Orthoboric acid, H₃BO₃

Orthoboric acid, H₃BO₃, was first prepared by Homberg in 1702. It may be readily prepared in the laboratory by treating a solution of borax (3 pts.) in hot water (12 pts.) with sulphuric acid (1 pt.). On cooling, orthoboric acid crystallises out. Tt is recrystallised from hot water, dried, fused to expel traces of sulphuric acid, and again dissolved in hot water and re- crystallised.

Orthoboric acid is prepared on a commercial scale. Originally, all the orthoboric acid on the European markets came from Italy, and a large quantity of the acid is still derived from that country. It occurs in the suffioni, or jets of steam which issue from volcanic vents near Monte Rotondo, Lago Zolforeo, Sasso, and Larderello, in Tuscany. Many borings have also been made in order to produce artificial suffioni. The suffioni are surrounded by brickwork basins, several of which are generally built on the side of a hill. Water from any convenient source is run into the uppermost basin and subjected to the action of the suffioni within it for a day. It is then run into the next lower basin, and so on, until the water contains about 2 per cent, of boric acid. Next it is made to flow in a thin stream over a large sheet of corrugated lead, 2 metres wide and 125 metres long, placed on a slight incline and heated from below by the vapours from suffioni too poor in boric acid to be utilised for the extraction of the acid. Water may be evaporated in this way at the rate of 20,000 litres per day. The liquid that runs from the end of the lead sheet is further concentrated in leaden pans until the boric acid commences to crystallise out, the gypsum that is invariably deposited during the evaporation being removed from time to time. Crude Tuscan boric acid contains 74 to 80 per cent, of boric acid, 8 to 14 per cent, of ammonium and magnesium sulphates, 4.5 to 7 per cent, of water, together with small quantities of gypsum, clay, sand, sulphur, organic matter, etc.

The origin of the boric acid in the suffioni is not at present understood. It has been conjectured that the boric acid arises from a reaction between boron sulphide and water; on the other hand, it has been supposed to be produced from boron nitride. According to Nasini, its source is the included tourmaline in the surrounding granite rocks, which yields boric acid when heated in superheated steam. The gases issuing from the suffioni are fadio-active, as also is the granitic rock from which they issue.

Boric acid is prepared on a commercial scale from the various naturally occurring borates. For this purpose they are sometimes dissolved in hot hydrochloric acid, and the boric acid, which crystallises out on cooling, recrystallised from water. Numerous other methods have been proposed. In one process, ulexite is ground to an impalpable powder, suspended in boiling water, and decomposed by passing sulphur dioxide into the liquid. In another, boracite is decomposed with the equivalent amount of sodium bisulphate (a by-product of the nitric acid manufacturing process), the boric acid crystallised out, and mother-liquor worked up for the sodium sulphate it contains. The American deposits of colemanite are converted mainly into borax.

Orthoboric acid crystallises from water in white, six-sided laminee which have a pearly lustre and are unctuous to the touch. The crystals are triclinic (a:b:c = 1.7329:1:0.9228, α = 92°30', β = 104°25', γ = 89°49'). According to Carnelley, orthoboric acid melts at 184° to 186°. The density at 15° is given by Stolba as 1.434; Ditte gives the following values: -

Temp. °C.	0°	12°	14°	60°	80°
Density	1.5463	1.5172	1.5128	1.4165	1.3828

The specific heat is 0.3535.

Orthoboric acid is sparingly soluble in cold water, but the solubility rapidly increases with rise of temperature. The percentages of orthoboric acid in its saturated aqueous solutions at various temperatures are as follows: -

Temp. °C	0°	10°	20°	30°	40°	50°	60°	70°	80°	100°
Grams. H3BO3 in 100 grams of solution	2.59	3.39	4.75	6.29	8.02	10.35	12.90	15.76	19.11	28.34

According to Herz and Knoch, a saturated solution of orthoboric acid contains 0.620 gram-molecules of acid per litre at 13°, 0.7915 at 20°, and 0.8999 at 25°. The cryohydric point is –0.76°, at which temperature the saturated solution contains 2.27 per cent, of orthoboric acid. The boiling-point of a saturated solution of orthoboric acid is 103.12°. The heat of solution of orthoboric acid in water is -5.395 Cals. A saturated solution of orthoboric acid has a density of 1.014 at 8°, and 1.0248 at 15° (Stolba).

The solubility of orthoboric acid in water is increased by the presence of potassium or rubidium chloride, but diminished by the presence of hydrogen, lithium, or sodium chlorides. The acid is very slightly soluble in ether, more soluble in alcohols and essential oils. One hundred grams of glycerol dissolve the following amounts of orthoboric acid: -

Temp. ° C	0°	20°	40°	60°	80°	100°
Grams H3BO3	20	28	38	50	61	73

The partition-coefficient of boric acid between water and ether is 34.2 at 16°; between isobutyl alcohol and water 2.74, and between amyl alcohol and water 3.37 at 15°, 3.34 at 25°, and 3.31 at 35°.

