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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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