BORON, one of the non-metallic elements, occurring in nature in the form of boracic (boric) acid, and in various borates such as borax, tincal, and boracite (symbol B, atomic number 5, atomic weight 10.8 2, isotopes i o and r r) . It was isolated by J. L. Gay Lussac and L. J. Thenard in 1808 by heating boron tri oxide with potassium in an iron tube. It may also be obtained by reducing the chloride by sodium or hydrogen, or the boro fluoride by potassium ; or boron trioxide may more conveniently be reduced by aluminium, in which case a large excess of the oxide must be used to minimise formation of a compound of aluminium and boron. The product is extracted by hydrochloric acid, and the remaining impurities are volatilised in a vacuum at 1,200°C.; the boron can then be melted at a still higher tempera ture in a current of hydrogen. The resulting metallic boron is a conductor of electricity and has a density of 2.5 ; the amorphous variety is a chestnut-coloured powder, density 2.45. Boron is totally unaffected by air at ordinary temperatures, but at high temperatures it burns to the nitride, BN, and the oxide, It combines directly with fluorine at ordinary temperatures, and with chlorine, bromine, and sulphur on heating. It does not react with the alkali metals, but combines with magnesium at a low red heat to give the boride, (see below). It reduces many metallic oxides, such as those of lead and copper, and decomposes water at a red heat. Hot sulphuric or nitric acid oxidises it to boric acid, and on fusion with alkali carbonates or hydroxides it gives the corresponding borate. As in the cases of silicon and carbon, its specific heat increases rapidly with increasing temperature. Boron dissolves in molten aluminium, and on cooling, transparent, colour less crystals are obtained with a lustre, hardness, and refractivity approaching that of the diamond. Its carbide, is equally hard (H. Moissan).
Many of the compounds of boron are of great interest in con nection with the theory of valency (q.v.), and they show a striking similarity to the corresponding compounds of silicon, in spite of the different valencies.
For many years the hydrides of boron baffled investigators, but they are now known as well-defined individuals. When magnesium boride is treated with hydrogen chloride, an evil-smelling gas is obtained which contains hydrogen and what was regarded as a hydride; it burns with a green flame to the oxide, and deposits elementary boron if passed through a hot tube. Sir W. Ramsay and H. S. Hatfield cooled the gas in liquid air and separated hy drides of supposed composition and it was left to A. Stock and his collaborators, however, in a series of brilliant re searches, to demonstrate that this gas is a mixture of hydrides, the chief of which is borobutane or tetraborane, They have now isolated a whole series of "hydroborons" analogous to the hydrocarbons. Boroethane or diborane, is the simplest, for appears to be incapable of existence ; it is formed from the above by keeping it over mercury at the ordinary tempera ture, by exposing it to ultra-violet light, or by heating it for some hours at ioo°C. It is a colourless, malodorous gas which condenses at —88°C. and freezes at —169'C.: it is much more stable and less inflammable than and it is decomposed quantitatively by water : Also it reacts with hydrogen chloride or iodide to give chloro- or iodo diborane. Tetraborane is synthesised from iododiborane and sodium: 2B in just the same way as butane from ethyl iodide (see CHEMISTRY : Organic) ; it condenses at r 6 ° C. and freezes at —112°C.; it reacts with water or caustic soda to give hydrogen and boric acid or sodium borate. Penta borane, dihydropentaborane, B ; hexaborane, and decaborane, are also known. The last is a frequent decom position product of other hydroborons, and is produced by the spontaneous decomposition of dihydropentaborane ; it melts at r oo ° C. Hexaborane boils at about r oo ° C. under atmospheric pressure. A more systematic nomenclature for hydroborons was suggested by A. Stock and E. Pohland (Berichte, lix., 1926).
