SODIUM CITRATE (C6H5O7NA3): STRUCTURE, USES, PROPERTIES - CHEMISTRY - 2025

The sodium citrate is the sodium salt of citric acid. It is made up of a citrate anion C ₆ H ₅ O ₇ ^{3 -} and three sodium Na ⁺ cations. Its molecular formula is C ₆ H ₅ O ₇ Na ₃ or in its expanded form: NaOOC-CH ₂ -C (COONa) (OH) -CH ₂ -COONa. Sodium citrate is the conjugate base of citric acid, that is, it is theoretically obtained from the latter by replacing each H ⁺ proton of –COOH with a sodium Na ⁺.

It can be prepared in anhydrous form (without water in its composition) or in its hydrated form with 2 or 5 molecules of water. It is a compound that when it is anhydrous (without water) tends to absorb water from the environment.

Trisodium citrate molecule. Author: Marilú Stea.

Sodium citrate acts as a buffer, this means that it stabilizes the pH, which is a measure of the acidity or basicity of an aqueous solution. It also has the property of forming a stable compound with calcium Ca ²⁺ ions, calcium citrate.

It is used in the food industry, for example, to prevent cow's milk from forming lumps that are difficult for infants to digest.

In bioanalysis laboratories it is used to prevent the blood that is drawn from patients for certain tests from clotting. It also prevents blood used in transfusions from clotting. It is widely used in phosphate-free detergents because it is not polluting.

Structure

Trisodium citrate is made up of three Na ⁺ sodium ions and one citrate ion.

The citrate ion is made up of a chain of three carbon atoms to which are attached 3 carboxylates –COO ^- (one for each carbon atom) and a hydroxyl –OH at the central carbon.

Each of the 3 Na ⁺ ions is attached to a –COO ^- group.

Structure of trisodium citrate. Jü. Source: Wikimedia Commons.

Nomenclature

- Sodium citrate

- Trisodium citrate

- trisodium 2-hydroxypropane-1,2,3-tricarboxylate

Properties

Physical state

White or colorless crystalline solid.

Molecular weight

258.07 g / mol

Melting point

300 ºC

Solubility

It is very soluble in water: 42.5 g / 100 mL at 25 ºC. It is practically insoluble in alcohol.

pH

A 5% aqueous solution of sodium citrate has a pH between 7.5 and 9.0, that is, it is slightly alkaline.

Chemical properties

Sodium citrate is a buffer, which means that it stabilizes the pH, which is a measure of the acidity or alkalinity of an aqueous solution. By stabilizing the pH, citrate does not allow the solution to become acidic or alkaline above certain values.

When there are excess protons, H ⁺ takes them to convert their –COO ^- groups into –COOH groups. Vice versa, if there are many OH groups ^{- it} gives up the protons of its –COOH groups and converts them into –COO ^-.

When it is transformed by the body, it provides 3 molecules of HCO ₃ ^- for every molecule of citrate.

Other properties

Its crystals are deliquescent, that is, they absorb water from the air. Sodium citrate dihydrate is more stable in air.

If the dihydrate is heated, when it reaches 150ºC it becomes anhydrous (without water).

Like citric acid, sodium citrate tastes sour.

Obtaining

Sodium citrate is usually made by adding sodium carbonate to a citric acid solution until effervescence ceases. Effervescence is due to the evolution of carbon dioxide CO ₂:

Na ₂ CO ₃ + Citric acid → Sodium citrate + CO ₂ ↑

The water is then evaporated from the solution to crystallize the product.

It can also be prepared by treating a sodium sulfate solution with calcium citrate:

Na ₂ SO ₄ + Calcium citrate → Sodium citrate + CaSO ₄ ↓

The solution is filtered to remove solid CaSO ₄ that has precipitated. The solution is then concentrated and thus the sodium citrate crystallizes.

Location in nature

Sodium citrate is the conjugate base of citric acid, which is a natural compound found in all living organisms as it is part of their metabolism, which is the mechanism for developing energy in cells.

Applications

In the food industry

Sodium citrate has various uses in the food industry, for example, as an acidity regulator, sequestrant, stabilizer, surfactant or emulsifier.

It is used in drinks, frozen desserts and special cheeses, among other products.

It allows to avoid the coagulation of milk, so it is added to cow's milk that is used for the feeding of lactating children. In this way, clots or hard curds that are difficult to digest are not created by the stomach of children.