Orthoboric acid is volatile in steam. The vapour given off from a saturated boiling aqueous solution of the acid contains 0.039 per cent, of acid. The acid may also be volatilised from its alcoholic solutions.

The salts of boric acid are called borates. Very few orthoborates are known, but numerous meta- and pyro-borates have been prepared. Further, salts of such hypothetical acids as H₄B₂O₅, H₆B₈O₁₅, etc., are known. Anhydrous borates may be prepared by fusing boron sesqui-oxide with metallic oxides. When an excess of boron sesqui-oxide is present, it is found that in some cases a homogeneous liquid mass is obtained which solidifies to a homogeneous glass; in other cases it separates on cooling into conjugate liquid phases; while in others it is not possible to obtain a homogeneous liquid melt, the mass separating into two non-miscible phases of metallic borate and boron sesqui-oxide respectively.

The borates, with the exception of the alkali borates, are practically insoluble in water. The most important borate is sodium pyroborate or borax. The important borates are described under the headings of the various metals.

Although the metallic orthoborates cannot be obtained by precipitation, and have only in a few cases been prepared in other ways, the tri-alkyl esters of boric acid, B(OR)₃, where R is an alkyl group C_nH_2n+1, are readily prepared by the interaction of boron trichloride and alcohols, e.g.: -

BCl₃ + 3C₂H₅.OH = B(OC₂H₅)₃ + 3HCl.

They are colourless liquids of low boiling-point and normal vapour density, and are readily hydrolysed by water. They combine with the metallic derivatives of the alcohols, forming crystalline compounds in which boron is presumably a pentad. The following are known: -

LiB(OMe)₄; KB(OMe)₄; NaB(OEt)₄; TlB(OEt)₄; NaB(OMe)₄; Ca[B(OMe)₄]₂; KB(OEt)₄; NaB(OPrα)₄

In aqueous solution, orthoboric acid is a very weak acid. It turns litmus a wine-red, turmeric a reddish-brown, and has no effect on methylorange. The aqueous solution is a very poor conductor of electricity, so that the acid is ionised only to a very slight extent. The molecular weight of the acid in solution is, in fact, that corresponding to the molecular formula H₃BO₃. That there is only one molecular species present to any appreciable extent is shown by the fact that the partition-coefficients are independent of the actual concentration of boric acid in the aqueous phase. In solution, orthoboric acid behaves as a monobasic acid. The ionisation of the acid follows Ostwald's dilution law for binary electrolytes, and the dissociation must therefore be represented as H₃BO₃ ⇔ H^• + H₂BO₃'. The ionic mobility of the anion at 18° is 28. The affinity constant (k) of orthoboric acid varies with the temperature as follows: -

Temp. °C.	15°	25°	37°	40°
k×1010	5.48	6.62	8.10	8.49

Hence boric acid is weaker than either carbonic or hydrosulphuric acid. Owing, however, to the superior volatility of these acids, a concentrated, boiling solution of boric acid can decompose certain metallic carbonates and sulphides.

Both metaboric acid and pyroboric acid (orthoboric acid dehydrated till it corresponds to the formula, H₂B₄O₇), when dissolved in water, are converted into orthoboric acid, since their aqueous solutions are identical with solutions of orthoboric acid of the same boron content. Moreover, aqueous solutions of alkali meta- and pyro-borates are in every respect identical with aqueous solutions prepared from the requisite quantities of orthoboric acid and alkali hydroxide. In conformity with the view already outlined that boric acid in solution is a monobasic acid, it reacts in aqueous solution with one equivalent of alkali. This has been shown both by freezing-point measurements and by thermochemical considerations. Accordingly, the ions in alkali borate solutions are essentially M and H₂BO₃' or BO₂', and such solutions, by double decomposition with metallic salts, precipitate sparingly soluble metaborates. There must, however, be an appreciable concentration of hydroxyl ions in an alkali borate solution, since, boric acid being exceedingly weak and the alkali hydroxides very strong, the salts produced by their interaction must be perceptibly hydrolysed. At ordinary temperatures, in fact, a decinormal solution of borax is hydrolysed to the extent of about 0.5 per cent. Accordingly, in sufficiently dilute solutions, precipitated metallic metaborates are contaminated with co-precipitated metallic hydroxides (or oxides) if the hydroxides are very sparingly soluble substances. Alkali borate solutions also contain complex polyborate ions, the precise nature of which is not known.

Boric acid and its most important salt, borax, receive many practical applications. As a mild antiseptic, the acid is largely used as a food preservative. It is also used in the preparation of candle wicks. Boric acid and borax are employed in the preparation of enamels, pottery glazes, hat varnishes, paint driers, borosilicate glass, certain kinds of optical glass, cosmetics, tooth powders, soaps, parchment paper, glazed paper and cards, safe linings, and fireproof textile fabrics; they are also used by tanners for dressing leather, by coppersmiths and jewellers as fluxes in brazing and soldering operations, by laundresses, etc.