Boric acid

From Crystal growing


   Boric acid   
Names: boric acid
ortoboric acid
boracic acid
hydrogen borate
boron(III) hydroxide

Formula: H3BO3
Molar mass: 61.832 g/mol
Density: 1.435 g/cm3
Crystal system: triclinic
a=7.04 Å, b=7.04 Å, c=6.56 Åα=92.5°, β=101.167°, γ=120°

Color: colorless
Melting point: 170.9 °C444.05 K <br />339.62 °F <br />799.29 °R <br />
Boiling point: 300 °C573.15 K <br />572 °F <br />1,031.67 °R <br />
Refractive index: 1.456
Magnetic properties: diamagnetic
χ=-3.41 · 10-5cm³/mol
Hardness: very fragile
(1 on Moh's scale)
Toxicity: non-toxic


Inorganic compound, weak monobasic acid. Doesn't form water hydrates.


Occurs in nature as sassolite mineral.

Where to buy

In pharmacy ("boric acid" in powder).


Reaction between borax and sulfuric, hydrochloric or acetic acid

Chemical equation:

Na2B4O7 + H2SO4 + 5H2O = 4H3BO3 + Na2SO4
Na2B4O7 + 2HCl + 5H2O = 4H3BO3 + 2NaCl
Na2B4O7 + 2CH3COOH + 5H2O = 4H3BO3 + 2NaCH3COO

For preparation of 100.00g of boric acid а 154.19g of sodium tetraborate decahydrate and 107.17g of 37% sulfuric or 79.69g of 37% hydrochloric or 69.37g of 70% acetic acid is required.
Add to the flask with hot tetraborate solution a acid solution with heating and stirring. Cooling this mixture will cause forming a large amount of crystalline precipitate. Filter precipitate and wash it wish small amount of water, alcohol or glycerol, then 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.

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 glycolгр/100,00 гр propanolгр/100,00 гр isobutanolгр/100,00 гр ammoniaгр/100,00 гр hydrazineгр/100,00 гр trichlorethylene
0°C273.15 K <br />32 °F <br />491.67 °R <br />2.52
10°C283.15 K <br />50 °F <br />509.67 °R <br />3.49
15°C288.15 K <br />59 °F <br />518.67 °R <br />4.10.016
20°C293.15 K <br />68 °F <br />527.67 °R <br />4.7255
25°C298.15 K <br />77 °F <br />536.67 °R <br />5.489.4417.930.617.51.5218.
30°C303.15 K <br />86 °F <br />545.67 °R <br />6.23
40°C313.15 K <br />104 °F <br />563.67 °R <br />8.08
50°C323.15 K <br />122 °F <br />581.67 °R <br />10.27
60°C333.15 K <br />140 °F <br />599.67 °R <br />12.97
70°C343.15 K <br />158 °F <br />617.67 °R <br />15.75
80°C353.15 K <br />176 °F <br />635.67 °R <br />19.1
90°C363.15 K <br />194 °F <br />653.67 °R <br />23.27
100°C373.15 K <br />212 °F <br />671.67 °R <br />27.53
Highly soluble in sulfuric acid. Soluble in dioxane.