Boric acid

Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula. It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolves in water, and occurs in nature as the mineral sassolite. It is a weak acid that yields various borate anions and salts, and can react with alcohols to form borate esters.
Boric acid is often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other boron compounds.
The term "boric acid" is also used generically for any oxyacid of boron, such as metaboric acid and tetraboric acid.

History

Orthoboric acid was first prepared by Wilhelm Homberg from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi. However, boric acid and borates have been used since the time of the ancient Greeks for cleaning, preserving food, and other uses.

Molecular and crystal structure

The three oxygen atoms form a trigonal planar geometry around the boron. The B-O bond length is 136 pm, and the O-H is 97 pm. The molecular point group is C_3h.
Two crystalline forms of orthoboric acid are known: triclinic with space group P, and trigonal with space group P3₂. The former is the most common; the second, which is a bit more stable thermodynamically, can be obtained with a special preparation method.
The triclinic form of boric acid consists of layers of molecules held together by hydrogen bonds with an O...O separation of 272 pm. The distance between two adjacent layers is 318 pm. While the layers of the triclinic phase are nearly trigonal with,, and , the stacking of the layers is somewhat offset in the triclinic phase, with and. The triclinic phase has and the trigonal one has.


The unit cell of boric acid	hydrogen bonding allows boric acid molecules to form parallel layers in the solid state

Preparation

Boric acid may be prepared by reacting borax with a mineral acid, such as hydrochloric acid:
It is also formed as a byproduct of hydrolysis of boron trihalides and diborane:

Reactions

Pyrolysis

When heated, orthoboric acid undergoes a three-step dehydration. The reported transition temperatures vary substantially from source to source.
When heated above 140 °C, orthoboric acid yields metaboric acid with loss of one water molecule:
Heating metaboric acid above about 180 °C eliminates another water molecule forming tetraboric acid, also called pyroboric acid :
Further heating leads to boron trioxide:

Aqueous solution

When orthoboric acid is dissolved in water, it partially dissociates to give metaboric acid:
The solution is mildly acidic due to the ionization of the acids:
However, Raman spectroscopy of strongly alkaline solutions has shown the presence of ions, leading some to conclude that the acidity is exclusively due to the abstraction of from water:
Equivalently,
Or, more properly,
This reaction occurs in two steps, with the neutral complex aquatrihydroxyboron as an intermediate:
This reaction may be characterized as Lewis acidity of boron toward, rather than as Brønsted acidity. However, some of its behaviour towards some chemical reactions suggest it to be a tribasic acid in the Brønsted-Lowry sense as well.
Boric acid, mixed with borax in the weight ratio of 4:5, is highly soluble in water, though they are not so soluble separately.

Sulfuric acid solution

Boric acid also dissolves in anhydrous sulfuric acid according to the equation:
The product is an extremely strong acid, even stronger than the original sulfuric acid.

Esterification

Boric acid reacts with alcohols to form borate esters, where R is alkyl or aryl. The reaction is typically driven by a dehydrating agent, such as concentrated sulfuric acid:

With vicinal diols

The acidity of boric acid solutions is considerably increased in the presence of cis-vicinal diols such as glycerol and mannitol.
The tetrahydroxyborate anion formed in the dissolution spontaneously reacts with these diols to form relatively stable anion esters containing one or two five-member rings. For example, the reaction with mannitol, whose two middle hydroxyls are in cis orientation, can be written as:
Giving the overall reaction:
The stability of these mannitoborate ester anions shifts the equilibrium to the right, thereby increasing the solution's acidity by five orders of magnitude compared to that of pure boric oxide. This lowers the pK_a from 9 to below 4 for a sufficient concentration of mannitol. The resulting solution is referred to as mannitoboric acid.
The addition of mannitol to an initially neutral solution containing boric acid or simple borates lowers its pH enough for it to be titrated by a strong base such as NaOH, including with an automated potentiometric titrator. This property is used in analytical chemistry to determine the borate content of aqueous solutions, for example to monitor the depletion of boric acid by neutrons in the water of the primary circuit of light-water reactor when the compound is added as a neutron poison during refueling operations.

