BORAX: HISTORY, STRUCTURE, PROPERTIES, USES, SYNTHESIS, RISKS - CHEMISTRY - 2025

The borax or sodium borate is a designation for a group of chemical compounds with a base formula of Na ₂ B ₄ O ₇. It is a hygroscopic salt capable of forming numerous hydrates, in addition to being structurally enriched by the presence of certain metallic impurities.

The main compounds that receive the name of borax are the following: anhydrous sodium tetraborate, Na ₂ B ₄ O ₇; sodium tetraborate pentahydrate (Na ₂ B ₄ O ₇).5H ₂ O; and sodium tetraborate decahydrate, Na ₂ B ₄ O ₇.10H ₂ O, or its equivalent, sodium tetraborate octahydrate, Na ₂ B ₄ O ₅ (OH) ₄.8H ₂ O.

Borax fragment. Source: Leon Hupperichs

As can be appreciated, all of the above solids differ only in their degree of hydration; some have more water molecules in their crystals than others. Physically, borax looks like sturdy pieces or crystals of chalk (top image).

Borax is naturally found in seasonal lake sediments, dried out by evaporation. Borax is produced in Turkey, Searles Lakes (California), in the Atacama Desert, Chile, Bolivia, Tibet, and Romania. Borax crystals can be obtained synthetically by a slow nucleation process; that is, one in which the growth of crystals is favored.

Sodium tetraborate, anhydrous and decahydrated, are poorly soluble in cold water; but its solubility increases with an increase in temperature. Na ₂ B ₄ O ₇.10H ₂ O is very soluble in ethylene glycol, moderately soluble in ethanol and slightly soluble in acetone; these are less polar solvents than water.

Borax has numerous applications, such as in the manufacture of detergents and soaps; as a fire retardant for cellulosic material; in the production of fiberglass; in cleaning hard surfaces such as metals, glass and ceramics; and in the inhibition of corrosion, among other uses.

Borax is irritating to the eyes, skin, and respiratory tract. By ingestion it can cause, among other symptoms, vomiting, diarrhea, flaccidity, decay and seizures. It is currently suspected that it may have harmful effects on human reproduction.

History

The history of borax is confusing. It is noted that it was discovered in the bed of a lake in Tibet, dried up by evaporation. It was transferred in the 8th century from Tibet to Saudi Arabia, following the so-called Silk Road.

The term "borax" became popular as a result of the numerous applications that were found for it and marketed under the name 20 Mule Team Borax Tradermax, which referred to the way in which borax was transported through the deserts of Nevada and California.

Another version of its discovery indicates that the Egyptians knew of its existence for a long time, using it in the welding of metals by heat. In addition, they used it in medicine and in the mummification process. Borax was transferred to Europe in the 13th century by Marco Polo.

In the mid-19th century, large deposits of borax were discovered in the Mogave Desert, which has made its use common ever since.

Borax structure

Structure of borax ions. Source: Smokefoot

The upper image shows the ions that make up borax, specifically corresponding to a solid with a compositional formula of Na ₂ · 8H ₂ O (or Na ₂ B ₄ O ₇ · 10H ₂ O).

First, the caged structure of the anion ^2- can be noted (although it should have two deprotonated -OH groups, -O ^-), in which an octagonal ring (four atoms of B and four of O) can be seen, with a BOB bridge dividing it in two halves.

Said cage does not remain static, but vibrates and adopts different shapes in space; however, to a lesser degree in the vicinity of the bridge. Also, it can be noticed that it is very "oxygenated"; that is, it has many oxygen atoms, capable of interacting with water molecules through hydrogen bonds.

It is not surprising then that borax crystals can hydrate and generate numerous hydrates. Thus, for the solid Na ₂ · 8H ₂ O, there are eight H ₂ O molecules interacting with the borate cage, and at the same time, with the metal centers of Na ⁺.

On the cation side, we have the aqueous complex ⁺ _n, where the value of n will depend on the anion / cation ratio. In the upper solid, n = 2, so we have: ²⁺, whose 2+ charge neutralizes the 2- charge of the borate anion.

Crystals

Monoclinic unit cell of borax. Source: Ben Mills.

