metaboric acid

Like metaboric acid, the metaborates exist in disparate structures.

It is also called "modification III" of the metaboric acids.

Metaboric acid is a white, cubic crystalline solid and is only slightly soluble in water.

Sodium and potassium metaborates are salts formed by deprotonation of orthorhombic metaboric acid.

Metaboric acid is the name for a family of inorganic compounds formed by the dehydration of boric acid.

Heating of boric acid at 80-100 C releases one equivalent of water to give orthorhombic metaboric acid:

Upon heating at 130-140 C in a sealed ampoule (to prevent dehydration), orthorhombic metaboric acid converts to the monoclinic form:

Components of the thermal water include sulphate, calcium, magnesium, bicarbonate and a significant amount of fluoride acid and metaboric acid.

Above 140 C, boric acid or the other forms of metaboric acid convert to cubic metaboric acid.

Metaboric acid melts at about 236 C, and when heated above about 300 C further dehydrates, forming tetraboric acid or pyroboric acid (HBO):

Another method is heating boric acid above 300 C. Boric acid will initially decompose into water steam and metaboric acid (HBO) at around 170 C, and further heating above 300 C will produce more steam and boron trioxide.