It is also a cleaning agent |
| Weak organic acid | Weak tribasic acid |
| It is white or light yellow solid | It is white solid |
| It has two enantiomers L and D | It has two form anhydrous and monohydrate |
| Molar mass is 176.124 g/mol | Molar mass is 192.123 g/mol (anhydrous) |
| Melting point is greater | Melting point is smaller |
| Soluble in ethanol, glycerol, propylene glycol | Soluble in acetone, ether, alcohol, ethyl acetate, DMSO |
| Insoluble in diethyl ether, chloroform, benzene, petroleum ether, oils, and fats | Insoluble in benzene, chloroform, hydrogen sulfide, toluene |
| Use as food additives, dietary supplements, preservation of food, metal stain remover, urinary acidifier, etc | Use in food and drinks, beverages, flavouring, for identification of reducing sugar, etc |
What is Ascorbic acid?
It is an organic compound with the chemical formula C6H8O6. It is also known as hexuronic acid which is a reducing agent. It has two forms of isomers (enantiomers) Levo and Dextro. The L or Levo isomer occurs in nature, especially in foods. This form also exists as vitamin C which is a very essential nutrient for humans and animals. Due to its antioxidant properties, it is used as oral medicine. However, the D or Dextro form has no medical importance.
Chemical Characteristics
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Significance
It is an antioxidant or mild reducing agent. It forms a radical cation when the loss of one electron takes place. The loss of the second electron from it gives the dehydroascorbic acid also known as an oxidized form of vitamin C.
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The acidity of ascorbic acid
It is a vinylogous acid that shows transmission of electronic effect through a conjugated system. When deprotonated on one of the hydroxyl groups, it results in the formation of the ascorbate anion. The ascorbate anion is stabilized by the resonance. Therefore, the ascorbic acid is very reactive toward the compounds that contain only isolated hydroxyl groups.
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Formation of salts
When ascorbate anion reacts with the metals like sodium, calcium, and potassium, they form sodium ascorbate, calcium ascorbate, and potassium ascorbate salts respectively.
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Formation of esters
When ascorbic acid reacts with the alcohols it gives esters, such as ascorbyl palmitate and ascorbyl stearate, etc. These esters are used in anti-aging treatments and sometimes they are considered better than vitamin C (ascorbic acid) itself.
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Formation of ketones
It results in the formation of 1,3 diketones when the nucleophilic attack takes place on a proton in the medium. The beta-diketones have the capability to form complexes with zinc and cobalt.
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Oxidation
On exposure to oxygen, it oxidatively decomposes into various products such as diketogulonic acid, xylonic acid, threonic acid, and oxalic acid.
There are reactive oxygen species or radicals that damage the cells at a molecular level in plants and animals. These species initiate radical chain reactions. This can be terminated by ascorbate through electronic transfer.
Uses of Ascorbic acid
Some uses of Ascorbic acid are listed below:
- The L-ascorbic acid is used as a food additive.
- Levo ascorbic acid is used for dietary supplements.
- Ascorbic acid is a reductant in photographic developing solutions.
- Preservatives for food and beverages are made of ascorbic acid.
- It is added as a catalyst in the polymer industry to assemble the molecular chains more quickly in the plastics.
- It is used to convert heroin into water-soluble salts.
- Ascorbic acid is also used to remove metal stains from swimming pool surfaces, etc.
- Used to increase fluorescent signal and photobleaching in fluorescence-based techniques.
- Biologically, it is used as a urinary acidifier to improve the antiseptic effect of methenamine.
Additional topics
What is Citric acid?
Citric acid is an organic compound that mostly occurs in citrus fruits. Its chemical formula is C6H8O7. It can be colorless or white solid in monoclinic crystalline form. It can be used in chelating agents, flavoring, acidifiers, etc. There are many derivatives of citric acid known as citrates. They can be salts, esters, and polyatomic anions. For example, trisodium citrate (salt), and triethyl citrate (ester).