Above is the monoclinic unit cell for Na ₂ · 8H ₂ O or Na ₂ B ₄ O ₇ · 10H ₂ O crystals. Here electrostatic attractions and hydrogen bonds govern or dominate the structural patterns originated in each span of the crystal.

The other hydrates and the anhydrous salt lose one or all of their water molecules, altering the composition and structure of the borax crystal. In all of them, the anion and cation remain the same (unless there are impurities or side reactions), so there can be no confusion in this regard.

Confusions with the borate anion

Na ₂ · 8H ₂ O and Na ₂ B ₄ O ₇ · 10H ₂ O are the same. Why?

In the first place, if its atoms are counted it will be seen that they coincide in number; and secondly, in the anion B ₄ O ₇ ^2- the absence of the two OH groups linked to the boron heads of the BOB bridges is considered; two OHs that, together with the two H ⁺ from B ₄ O ₇ ^2-, add up to 2 H ₂ O, giving a total of 10 H ₂ O (8 + 2).

Properties

Names

- Borax.

- Sodium borate.

- Sodium tetraborate.

- Disodium tetraborate.

Molecular mass

Anhydrous: 201.22 g / mol.

Decahydrate: 381.38 g / mol.

Physical appearance

A white crystalline solid.

Density

Anhydrous: 2.4 g / cm ³

Decahydrate: 1,739 g / cm ³

Melting point

Anhydrous: 743 ° C (1,369 ° F, 1,016 K).

Decahydrate: 75 ºC.

Boiling point

Anhydrous: 1,575 ° C (2,867 ° F, 1,848 K).

Water solubility

31.78 g / L (for both anhydrous and decahydrate).

Solubility in organic solvents

It is very soluble in ethylene glycol, moderately soluble in diethylene glycol and methanol, and slightly soluble in acetone.

Refractive index (ηD)

Anhydrous: η ₁ = 1.447; η ₂ = 1.469.

Decahydrate: η ₂ = 1.472.

Stability

It is stable under proper storage conditions. Although it has a low vapor pressure, it increases with temperature, leading to crystallization and condensation, so excessive fluctuations in temperature and humidity should be avoided.

Light emission

Sodium tetraborate, when subjected to the action of fire, emits a green light.

Reactions

- When dissolved in water, it creates alkaline solutions with a pH of around 9.2.

- Borax reacts to form other borates, such as perborate (PBS).

- It also produces boric acid by reacting with hydrochloric acid:

Na ₂ B ₄ O ₇ · 10H ₂ O + HCl => 4 H ₃ BO ₃ + 2 Na ⁺ + 2 Cl ^- + 5 H ₂ O

- Sodium tetraborate behaves like an amphoteric substance, since in aqueous solution it hydrolyzes and is capable of neutralizing acids. In the same way, it is capable of neutralizing high concentration alkalis, avoiding an excessive rise in the pH of the medium.

Applications

Borax has numerous applications, both in humans and in all the activities that they carry out.

Antioxidant action

Borax has the ability to dissolve oxides. For this reason it is used in the recovery of metals such as brass, copper, lead and zinc.

It is also used to prevent the oxidation of metal bars in foundries. Borax covers its surface, excluding air, and therefore, preventing oxidation. In addition, it prevents the corrosion of ferrous material in automobile manufacturing.

The solubility of borax in ethylene glycol has been useful in antifreeze formulations. Borax neutralizes the acidic residues that are produced during the decomposition of ethylene glycol, minimizing the oxidation that the surface of metals can undergo.

Manufacture of adhesives

- Borax is part of a starch adhesive formulation for corrugated paper and cardboard.

- It is a peptizing agent in the manufacture of adhesives based on casein and dextrin.

- It works by strengthening the adhesive materials through the crosslinking of the conjugated hydroxyl groups.

Fire retardant

It is used as a fire retardant in cellulosic materials and other polymers that contain hydroxyl groups. It exerts its action through various mechanisms, including the formation of a vitreous layer that limits the access of fuels.

Manufacture of borosilicate glass (Pyrex)

Borax is a source of boron used in the manufacture of borosilicate glass; characterized by a high melting point, low coefficient of expansion and high resistance to thermal shocks.

This allows it to be subjected to high temperatures without experiencing fractures, receiving the name of refractory glass.

