Sodium borate

Sodium borate is a generic name for any salt of sodium with an anion consisting of boron and oxygen, and possibly hydrogen, or any hydrate thereof. It can be seen as a hydrated sodium salt of the appropriate boroxy acid, although the latter may not be a stable compound.

Many sodium borates have important industrial and household applications; the best known being borax, (Na+)2[B4O5(OH)4]2−·8H2O = Na2B4H20O17.

The ternary phase diagram of the Na2O–B2O3–H2O phase diagram in the 0–100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products.[1]

Sodium borates, as well as boroxy acids, are often described as mixtures xNa2O·yB2O3·zH2O = Na2xB2yH2zOx+3y+z, with x, y, and z chosen to fit the elemental formula, or a multiple thereof. Thus, for example, borax Na2B4H20O17 would be 1Na2O·2B2O3·10H2O, and boric acid B(OH)3 would be 0Na2O·1B2O3·1H2O = 2[B(OH)3].

The elemental formula was often interpreted as a z-hydrate of an "anhydrous" salt without any hydrogen, namely Na2xB2yO3y·zH2O. However, later research uncovered that many borates have hydroxyl groups HO− bound covalently to the boron atoms in the anion. Thus borax, for example, is still often described as a decahydrate Na2B4O7·10H2O, with the implied anion [B4O7]2−, whereas the correct formula is Na2B4O5(OH)4·8H2O, with anion [B4O5(OH)4]2−.

The following table gives some of the crystalline sodium borates in this family.[2] The column x/(x+y) is the formal mole fraction of Na2O in the "anhydrous" version.

x:y:z x/(x+y) Elemental formula As hydrate Correct formula Name
3:1:0 0.750 Na6B2O6 = Na3BO3 Na3BO3 trisodium orthoborate (anhydrous)
1:1:0 0.500 Na2B2O4 = NaBO2         Na3B3O6 sodium metaborate
1:1:1 0.500 Na2H2B2O5 Na2B2O4·H2O ? ?[2]
1:1:4 0.500 Na2H8B2O8 = NaBH4O4 NaBO2·2H2O NaB(OH)4 sodium tetrahydroxyborate[2]
1:1:8 0.500 Na2H16B2O12 = NaBH8O6 NaBO2·4H2O ? ?[2]
1:2:0 0.333 Na2B4O7 Na2B4O7 borax (anhydrous)
1:2:4 0.333 Na2H8B4O11 Na2B4O7·4H2O Na2B4O5(OH)4 borax "tetrahydrate"
1:2:5 0.333 Na2H10B4O12 = NaB2H5O6 Na2B4O7·5H2O Na2B4O5(OH)4·H2O borax "pentahydrate"[2]
1:2:10 0.333 Na2H20B4O17 Na2B4O7·10H2O Na2B4O5(OH)4·8H2O borax "decahydrate"[2]
1:3:0 0.250 Na2B6O10 = NaB3O5 NaB3O5 ?
1:4:0 0.200 Na2B8O13 Na2B8O13 disodium octaborate (anhydrous)
2:9:11 0.182 Na4H22B18O31                     Na4B18O29·11H2O Na2[B8O11(OH)4]·[B(OH)3]·2H2O [1] disodium enneaborate
1:5:2 0.167 Na2H4B10O18 = NaH2B5O9 NaB5O8·H2O ? sodium pentaborate "monohydrate"[1]
1:5:4 0.167 Na2H8B10O20 = NaH4B5O10   NaB5O8·2H2O Na[B5O7(OH)2]·H2O sodium pentaborate "dihydrate"[3]
1:5:10 0.167 Na2H20B10O26 = NaH10B5O13 NaB5O8·5H2O ? sodium pentaborate "pentahydrate"[4]
3:5:4 0.125 Na6H8B10O22 = Na3H4B5O11 Na3B5O7·2H2O Na3B5O8(OH)2·H2O trisodium pentaborate[5]

Some of the borates above may have more than one isomeric or crystalline form. Some may decompose when dissolved in water. Note that the anion of the "anhydrous borax" is different from that of its "hydrates".

Some of the anhydrous borates above can be crystallized from molten mixtured of sodium oxide and boric oxide.[6]

Some sodium borates hower cannot be analyzed as combinations xNa2O·yB2O3·zH2O of the three ordinary oxides. The most important example is sodium perborate, originally described as NaBO3·H2O but actually (Na+)2[B2O4(OH)4]2−. The anion of this compound has two peroxide bridges −O−O− which make it oxygen-rich compared to the general family above.

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  2. ^ a b c d e f Cite error: The named reference nies1967 was invoked but never defined (see the help page).
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