Chemical Characteristics
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Forms of citric acid
There are two forms of citric acid such as anhydrous (water-free) and monohydrate citric acid. When citric acid crystallizes with hot water, it results in the formation of anhydrous citric acid. However, monohydrate citric acid is formed by crystallizing it in cold water. This form of citric acid can be converted into anhydrous citric acid at 78 oC.
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Acidic salts of citric acid
There are different acidic salts that can be prepared by changing or adjusting the pH before crystallizing the compounds. For example, sodium citrate, calcium citrate, etc.
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Formation of esters
Citric acid gives a variety of esters (mono, di, tri) like triethyl citrate, etc. They are commonly called CAEs (Citric acid esters).
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Complexes of citrate ion
Citrate ions make complexes with metallic cations. Due to the chelating effect, the stability constant for the formation of these complexes is too large. The formation of chelate complexes usually takes place by using all carboxylate groups making chelate rings of seven and eight members. These rings are thermodynamically less stable than smaller chelate rings.
Uses of Citric acid
Some uses of citric acid are listed below:
- Citric acid is used in foods and drinks for flavoring and preservation, etc.
- It is used in ice cream as an emulsifying agent which keeps fats separate from caramel.
- Identification of reducing sugars done by Benedict’s solution uses citric acid.
- It is palleted with sodium bicarbonate to form soluble effervescence, for ingestion.
- It has applications in food coloring for balancing the pH of the dye.
- The solubility of brown heroin is considerably increased in citric acid.
- Citric acid is an excellent chelating agent.
- The acidity of this acid is used for cleaning purposes.
- It has good dissolution properties so, can be used to dissolve rust from steel.
- It is an odorless alternative to white vinegar for dying fabric.
- Ascorbic acid is widely used in cosmetics as an acidulant in creams, and gels because it has alpha hydroxy acid that can be used for exfoliating formulations to help remove dead skin cells. Hence, it reveals fresh, softer, and smoother skin.
Key takeaway(s)
Concepts Berg
What’s the difference between Citric Acid, Ascorbic Acid, and Vitamin C?
Ascorbic acid is also known as vitamin C. Both vitamin C and citric acid are organic compounds. Vitamin C has chemical formula is C6H8O6, while citric acid is a tribasic organic acid with a chemical formula of C6H8O7.
How is ascorbic acid different from citric acid?
Although these two compounds have many similar physical and chemical properties, they have different molecular and structural formulas. Ascorbic acid contains six oxygen atoms while citric acid contains seven oxygen atoms. Ascorbic acid has a solubility in water, great than citric acid.
What can I use instead of ascorbic acid?
Magnesium ascorbate and calcium ascorbate can be used instead of ascorbic acid.
Is ascorbic acid a good vitamin C?
Ascorbic acid is the purest form of vitamin C.
Which form of vitamin C is best?
The Levo form of vitamin C or L-ascorbic acid is its best form because it prevents and treats scurvy and acts as an antioxidant.
What are some common household acids?
Some common household acids are:
- Vinegar
- Lemon juice (citric acid)
- Tartaric acid
- Oxalic acid
- Maleic acid
- Carbonated soda
- Phosphoric acid
- Carbonic acid, etc
What are natural sources of citric acid?
Some natural sources of citric acid are:
- Lemons
- Limes
- Oranges
- Grapes
- Berries, etc
What are the risks of citric acid?
The risks of citric acid are:
- Eye pain
- Teeth problems, etc
What happened if citric acid is ingested?
Ingested citric acid can create the following problems:
- Upset stomach
- Chest pain
- Increases heart rate
- Numbness of hands and feet
- Fatigue, etc
References
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Food Antioxidants: Technological: Toxicological and Health Perspectives By D.L. Madhavi (University of Illinois), S.S. Deshpande (Idetek Inc, California), D.K. Salunkhe (Utah State University)