Borosilicate glass is used in the manufacture of materials to be used in the home to bake food. It is also used in laboratories in beakers, flasks, measuring cylinders, pipettes, etc.

Emulsifier

Borax, along with other compounds, is used to emulsify soaps and other paraffins. It is also used as a base for the production of lotions, creams and ointments.

Cleaning supplies

- It is added in formulations to clean firm surfaces such as metal, glass and ceramic.

- It is used in hand cleaners, polishing material and detergent soaps for use in laundries and at home. In addition, it facilitates the removal of oily deposits from factory floors.

Medical uses

- It is used in the treatment of fungal foot infections.

- It is part of the formula of nutrients that supply boron. This element is thought to be involved in the metabolism of calcium, phosphorus and magnesium, and may be necessary for bone structure.

- It is involved in the determination of glycosylated hemoglobin (HbA1C), a test used to determine the long-term development of diabetes in a patient, since the half-life of an erythrocyte is 120 days.

This information is more useful to the clinician than individual blood glucose measurements from the patient.

PH buffer

It is used in various pH buffer systems used in biochemistry; among them, Tris (hydroxymethyl) aminomethane-borate, used in polyacrylamide electrophoresis of DNA and RNA nucleic acids.

Decreased hardness of water

Borax is capable of combining with the calcium and magnesium ions present in water, reducing their concentration. This reduces the hardness of the water.

Fertilizer and compost

Borax is used as a source of boron. Calcium and boron are involved in the formation of the plant cell wall. In addition, it has been pointed out that boron favors pollination, which is why it intervenes in the fertilization and proliferation of plants.

Wood preservation

Borax fights woodworm, a beetle that consumes and destroys wood. It is also used in the conservation and maintenance of wooden boats.

Flux and flux

It is used as a flux in the welding of steel and iron, causing a decrease in the melting point and the elimination of iron oxide; an undesirable contaminant. For a similar purpose, he participates in the welding of gold and silver.

Others

- It is used as a mild pesticide; for example, to combat moths on wool.

- Spraying is used to control pests present in cabinets, internal tanks, in the gaps in the walls, and in general, where the use of more polluting pesticides is not desired.

- It is used in nuclear reactors to control reactions that could produce, in extreme cases, a chain reaction.

- In car radiators, borax is used to block places where there are water leaks.

Synthesis

Borax is obtained by reacting the mineral ulexite, composed of the elements boron, sodium and calcium, with the formula NaCaB ₅ O ₉.8H ₂ O, with sodium carbonate and sodium hydrocarbonate in aqueous medium.

In fact, the synthesis of sodium tetraborate or borax is little used; Most of the compound produced is obtained by its extraction of the minerals that contain it.

The process can be summarized in a dissolution of the ore in water, followed by a filtration in clay and finally an evaporation of the aqueous solution.

Risks

Borax is a natural product, however, it can cause a series of damages to human health. For example, contact with borax powder can cause skin irritation that could lead to dermatitis.

Similarly, borax causes irritation to the eyes and upper respiratory tract. Symptoms of its inhalation include cough and sore throat.

Meanwhile, the intake of borax presents the following symptoms: vomiting, nausea, diarrhea, flaccidity, headache and seizures. In addition, the central nervous system and kidney function may be affected.

From studies carried out on male rats, fed with borax, testicular atrophy was observed in them. While the studies carried out with pregnant female rats indicated that borax could cross the placenta, producing an alteration of fetal development, manifested by a low birth weight.

Exposure to borax is believed to affect male reproductive function in men, manifested by a reduction in sperm count.

A fatal dose of 10 to 25 grams is estimated for borax intake in adults.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. National Center for Biotechnology Information. (2019). Argon. PubChem Database. CID = 23968. Recovered from: pubchem.ncbi.nlm.nih.gov
3. Karolyn Burns, Seneca Joseph, & Dr. Ryan Baumbach. (sf). Synthesis and Properties of Borax Crystals and Intermetallic Alloys. Recovered from: nationalmaglab.org
4. Wikipedia. (2019). Borax. Recovered from: en.wikipedia.org
5. Rio Tinto. (2019). Borax decahydrate. 20 Mule Team Borax. Recovered from: borax.com
6. Fletcher Jenna. (January 14, 2019). Is borax safe to use? Recovered from: medicalnewstoday.com