Sodium tetraborate

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   Sodium tetraborate   
Names: sodium tetraborate
disodium tetraborate
sodium borate

Formula: Na2B4O7 (anhydrous)
Na2B4O7 · 5H2O (pentahydrate)
Na2B4O7 · 10H2O (decahydrate)
SMILES: B1(OB2OB(OB(O1)O2)[O-])[O-].[Na+].[Na+]
Molar mass: 201.217 g/mol (anhydrous)
291.291 g/mol (pentahydrate)
381.366 g/mol (decahydrate)
Density: 2.37 g/cm3 (anhydrous)
1.815 g/cm3 (pentahydrate)
1.71 g/cm3 (decahydrate)
Crystal system: monoclinic (anhydrous)
a=5.507 Å, b=17.796 Å, c=8.377 Åα=90°, β=96.566°, γ=90°
orthorhombic (pentahydrate)
α=90°, β=90°, γ=90°
monoclinic (decahydrate)
a=11.884 Å, b=10.602 Å, c=12.2111 Åα=90°, β=106.79°, γ=90°

Color: colorless
Melting point: 742 °C1,015.15 K <br />1,367.6 °F <br />1,827.27 °R <br /> (anhydrous)
140 °C413.15 K <br />284 °F <br />743.67 °R <br /> (pentahydrate)
Decomposition point: 1575 °C1,848.15 K <br />2,867 °F <br />3,326.67 °R <br /> (anhydrous)
Refractive index: 1.462 (decahydrate)
1.468 (pentahydrate)
Magnetic properties: diamagnetic
Stability: slightly erodes (decahydrate)
stable (pentahydrate)
Hardness: relatively strong
(2 on Moh's scale)
Toxicity: non-toxic


Inorganic compound, salt of alkaline metal sodium and inorganic tetraboric acid. From water solutions crystallizes as decahydrate and pentahydrate.


Occurs in nature as tinkal mineral.

Where to buy

In pharmacy ("borax" as powder or glycerol solution).


Reaction between baking soda, sodium carbonate or hydroxide and boric acid

Chemical equation:

4H3BO3 + 2NaHCO3 = Na2B4O7 + 7H2O + 2CO2
4H3BO3 + Na2CO3 = Na2B4O7 + 6H2O + CO2
4H3BO3 + 2NaOH = Na2B4O7 + 7H2O

For preparation of 100.00g of sodium tetraborate decahydrate а 44.06g of soda or 27.79g of carbonate or 20.98g of sodium hydroxide and 64.85g of boric acid is required.

Add acid to the flask, then add small parts of sodium compound with stirring until heating will stop (attention, the solution may boil!) or, if you use carbonate, until carbon dioxide emission will stop. After reaction will stop, filter the solution and use it for crystal growing.

Influence of temperature

Very slow crystallization and low transparency at room temperature, but growing from hot solution eliminates these drawbacks. If temperature is over decahydrate decomposition temperature, a pentahydrate crystallization starts with another crystalline final form.

Influence of impurities


Crystal can thicken after several months due to eroding.

Storage conditions

Store in its original form or under several layers of varnish at average humidity and room temperature. Do not store in matchboxes or cotton wool and do not heat.


Temperatureгр/100,00 гр waterгр/100,00 гр ethanolгр/100,00 гр methanolгр/100,00 гр acetoneгр/100,00 гр glycerolгр/100,00 гр ethyl acetateгр/100,00 гр ethylene glycol
0°C273.15 K <br />32 °F <br />491.67 °R <br />1.112.04
10°C283.15 K <br />50 °F <br />509.67 °R <br />3.19
15°C288.15 K <br />59 °F <br />518.67 °R <br />3.942.54
20°C293.15 K <br />68 °F <br />527.67 °R <br />4.94110.97
25°C298.15 K <br />77 °F <br />536.67 °R <br />6.1624.840.60.1471.23
30°C303.15 K <br />86 °F <br />545.67 °R <br />7.76
40°C313.15 K <br />104 °F <br />563.67 °R <br />12.64
50°C323.15 K <br />122 °F <br />581.67 °R <br />15.8721.82
60°C333.15 K <br />140 °F <br />599.67 °R <br />~17.943.53
70°C343.15 K <br />158 °F <br />617.67 °R <br />2058.58
80°C353.15 K <br />176 °F <br />635.67 °R <br />2379.57
90°C363.15 K <br />194 °F <br />653.67 °R <br />28113.58
100°C373.15 K <br />212 °F <br />671.67 °R <br />52.535191.04