These hydroborons readily give rise to a number of compounds with ammonia, either by direct addition or by decomposition. Thus diborane gives 2NH3, or, more probably, tetraborane gives the same compound together with the ammine of dehydrodiborane : NH3+B2H4 (NH3),; pentaborane yields a tetrammine, ; and this on treat ment with hydrogen chloride gives first tetrachloropentaborane tetrammine and then heptachloropentaborane tetrammine : 3HC1 = Excess of ammonia at 200°C. with di borane gives boroimide, B2 but if the ammine (NH3)2 is heated, there is insufficient ammonia to replace all the hydrogen atoms by imino-groups and a relatively stable compound, is obtained, probably BH NH•BH NH it forms a -NH•BH/ Boron fluoride was first prepared by Gay Lussac and Thenard, and is best prepared by heating a mixture of the oxide and fluor spar with concentrated sulphuric acid. It is a colourless, pungent, fuming gas which does not attack glass but is decomposed by sodium or potassium (see preparation of boron). It neither burns nor supports combustion, but reacts readily with water to give hydrofluoboric (hydroborofluoric) acid : this acid cannot be isolated in the free state, but many of its salts are known as borofluorides or fluoborates. Boron fluoride also reacts with gaseous ammonia to give a white crystalline solid, with excess of ammonia, colourless liquids, and are formed, but they lose ammonia on heating and are converted into the solid Boron trichloride is prepared by heating the element in a stream of chlorine, or better, by passing chlorine over a heated mixture of the oxide with charcoal, the volatile chloride being condensed in a freezing mixture. It is a colourless, fuming liquid boiling at 18°C., and readily reacting with water to give boric and hydro chloric acids ; it also unites with ammonia to give first the white crystalline compound, and then, with excess, bora mide, The bromide may be prepared by the same methods as the chloride and has similar properties : it boils at 91°C. The iodide cannot be formed by direct union of the ele ments, but by the action of gaseous hydrogen iodide upon the amorphous element ; it is also formed by the action of iodine on diborane. It is a white crystalline solid melting at 48°C. and boil ing at 210°C. With a solution of phosphorus in carbon disul phide it gives a red powder, PBI,, which may be sublimed in a vacuum at 200°C., condensing as red crystals, and when heated in a current of hydrogen yields the phosphide PB.
Boron nitride is formed when boron burns in air or nitrogen, or better by heating anhydrous borax with twice its weight of ammonium chloride in a platinum crucible. It is a white powder, insoluble in water and unaffected by most reagents, but dissolves slowly in hydrofluoric acid, and is slowly converted into the oxide and boric acid or a borate by the action of steam or caustic alkali solution.
Boron sulphide, can be obtained by the direct union of the elements at a white heat or from the iodide and boiling sul phur, but is better prepared by heating a mixture of the oxide and charcoal in a current of carbon disulphide vapour, or by heating the oxide with aluminium sulphide in an atmosphere of nitrogen. The almost colourless crystals have a disagreeable smell and are rapidly decomposed by water to hydrogen sulphide and boric acid. It combines with ammonia to give which decomposes on gentle heating to give the imide or on stronger heat ing to the nitride. A pentasulphide, is also known.
Boron forms only one oxide which is best prepared by strongly igniting the acid; it rapidly absorbs water to regenerate boric acid. It is not volatile below a white heat and is therefore able to displace sulphur trioxide when it is heated with alkali sulphates, forming the corresponding borate. It also forms compounds with most metallic oxides, and as many of these possess characteristic colours, they are utilised in the "borax bead" test in analysis (see CHEMISTRY : Analytical). It dissolves in fairly concentrated sulphuric and phosphoric acids to give sulphates 4H20 and 3H.,0, according to M. Levi and L. F. Gil bert) and a phosphate, ; all these are very difficult to crys tallise and very hygroscopic. Borax and boric acid are described under separate headings, and the perborates are included with the former.
Many organic compounds of boron are known; thus, ethyl and methyl borates, and are produced from the appropriate alcohol and either boron trichloride or sulphuric acid and a borate; they are colourless liquids of boiling points 119°C. and 72°C., respectively and are readily decomposed by water. The latter is utilised in the analysis of boron compounds (see below). Ethyl borate and zinc ethyl or methyl react to give boron triethyl or trimethyl, B and B These both com bine with ammonia to give, e.g., B compare Mixed alkylen, alkyloxy- and alkylen-hydroxy-compounds are known, and also compounds with acetylacetone : [ (C5H;02),B] Cl, which ionises, and which is non-ionic (G. T. Morgan and R. B. Tunstall). The later are of interest in that they show boron to have a co-ordination number of four. Phenyl boric acid, C,H,B (OH) 2, is a powerful antiseptic.
Atomic weight determinations based on borax are untrust worthy since this suffers slight loss of sodium oxide on heating (see BORAX) ; determinations based on the boron trichloride are therefore preferred, and these give values of 10.82-10•84 (H. V. A. Briscoe and P. L. Robinson).
Soluble borates give a white precipitate with silver nitrate, and from their concentrated solutions sulphuric or hydrochloric acid precipitates boric acid. The acid gives a characteristic brown colour with turmeric paper, becoming bluish-black on addition of alkali. Borates heated with concentrated sulphuric acid and methyl alcohol give methyl borate (above), the vapour of which burns with a green flame. If this vapour is passed over heated lime, it is retained as and this method may be used quan titatively. Borates may also be estimated as potassium boro fluoride, which is insoluble in an alcoholic solution of potassium acetate ; if substances other than boric acid or potassium borate are present in solution, the boron must be separated by means of the methyl ester, which is absorbed in potash. Direct alkalimetric titration of boric acid is impossible owing to its extreme weak ness as an acid, but if excess of glycerol or mannitol is added, and the solution is made just neutral to methyl-orange, the com plex acid formed can be titrated as a monobasic acid, phenol phthalein being used as an indicator. (A. D. M.)