Toxicology

Based on mammalian median lethal dose rating of 2.66 g/kg body mass, boric acid is only poisonous if taken internally or inhaled in large quantities. The Fourteenth Edition of the Merck Index indicates that the LD₅₀ of boric acid is 5.14 g/kg for oral dosages given to rats, and that 5 to 20 g/kg has produced death in adult humans. For a 70 kg adult, at the lower 5 g/kg limit, 350 g could produce death in humans. For comparison's sake, the LD₅₀ of salt is reported to be 3.75 g/kg in rats according to the Merck Index. According to the Agency for Toxic Substances and Disease Registry, "The minimal lethal dose of ingested boron was reported to be 2–3 g in infants, 5–6 g in children, and 15–20 g in adults. However, a review of 784 human poisonings with boric acid reported no fatalities, with 88% of cases being asymptomatic." Human studies in three borate exposure-rich comparison groups produced no indicators of developmental toxicity in blood and semen tests. The highest estimated exposure was 5 mg B/kg/day, likely due to eating in contaminated workplaces, more than 100 times the average daily exposure.
Long-term exposure to boric acid may be of more concern, causing kidney damage and eventually kidney failure. Although it does not appear to be carcinogenic, studies in dogs have reported testicular atrophy after exposure to 32 mg/ for 90 days. This level, were it applicable to humans at like dose, would equate to a cumulative dose of 202 g over 90 days for a 70 kg adult, not far lower than the above LD₅₀.
According to the CLH report for boric acid published by the Bureau for Chemical Substances Lodz, Poland, boric acid in high doses shows significant developmental toxicity and teratogenicity in rabbit, rat, and mouse fetuses, as well as cardiovascular defects, skeletal variations, and mild kidney lesions. As a consequence in the 30th ATP to EU directive 67/548/EEC of August 2008, the European Commission decided to amend its classification as reprotoxic category 2 and to apply the risk phrases R60 and R61.
At a 2010 European Diagnostics Manufacturing Association Meeting, several new additions to the substance of very high concern candidate list in relation to the Registration, Evaluation, Authorisation and Restriction of Chemicals Regulations 2007 were discussed. Following the registration and review completed as part of REACH, the classification of boric acid CAS 10043-35-3 / 11113-50-1 is listed from 1 December 2010 is H360FD .

Physics

Sound absorption in oceans not only depends on water molecules but also on dissolved salts present in low concentration in seawater. Boric acid and borate in seawater) relaxation contributes to absorbing sounds in the low‐frequency region. At higher frequencies, between 10 and 1000 kHz magnesium sulfate is the main contributor to the absorption of acoustic waves in seawater.

Uses

Industrial

The primary industrial use of boric acid is in the manufacture of monofilament fiberglass, which is usually referred to as textile fiberglass. Textile fiberglass is used to reinforce plastics in applications that range from boats to industrial piping to computer circuit boards.
In the jewelry industry, boric acid is often used in combination with denatured alcohol to reduce surface oxidation and formation of firescale on metals during annealing and soldering operations.
Boric acid is used in the production of glass in LCD flat panel displays.
In electroplating, boric acid is used as part of some proprietary formulas. One known formula uses about a 1 to 10 ratio of to nickel sulfate|, a very small portion of sodium lauryl sulfate and a small portion of sulfuric acid|.
The solution of orthoboric acid and borax in 4:5 ratio is used as a fire retarding agent of wood by impregnation. Also, it is used in combination with other chemicals for the fire retardancy of wood-based materials.
It is also used in the manufacturing of ramming mass, a fine silica-containing powder used for producing induction furnace linings and ceramics.
Boric acid is added to borax for use as welding flux by blacksmiths.
Boric acid, in combination with polyvinyl alcohol or silicone oil, is used to manufacture Silly Putty.
Boric acid is also present in the list of chemical additives used for hydraulic fracturing in the Marcellus Shale in Pennsylvania. It is often used in conjunction with guar gum as cross-linking and gelling agent for controlling the viscosity and the rheology of the fracking fluid injected at high pressure in the well. It is important to control the fluid viscosity for keeping in suspension on long transport distances the grains of the propping agents aimed at maintaining the cracks in the shales sufficiently open to facilitate the gas extraction after the hydraulic pressure is relieved. The rheological properties of borate cross-linked guar gum hydrogel mainly depend on the pH value.
Boric acid is used in some expulsion-type electrical fuses as a de-ionization/extinguishing agent. During an electrical fault in an expulsion-type fuse, a plasma arc is generated by the disintegration and rapid spring-loaded separation of the fusible element, which is typically a specialized metal rod that passes through a compressed mass of boric acid within the fuse assembly. The high-temperature plasma causes the boric acid to rapidly decompose into water vapor and boric anhydride, and in turn, the vaporization products de-ionize the plasma, helping to interrupt the electrical fault.