Sodium citrate can be added to milk so that no lumps form in the stomach. Author: Couleur. Source: Pixabay.

It is used to accelerate the color fixation in cured pork or beef, that is, meat that has been treated to prevent spoilage. Up to 50% ascorbic acid is replaced in these.

Cured pork that probably contains sodium citrate. Author: Ben Kerckx. Source: Pixabay.

It also helps prevent blood clotting in fresh beef (such as steaks).

In medicine

It is used as an alkalizing agent, as it neutralizes excess acid in the blood and urine. It is indicated to treat metabolic acidosis that occurs when the body produces too much acid.

Once the citrate ion is ingested, it becomes bicarbonate ion which is a systemic alkalizing agent (that is, of the whole organism) and as a consequence, the H ⁺ ions are neutralized, the pH of the blood increases and acidosis is reversed or eliminated.

Sodium citrate serves as a neutralizing agent for an upset stomach.

It is used as an expectorant and sudorific. It has diuretic action. It increases urinary calcium excretion, which is why it is used in hypercalcemia, which is when there is an excess of calcium in the blood.

It also serves to facilitate lead removal when lead poisoning occurs.

If administered in excess, it can cause alkalosis, muscle spasms due to malfunction of the parathyroid gland and depression of heart function due to a decrease in the level of calcium in the blood, among other symptoms.

Sodium citrate is used in transfusions. It is added to the blood along with dextrose to prevent it from clotting.

Stored blood for transfusions. It probably contains sodium citrate. Author: Kshirl02. Source: Pixabay.

Normally the liver rapidly metabolizes the citrate obtained in the transfusion, however, during transfusions of large quantities, the liver's capacity may be exceeded.

In such cases, since citrate forms a complex with calcium, a decrease in the calcium ion Ca ²⁺ in the blood results. This can lead to lightheadedness, tremors, tingling, etc. Therefore transfusions must be performed slowly.

In the pharmaceutical industry

Among many other uses it is used to prevent the darkening of drugs in which there are iron and tannins.

In blood tests

It is used as an anticoagulant in blood collection or when it must be stored, as it acts as a chelator of calcium Ca ²⁺ ions, that is, it binds to calcium ions, forming calcium citrate which is not ionized.

It is used in coagulation tests and in determining the sedimentation rate of red blood cells.

Blood test. Author: Bokskapet. Source: Pixabay.

It is used as an anticoagulant during plasmapheresis, a procedure to remove excess substances that are harmful to the body from the blood.

In the synthesis of nanoparticles

It is used as a stabilizer in the synthesis of gold nanoparticles. Trisodium citrate dihydrate is added to chloroauric acid, forming a wine-red suspension.

Sodium citrate serves as a reducing agent and as an antiaggregation agent, as it adsorbs onto the nanoparticles.

Due to the negative charge of citrate the particles repel each other avoiding agglomeration and forming a stable dispersion. The higher the citrate concentration, the smaller the particles.

These nanoparticles are used to prepare medical biosensors.

In environmentally friendly detergents

Sodium citrate is widely used in phosphate-free liquid detergents. This is because its environmental behavior is harmless as it is a metabolite found in all living organisms.

It is easily biodegradable and weakly toxic to aquatic life. For this reason it is considered to be a cleaning agent with environmentally friendly characteristics.

In the improvement of the exploitation of tar sands

Oil sands are sandy formations rich in bitumen or tar, a material similar to petroleum.

Sodium citrate has been successfully tested in conjunction with sodium hydroxide NaOH to extract oil from tar sands.

It is thought that by joining the citrate with the sandstone, the –COO ^- ions of the citrate form a large number of negative charges on the silica particles. This produces a better separation of sand from bitumen by simply repellency of the negative charges in gravel from the negative charges in bitumen.

In other applications

It is used in photography, to remove trace metals, in electroplating and to remove SO ₂ from waste gases.

References

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4. Xiang, B. et al. (2019). Study of the Role of Sodium Citrate in Bitumen Liberation. Energy Fuels 2019, 33, 8271-8278. Recovered from pubs.acs.org.
5. Heddle, N. and Webert, KE (2007). Transfusion Medicine. In Blood Banking and Transfusion Medicine (Second Edition). Recovered from sciencedirect.com.
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