Sodium tetrahydroxyborate is a

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

(ionic compound) of with

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

or . It is one of several

sodium borate Sodium borate is a generic name for any salt of sodium with an anion consisting of boron and oxygen, and possibly hydrogen, or any hydrate thereof. It can be seen as a hydrated sodium salt of the appropriate boroxy acid, although the latter may not ...

s. At

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

it is a colorless transparent crystalline solid. The element ratio corresponds to the oxide mixture , but the structure of the solid is quite different from that suggested by this formula.

Structure

Sodium tetrahydroxyborate has been crystallized from aqueous solutions in two

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

forms. Both contain the

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

tetrahydroxyborate Tetrahydroxyborate is an inorganic anion with the chemical formula or . It contributes no colour to tetrahydroxyborate salts. It is found in the mineral hexahydroborite, , originally formulated . It is one of the boron oxoanions, and acts as a ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, which is formed from (ortho)boric acid in water solutions by binding an

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

anion instead of loss of a

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

. These anions lie in layers perpendicular to the (010) plane, and form a tridimensional lattice held together by

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

s between the

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

atoms in each anion and the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

atoms in adjacent anions.

Monoclinic form

The first form, described in 1993, crystallizes in the

monoclinic crystal system In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic sy ...

with

symmetry group In group theory, the symmetry group of a geometric object is the group of all transformations under which the object is invariant, endowed with the group operation of composition. Such a transformation is an invertible mapping of the ambient ...

''P''2₁/''a'' and

parameters A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...

''a'' = 588.6 pm, ''b'' = 1056.6 pm, ''c'' = 614.6 pm, ''β'' = 111.6 °, formulas per cell ''Z'' = 4, cell volume ''V'' = 0.3554 nm³. In the monoclinic form the tetrahydroxyborate anions within each anion layer have alternating orientations, related by central inversion. Three of the oxygen atoms in each anion lie at most 4.2 pm away from a (010) plane, and the fourth is about 270 pm away from that plane. The sodium

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s lie approximately on the same anion layers, 34 pm away from the layer's midplane. Each sodium ion is surrounded by five oxygen atoms in an approximate square

pyramid A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...

al arrangement, at distances of 233 to 239 pm, with a sixth oxygen at 313 pm. These six oxygens belong to four anions.

Orthorhombic form

The second form, described in 2009, crystallizes in the

orthorhombic crystal system In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

with group ''P''2₁2₁2₁ and cell parameters ''a'' = 532.3 pm, ''b'' = 949.6 pm, and ''c'' = 659.6 pm, ''Z'' = 4, volume ''V'' = 0.3334 nm³ In this version, all anions inside each (010) anion layer have one of the four bonds oriented in the same direction, approximately parallel to the (100) axis, opposite to that of the anions in the adjacent layers. Three oxygen atoms of each anion lie at most 4 pm away from their mean (100) plane. The anions form columns parallel to the (100) direction, with each column rotated by 180 degrees about that axis relative to the adjacent ones. The layer spacing is 224 pm. The sodium atoms are located in layers halfway between the anion layers. Each sodium ion is surrounded by five oxygen atoms, in a square pyramid arrangement, at distances between 233 and 239 pm. The oxygen atoms at the base are displaced at most 4 pm from their mean plane, the sodium atom is displaced from it by 30 pm, and the fifth oxygen is 270 pm from it.

Synthesis and properties

The monoclinic form has been crystallized by mixing

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

14.5% (weight), boric acid 9.0%,

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...

10.7%, and water 65.8% and letting it stand for a long time at 293 K. The orthorhombic form has been crystallized, as thin needles, from a solution of sodium hydroxide and boric acid at pH 12 (which meant a boron to sodium mole ratio of 3:2 in the solution), left to evaporate at room temperature. The starting solution for evaporative crystallization can be prepared also from

sodium metaborate Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula . However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is or . The formula can be written also as · to highlight the ...

tetrahydrate . The solid is fairly stable but slowly reacts with

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

from air. The crystallization process has been studied by

Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...

of nanometre-size droplets of solution as the water evaporates in air. Sodium tetrahydroxyborate is end product of the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds appli ...

at 45–65

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

, through the formation of neutral

borane Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula . The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated ...

and then boric acid .

References

Laszlo J. Csetenyi, Fredrik P. Glasser, and R. Alan Howie (1993): "Structure of sodium tetrahydroxyborate". ''Acta Crystallographica Section C'', volume C49, pages 1039-1041. A. E. Lapshin, E. F. Medvedev, I. G. Polyakova & Yu. F. Shepelev (2009): "Synthesis and crystal structure of a new modification of sodium tetrahydroxyborate NaB(OH)4". ''Glass Physics and Chemistry'', volume 35, pages 308–312. Fayan Zhu, Wenqian Zhang, Hongyan Liu, Xiufang Wang, Yongquan Zhou, Chunhui Fang, Yunhong Zhang (2020): "Micro–Raman and density functional theory analyses of ion pairs in concentrated sodium tetrahydroxyborate droplets". ''Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy'', Volume 224, article 117308. André Gonçalves, Pedro Castro, Augusto Q. Novais, Carmen M. Rangel, and Henrique Matos (2008): "Modeling of catalytic hydrogen generation from sodium borohydride". ''Computer Aided Chemical Engineering'', volume 25, pages 757-762. {{Borates Borates Sodium compounds