Structure of a Gold(III) Hydroxide and Determination of Its Solubility

A representative gold(III) complex ion ([AuCl4](-)), successively hydrolyzes to form [AuCl4-n(OH)(n)](-) (n = 0-4) in aqueous solution as the pH increases and, finally, precipitates. It has been referred to as "gold(III) hydroxide" for the past fifty years. However, whether the precipitate is gold(III) hydroxide (Au(OH)(3)) or hydrous gold(III) oxide (Au2O3 center dot nH(2)O) has remained uncertain. Few studies have been conducted to determine its chemical and physical properties. The aim of this investigation is to identify the precipitate and to determine its solubility. The precipitate was X-ray amorphous. Based on the transmission electron microscopy (TEM) observation, the shape was spherical and the particle diameter was approximately 9 nm. From the results of Au-197 Mossbauer spectroscopy, Au L-3-edge X-ray absorption (XA) spectroscopy and thermogravimetry/differential thermal analysis (TG/DTA) measurements, the precipitate composition was estimated to be Au(OH)(3). Based on the composition, its structure was proposed to be a linear polymer. Moreover, its solubility at 298K was determined to be 0.00120 g/100 g of H2O. This chemical and physical properties information of Au(OH)(3) is essential for gold chemistry, for example, for preparing supported gold catalyst.