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Gluconic acid: structure, properties, synthesis, uses

Gluconic acid is a non-toxic compound that is present in honey, wine, fruits, etc. It is the acidic version of glucose. It is very soluble in water and poorly soluble, in general, in organic solvents.

Gluconic acid was discovered by Hlasiwetz and Habermann, in 1870, through the oxidation of glucose with chlorine. Decades later, in 1922, Molliard determined the presence of gluconic acid in a culture medium for the fungus Sterigmatocystis nigra, now known as A. niger.

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Structure of gluconic acid

Different representations for the gluconic acid molecule. Source: Yikrazuul / Public domain.

In the first image the structural formula for the open chain of gluconic acid was shown. But like glucose, the molecule of this acid closes to form a pyranous ring, as represented in the image above by various models or projections. From left to right: Tollens, Haworth, chair, and absolute stereochemical projection.

Note how much it resembles glucose, with the only difference that the –CH 2 OH end has been oxidized to –COOH; that is, it has ceased to be an alcohol to become a carboxylic acid . This extreme is the one that donates the H + ion and therefore defines the acidity of gluconic acid.

Properties

Chemical formula

6 H 12 O 7

Molar mass

196.155 g / mol

Physical appearance

Colorless or slightly yellow crystals. In the same way, when it dissolves, it generates a colorless liquid with yellowish colorations, whose consistency is that of a syrup.

Flavor

Slightly acidic.

Melting point

131 ° C. Above this temperature it begins to decompose to release carbon dioxide, which is why its boiling point is not conclusively reported.

Water solubility

316 g / L at 25 ºC

Solubility in other solvents

Slightly soluble in alcohol, but insoluble in ether and many organic solvents.

Density

1.23 mg / mL

Octanol / water partition coefficient

Log P = – 1.87 (estimated)

Dissociation constant

Ka = 2.5 · 10 -4  at 25 ºC

Acidity (pKa)

3.6 at 25 ºC

Refractive index 

1.4161

Synthesis of gluconic acid

The methods of synthesis or production of gluconic acid can be classified into chemical methods and biological methods.

Chemical methods

Among these methods we have the following:

-Chemical oxidation of glucose by a hypochlorite solution.

-Hydrolysis of α-D-glucose with a mixture of bromide and sulfuric acid.

-Irradiation of D-glucose with gamma radiation.

The Cannizaro reaction applied to glucose, under alkaline conditions, gives rise to the compounds sorbitol and gluconic acid. Likewise, gluconic acid can be synthesized by directly oxidizing glucose in the presence of the palladium catalyst.

The fermentation of glucose is used for the synthesis of gluconic acid, carried out either by filamentous fungi, such as A. niger, or by bacteria of various genera, including Pseudomonas and Zymomonas.

The presence of gluconic acid in a culture medium of A. niger, from Molliard in 1922, led us to think that the fermentation of glucose by the action of the fungus A. niger could be a convenient method for the production of this sweet acid. . In fact, this method is still used today.

The cultivation conditions of A. niger for glucose fermentation include: a high concentration of dissolved oxygen, and a pH between 5.5 and 6.5. This pH is maintained in this range by adding calcium carbonate as a neutralizing agent.

The glucose concentration in the culture is high (110-250 g / L), while the nitrogen and phosphorous concentration is low (<20 mmol / L). Also, a low concentration of metal ions is required. Following these conditions, glucose fermentation is 90% complete in less than 24 hours.

Applications

Doctors and Veterinarians

Gluconic acid is an essential reagent for obtaining calcium gluconate. This salt is used in the treatment of patients with hypocalcemia. Also, in the form of a gel, it is used in the treatment of burns caused by hydrofluoric acid.

On the other hand, quinine gluconate is used in the treatment of malaria. Likewise, iron gluconate has been used in the treatment of iron deficiency anemia.

Finally, gluconic acid is a base reagent for the formulation of many gluconate salt drugs.

Cleaning

Gluconic acid and its salts have a chelating action, so they can trap metals, such as iron, copper, aluminum, lanthanum, among others. This property allows its use in industrial and domestic cleaners.

Calcium gluconate, due to its property of forming complexes with different metals, is used to remove metallic oxides from metallic surfaces, or to remove lacquers and paints from objects. Gluconic acid and its salts are also used in bottle rinse formulations.

Sodium gluconate is used as a metal trap in many detergents. Gluconic acid, and several of its salts, are used in the tanning and textile industries, especially with regard to the cleaning of their products, being used especially in the removal of metal deposits.

Food

Gluconic acid is used as a companion to yeast, in cake mixes, and as an acid component in dry mix desserts. Glucono-δ-lactone is used in the food industry as a mild acidulant.

Gluconic acid prevents cloudiness in beverages and the formation of lumps in beverages due to the presence of calcium compounds. It improves the taste of foods, giving them a bittersweet taste, while also helping to control the bitterness present in some of them.

Other uses

A gluconate acts as a retarder in the concrete setting process, allowing the necessary time for a correct pouring of the concrete, thus avoiding the production of high temperatures that can cause the formation of cracks.

And finally, gluconic acid and some of its salts are used as components of some cell culture media.

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