Bismuthyl Perchlorates

Muir, in one of his early investigations on bismuth compounds, claimed to have prepared bismuthyl perchlorate, BiOClO₄, by heating powdered bismuth in a dilute solution of perchloric acid. The solution of bismuth in perchloric acid is, however, accompanied by the reduction of the acid to chloric acid and the liberation of the explosive chlorine dioxide; hence the experiment can be carried out safely only by employing small quantities of 40 per cent, acid and heating very carefully. A similar product is obtained by dissolving bismuth oxide in the acid. The normal perchlorate can also be obtained by careful employment of the latter method. In this way the pentahydrate, Bi(ClO₄)₃.5H₂O, has been obtained in the form of small, hexagonal plates, which are extremely reactive to water, yielding a bismuthyl salt. An unstable trihydrate of bismuthyl perchlorate, BiOClO₄.3H₂O, is obtained by evaporating a solution of bismuth oxide in a more dilute acid, or by adding water to the normal salt and evaporating over calcium chloride; this passes readily into well-formed hygroscopic rhombohedra of the monohydrate, BiOClO₄.H₂O, which is the most stable body of this series. By careful drying at 80° to 100° C., the anhydrous perchlorate is obtained as a white powder. All these oxychlorates are soluble in water, yielding clear solutions, without appreciable hydrolysis. A basic perchlorate, whose composition approximates to the formula BiOH.ClO₄.H₂O, has been obtained by saturating concentrated perchloric acid with bismuth trioxide.

Conductivity measurements on solutions of bismuth perchlorate indicate that a very soluble basic salt, Bi(OH)₂.ClO₄, is stable even in the presence of a moderate excess of perchloric acid, and that it is converted into less basic salts such as Bi(OH)(ClO₄)₂ or Bi(ClO₄)₃ only slowly on addition of excess of acid. The first compound has the conductivity of a non-hydrolysed univalent salt.