ヒ酸

ヒ酸 化学特性,用途語,生産方法

• 解説

  化学式H3AsO4。オルトヒ酸orthoarsenic acidともいう。五酸化二ヒ素As2O5を俗にヒ酸ということもある。　ヒ素または三酸化二ヒ素As2O3を硝酸に入れて酸化窒素の発生がやむまで熱して酸化する。一度ろ過してから130℃まで徐々に熱して蒸発濃縮し，100℃以下に冷却するとH3AsO4･1/2H2Oの結晶が得られる。これは無色の板状晶で，融点36℃，潮解性。水に易溶，エチルアルコールに可溶である。
  株式会社平凡社 世界大百科事典 第２版について　情報

• 説明

  Arsenic acid has the molecular formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analog of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but only found in solution where it is largely ionized. Its hemihydrate form (H3AsO4·1/2H2O) does form stable crystals. Crystalline samples dehydrate with condensation at 100 °C.
  Arsenic acid is a tetrahedral species of idealized symmetry C3v with As—O bonds lengths ranging from 1.66 to 1.71? . Being a triprotic acid, its acidity is described by three equilibria:
  H3AsO4 ? H2AsO4^- + H⁺ (pk1 =10^-2.19)
  H2AsO4 ? 5HasO4 ^2- + H⁺ (pk2 = 10^-6.94)
  HasO4 2 ? 5AsO4 ^3- + H⁺ (pk3 = 10^-11.5)

• 製造方法

  Arsenic acid is prepared by treating arsenic trioxide with concentrated nitric acid or by combination of arsenic acid with water. The latter reaction is very slow. It is also formed when metaarsenic or pyroarsenic acid is treated with cold water.