Sodium borates

The anhydrous metaborate, NaBO₂, is formed by fusing borax with sodium carbonate, being obtained in hexagonal prisms, m.p. 966° C., density 0.2571 between 17° and 97° C. When the syrup-like solution obtained by concentrating equivalent proportions of sodium hydroxide and boric acid or borax is allowed to crystallize over concentrated sulphuric acid, the tetrahydrate separates in triclinic crystals. The dihydrate and other hydrates have also been described. The aqueous solution of the metaborate has an alkaline reaction, due to hydrolytic dissociation. The orthoborate is formed by fusing boric anhydride with sodium peroxide.

The most important borate of sodium is disodium tetraborate or borax, Na₂B₄O₇,10H₂O, found in Thibet as the mineral tincal, and also in California. The native variety is purified by crystallization, but most of the borax of commerce is obtained by fusing native boric acid with sodium carbonate.

The anhydrous salt is obtained by heating the decahydrate, and is a white substance of density 2.367. Its melting-point is given as 561° C., 730° C., 741° C., and 878° C., the fused substance solidifying to a colourless, transparent, vitreous mass known as the " borax bead." In the melted state it dissolves metallic oxides, producing characteristic colorations, and finds application in analysis. It is also employed in soldering metals to remove the superficial layer of oxide, and to prevent oxidation during the process by excluding the air. Its specific heat is 0.229 between 17° and 47° C. (Kopp), and 0.2382 between 16° and 98° C. (Regnault). Its heat of formation is 748.1 Cal.

The decahydrate or ordinary borax forms monoelinic crystals of density 1.723 (Hassenfratz); or 1.694 at 17° C., and 1.728 at the temperature of liquid air (Dewar). Its specific heat is 0.385 between 19° and 50° C. A pentahydrate belonging to the rhombohedral division of the hexagonal system crystallizes above 60° C. Its density is 1.815. The pentahydrate is stable from 60° C., the transition-point from the decahydrate, to 125° C. At 130° C. the stable form is the trihydrate at 150° C. it is the dihydrate; and at 180° C. it is the monohydrate. The last molecule of water is retained up to 318° C.

By fusing borax with boron trioxide, two definite acid borates are produced. One of them has the formula Na₂O,3B₂O₃, and melts at 694° C.; the other melts at 783° C., and has the formula Na₂O,4B₂O₃. The crystallization of solutions with the composition ratio

Na₂O:B₂O₃=1:5

yields a disodium pentaborate, Na₂O,5B₂O₃,10H₂O.

Sodium perborate, NaBO₃,4H₂O. - A perborate of the formula indicated is formed by the action of excess of hydrogen peroxide on a solution of borax and sodium hydroxide, and the action of a mineral acid or hydrogen peroxide on a mixture of sodium peroxide and boric acid:

2B(OH)₃ + 2Na₂O₂+H₂SO₄+4H₂O = 2(NaBO₃,4H₂O)+Na₂SO₄;
2B(OH)₃+Na₂O₂+H₂O₂ + 4H₂O=2(NaBO₃,4H₂O).

It is also produced in the electrolysis of an aqueous solution of sodium metaborate, or of sodium carbonate and borax. Alsgaard recommends electrolyzing a solution containing borax, sodium carbonate, sodium hydrogen carbonate, potassium dichromate, and sodium silicate with a copper pipe as cathode and a platinum anode, the current being 6 amperes at 7 to 8 volts, and the temperature 10° C. Foerster considers the formula NaBO₂,H₂O₂,3H₂O to be more in accord with the properties of the compound than NaBO₃,4H₂O.

Sodium perborate tetrahydrate is stable in air, but in aqueous solution it loses oxygen. The solution is alkaline in reaction. The velocity of decomposition and catalysis of sodium perborate have been studied by Seorgi and Nocentini. Addition of alcohol precipitates a salt, probably NaBO₄, readily decomposed with evolution of oxygen either in the dry state or on solution in water. The tetrahydrate loses its water of crystallization, forming a stable monohydrate. At 15° C. the solubility is 2.55 grams NaBO₃ in 100 grams of water. The salt finds application as a therapeutic agent, and is employed in bleaching. Its behaviour in solution is similar to that of hydrogen peroxide.

From a solution in water of sodium peroxide and boric acid another perborate, or " perborax," Na₂B₄O₈,10H₂O, separates. On solution in water it yields hydrogen peroxide. At 22° C. its solubility is 71 grams of Na₂B₄O₈ in 100 grams of water, the solution being strongly alkaline.