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 = Na_2xB_2yH_2zO_x+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 Na_2xB_2yO_3y·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.