(Pengsha)
Natural sodium tetraborate crystal, chemical formula: Na2B4O7 10H2O. Boric acid reacts with alkali to form borate. When the solution is strongly alkaline (pH = 1 1 ~ 12), metaborate is mainly generated:
H3BO3+NaOH=NaBO2+2H2O
When the alkalinity is weak (pH < 9.6), tetraborate is generated:
4 h3bo 3+2 NaOH+3H2O = na 2 B4 o 7 10H2O
According to its structure, the chemical formula of borax should be written as Na2B4o5 (OH) 4 8H2O.
Borax is a colorless crystal, which is weathered in the air. When heated to 400 ~ 500℃, it can be dehydrated into anhydrous sodium tetraborate and melted into glass at 878℃. Because the melt contains acid oxide B2O3, the metal oxide can be dissolved. When welding metal, this property can be used to remove oxides on the metal surface. Many metal oxides often show characteristic colors after being dissolved in molten borax, such as cobalt metaborate Co(BO2)2 is blue, nickel metaborate Ni(BO2)2 is brown, and chromium metaborate Cr(BO2)3 is blue-green. For example:
Sodium borate+disodium carbonate+cobalt oxide
This is also called borax bead reaction, which is often used to detect some metal ions in analytical chemistry. Borax has great solubility in water, which increases with the increase of temperature, and it is easy to hydrolyze in aqueous solution and become alkaline. The hydrolysis reaction of borate is as follows:
4BO-2+H2O=B4O2-7+2OH-
B4O2-7+7H2O=4H3BO3+2OH-
The balance of alkali addition shifts to the right, and boric acid can be made into borate, which exists in the form of metaborate under the condition of strong alkali. Sodium tetraborate aqueous solution can be regarded as a buffer system consisting of dibasic boric acid (or metaboric acid) and disodium hydrogen metaborate, and the pH value of the solution is about 9.4 ~ 9.6. As long as the pH value of the prepared alkaline solution is reduced to about 9.6, borax can be obtained. Tibet is rich in natural borax, and the northeast is also rich in boron and magnesium minerals. In industry, the method of decomposing ore with caustic soda is usually used to prepare borax from ludwigite. Firstly, sodium metaborate was obtained, and then carbon dioxide was introduced to reduce its alkalinity to obtain borax. The main reactions are as follows:
H2O+2NaOH=2Mg(OH)2+2NaBO2
4 sodium borohydride+carbon dioxide+10H2O
= na 2 B4 o 5(OH)4·8H2O+na2co 3
Borax has a wide range of uses, which can be used as a filler for washing powder and soap, as well as a raw material for manufacturing optical glass, enamel and porcelain glaze, and can also be used for manufacturing artificial gem and flux. After refining, it can be used to clear heat and detoxify. It is cool and tastes sweet and salty. Can be used for treating sore throat, dental abscess, aphtha, blindness and other diseases.