Boric acid was first prepared by Wilhelm Homberg (1652-1715) from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi. The presence of boric acid or its salts has been noted in sea-water, whilst it is also said to exist in plants and especially in almost all fruits (A. H. Allen, Analyst, 1904, 301). The free acid is found native in certain volcanic districts such as Tuscany, the Lipari Islands and Nevada, issuing mixed with steam from fissures in the ground; it is also found as a constituent of many minerals (borax, boracite, boronatrocaicite and colemanite).Boric acid is soluble in boiling water. When heated above 170°C it dehydrates, forming
metaboric acid HBO2. Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water. It melts at about 236°C, and when heated above about 300°C further dehydrates, forming
tetraboric acid or
pyroboric acid, H2B4O7. Boric acid can refer to any of these compounds. Further heating leads to boron trioxide.
Boron is a nonmetallic element, group III in the periodic table. Symbol B; aomic number 5; atomic mass 10.811; melting point ca 2,300 C; sublimation point ca 2,550 C; specific gravity 2.37 or 2.34; valence +3; electronic config. [He]2s22p1. There are two allotropes of boron; amorphous boron is a dark brown to black amorphous powder, but metal-like crystalline solid is an extremely hard (9.3 on Mohs' scale), black to silver-gray, brittle, lustrous and has a bad conductor in room temperatures. The specific gravities of amorphous and crystalline forms are 2.37 and 2.34 respectively. The crystalline form is far less reactive than the amorphous form.
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