Metaboric acid

Molten boric oxide reacts readily with water vapor above 1000°C to form metaboric acid in the vapor state. 

The name boric acid is usually associated with orthoboric acid, which is the only commercially important form of boric acid and is found ia nature as the mineral sassoflte. Three crystalline modifications of metaboric acid also exist. AH these forms of boric acid can be regarded as hydrates of boric oxide and formulated as B2O3 3H20 for orthoboric acid and B2O3 H20 for metaboric acid. 

Forms of Boric Acid. Orthoboric acid, B(OH)3, formula wt, 61.83, crystaUi2es from aqueous solutions as white, waxy plates that are triclinic ia nature sp gi 4, 1.5172. Its normal melting poiat is 170.9°C, however, when heated slowly it loses water to form metaboric acid, HBO2, formula wt, 43.82, which may exist ia one of three crystal modifications. Orthorhombic HBO2-III or a-form d = 1.784 g/mL, mp = 176° C) forms first around 130°C and gradually changes to monoclinic HBO2-II or P-form (d = 2.045 g/mL, mp = 200.9° C). Water-vapor pressures associated with these decompositions foUow. To convert kPa to mm Hg, multiply by 7.5. 

At temperatures above 150°C, dehydration continues to yield viscous Hquid phases beyond the metaboric acid composition (39). The most stable form of metaboric acid, cubic HBO2-I or y-form (d = 2.49 g/mL, mp = 236° C) crystaUi2es slowly when mixtures of boric acid and HBO2-III are melted ia an evacuated, sealed ampul and held at 180°C for several weeks (41). 

The general formula for boric acid esters is B(OR)2. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7] ethyl borate [130-46-9J, and phenyl borate [1095-03-0] respectively. Some of the most common boric acid esters used in industrial appHcations are Hsted in Table 1. The nomenclature in the boric acid ester series can be confusing. The lUPAC committee on boron chemistry has suggested using trialkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older Hterature as boron alkoxides and aryloxides. CycHc boric acid esters, which are trimeric derivatives of metaboric acid (HBO2), are known as boroxines (1). 

Boric acid (boracic acid) [10043-35-3] M 61.8, m 171 , pK 9.23. Crystd three times from H2O (3mL/g) between 100° and 0°, after filtering through sintered glass. Dried to constant weight over metaboric acid in a desiccator. It is steam volatile. After 2 recrystns of ACS grade it had Ag at 0.2 ppm. 

Partial dehydration of B(OH)3 above 100° yields metaboric acid HBO2 which can exist in several crystalline modifications ... 

Figure 6.26 Layer structure of orthorhombic metaboric acid HBOafni), comprising units of formula BaOafOHla linked by O H O bonds.

The related trialkoxyboroxines (ROB 0)3 can be prepared by esterifying B(OH)3, B2O3 or metaboric acid BO(OH) with the appropriate mole ratio of ROH. 

The enthalpy of decomposition of this peracid is relatively high (AHcj = -1.83 kJ/g - average risk according to CHETAH criterion C,). Its aqueous solutions are unstable. Solutions that contain 80% of peracid detonate when they are stirred (even at -10°C). The usual way of preparing this peracid involves the effect of hydrogen peroxide when metaboric acid is present however, although this operating method was followed, serious accidents have occurred. 

Formic acid, Metaboric acid See Peroxyformic acid... 

Borate. Dehydration is the first reaction occurring on heating the borate as in the case of the phosphate but at a much lower temperature (130-270 C, step 1 Figure 17). Boric acid heated alone in the same conditions in TG, eliminates water between 80-300 C in two main overlapping steps with maximum rate at 170 and 190 C corresponding to formation of metaboric acid and boric anhydride respectively ... 

The patented preparation of peroxyacids [2] by interaction of carboxylic acids with hydrogen peroxide in presence of metaboric acid needs appropriate safeguards to prevent accidental separation of the cone, peroxyacids [3], Much descriptive data on stabilities of a wide selection of peroxyacids has been summarised [4], A general method of preparation of peroxyacids involving addition of e.g. the anhydrides of acetic, maleic, phthalic or trifluoroacetic acids to a suspension of 90%... 

TABLE A2 Thermochemical Data of Selected Chemical Compounds Metaboric acid (BH02), ideal gas, molecular weight = 43.81674... 

The Beckmann rearrangement of cyclohexanone oxime catalysed by solid metaboric acid (286) has also been investigated (equation 94). When ketoximes, mixed with 286 (formed from boric acid at 100°C/0.1 Torr), are heated (140°C/7-42 h) the corresponding amides or lactams are produced in excellent yields (62-92%). Under the... 

Boric acid on slow heating loses water around 130°C, forming metaboric acid HBO2 which converts into different crystal modifications that depend on the temperature. Further heating dehydrates metaboric acid to pyroboric acid, H2B4O7 and boric oxide, B2O3. 

The reaction is exothermic (AH°/ hydration is -18.15 kcal/mol B2O3). In the molten state, it reacts with water vapor to form gaseous metaboric acid, HBO2. 

Fig. 4. Orthorhombic metaboric acid (a) and triborate mono-, di-, and trianions (b-d)...

B2O3. Production processes have been developed for metaboric acid, but to date it has not become commercially important and is not currently produced in any substantial quantities. 

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