MetaBorate, sodium

Sodium Borohydride. Sodium borohydride [16940-66-2] is a thermally stable, white crystalline soHd that decomposes in vacuo above 400°C. The heat of formation is —192 kJ/mol (—45.9 kcal/mol). NaBH is hygroscopic and absorbs water rapidly from moist air to form a dihydrate that decomposes slowly to sodium metaborate and hydrogen. It is soluble in many solvents including water, alcohols, Hquid ammonia and amines, glycol ethers, and dimethyl sulfoxide. 

Although there is Httle toxicity information pubHshed on hydrides, a threshold limit value (TLV) for lithium hydride in air of 25 fig/has been established (52). More extensive data are available (53) for sodium borohydride in the powder and solution forms. The acute oral LD q of NaBH is 50-100 mg/kg for NaBH and 50-1000 mg/kg for the solution. The acute dermal LD q (on dry skin) is 4-8 g/kg for NaBH and 100-500 mg/kg for the solution. The reaction or decomposition by-product sodium metaborate is slightly toxic orally (LD q is 2000-4000 mg/kg) and nontoxic dermally. 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

Fig. 5. Solubility—temperature curves for boric acid, borax, sodium pentaborate, and sodium metaborate (71).

Sodium Metaborate Tetrahydrate. Sodium metaborate tetrahydrate, NaB02 4H2O or Na20 B2O3 -8H20 formula wt, 137.86 triclinic sp gr, 1.743 is easily formed by cooling a solution containing borax and an amount of sodium hydroxide just in excess of the theoretical value. It is the stable... 

Heat capacity data for metaborate solutions have been reported (87). The solubiUty of sodium metaborate tetrahydrate in methanol at 40°C is 26.4 wt % (61). 

Sodium metaborate absorbs CO2 from the atmosphere, forming borax and sodium carbonate. Crystals of the tetrahydrate melt in its water of crystallisation at about 54°C. The soHd-state stmcture of the tetrahydrate, Na[B(OH)J 2H20, consists of discrete tetrahedral B(OH) 4 groups (90). 

The dihydrate loses water slowly at room temperature. Its heat of dehydration to NaB02 0.5H2O has been calculated as 58.1 kJ/mol (13.9 kcal/mol) of H2O (88). Sodium metaborate dihydrate reacts with atmospheric CO2 to produce sodium carbonate and borax. The melting point is 90—95°C, compared to 54°C for the tetrahydrate. Some crystallographic work has been done (91). 

Commercial preparation of sodium perborate tetrahydrate is by reaction of a sodium metaborate solution, from sodium hydroxide and borax pentahydrate, and hydrogen peroxide followed by crystallization of tetrahydrate (95). The tnhydrate and monohydrate can be formed by reversible dehydration of the tetrahydrate. 

Sodium perborate tnhydrate, NaBO 3H2O or Na2B2(02)2(0H)4 4H2O, triclinic, contains 11.8 wt % active oxygen (96). It has been claimed to have better thermal stabiUty than the tetrahydrate but has not been used commercially. The tnhydrate can be made by dehydration of the tetrahydrate or by crystallization from a sodium metaborate and hydrogen peroxide solution in the present of tnhydrate seeds. Between 18 and 50°C the tnhydrate is more stable but slower to crystallize than the tetrahydrate. Below 15°C the tnhydrate is spontaneously converted into the tetrahydrate. 

Other Sodium Borates. Prices for disodium octaborate tetrahydrate (SOLUBOR, TIM-BOR, POLYBOR), sodium metaborate tetrahydrate, and sodium metaborate dihydrate in 1985 were 924/1, 880/t, and 1034/1, respectively. The corresponding prices in 1990 were 1056/t, 990/t (technical grade), and 1298/t (technical grade), respectively (33). 

Sodium metaborate hydrates are more alkaline than borax and greater care is required in handling. The metaborate material is harm fill to the eyes and can cause skin irritation. Gloves, goggles, and a simple dust mask should be used when handling sodium metaborate powder. 

Sodium Metaborate Tetrahydrate and Dihydrate, The sodium metaborates are components in textile finishing, sizing and scouring compositions, adhesives, and detergents. They are also used in many photographic apphcations. In agriculture they are used in both herbicides and fertilizer sprays. The dihydrate is less affected by heat. 

Commercially, sodium tetraborate decahydrate or borax (Na CylOHjO) is usually employed, although sodium metaborate hydrate (NaB02 xH20) may be used. 

You have added a solution of calcium chloride to a solution of sodium metaborate. A crystalline precipitate resulted. In attempting to analyze your product. 

Many other peroxy compounds can analogously be produced in the region of HAP for instance, sodium perborate Na2(B03)2 (from sodium metaborate NaB02) and peroxycarbonates. These compounds are used as stable oxidizing and bleaching agents. 

What is particularly problematic about this phase separation process from a chemical corrosion point of view is that the B-rich phase (which can be modeled as a sodium-metaborate component Na20-2B203 or Na2B407) is highly water-soluble. As dissolution of sodium-metaborate is independent of dissolved silica activity ... 

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