Boron, produced by borax and carbon, was found in 1808. Boron is a nonmetal, which is famous for borax (sodium tetraborate) and boric acid. The latter is an acid with cleaning and sterilization functions, which is good for eyes. The annual demand for boron in various industries in the United States is more than 240,000 t. In agriculture, boron can be made into fertilizer and is also a good herbicide.
Although people have been dealing with boron for a long time, for example, borax was used as a flux when making glass in ancient Egypt, and it was also used by ancient alchemists, but the chemical composition of boric acid was still a mystery at the beginning of19th century.
1808, British chemist Sir humphry davy (1778- 1829) discovered potassium by electrolysis, and then made brown boron from electrolytic molten boron trioxide. In the same year, French chemists Joseph-Louis Grey-Lussac (1778- 1850) and Louis Jacques Nader (1777- 1857) reduced the anhydrous boric acid to produce boron.
Boron is named boron, and its name comes from Arabic, which means "flux". It shows that ancient Arabs have known that borax has the ability to melt metal oxides and is used as a flux in welding. The elemental symbol of boron is B, and the Chinese translation is boron.
B does not react with hydrochloric acid, but reacts with hot concentrated H2SO4 and hot concentrated HNO3 to generate boric acid;
2B+3H2SO4 (concentrated) = = 2b (OH) 3+3SO2 =
B+3HNO3 (concentrated) = = B (OH) 3+3NO2 =
Properties of boric acid
(1)H3BO3 is a white flaky crystal, slightly soluble in water (the solubility is 6.35g/( 100gH2O) at 273K). When heated, the solubility of the crystal increased (27.6 g at 373K) (100 gh2o).
(2)H3BO3 is a weak acid with Ka=5.8× 10- 10. The reason why it is weakly acidic is not because it ionizes proton H+, but because B is an electron-deficient atom, and H+ ions are released by adding OH- from H2O molecules (in which the lone pair on O atom coordinates with the empty P orbit of B atom).
(3) This ionization mode of boric acid indicates that boron compounds are electron-deficient. Therefore, boric acid is a typical Lewis acid, and its acidity can be greatly enhanced by adding mannitol or glycerol. For example, the pH value of boric acid solution is ≈ 5 ~ 6, and the pH value is ≈ 3 ~ 4 after adding glycerol.
It shows the properties of monobasic acid and can be titrated with strong base.
(4) Boric acid reacts with methanol or ethanol in the presence of concentrated H2SO4 to form borate ester, which burns and volatilizes at high temperature, producing a unique green flame. This reaction can be used to identify boric acid, borate and other compounds.
(5) In the process of heating, dehydration and decomposition, boric acid is firstly transformed into metaboric acid HBO3 _ 3 _ 3, and then it is continuously heated to generate B2O3.
(6) When reacting with extremely strong acidic oxides (such as P2O5 or AsO5) or acids, H3BO3 is forced to show weak alkalinity: