Gold(III) chlorohydroxo complexes in alkaline aqueous solutions

The stepwise substitution of OH- for Cl- in AuCl4- (25 degrees C, I = 1 mol /l (NaCl)) was studied by potentiometric titration. The equilibrium constan ts of substitution were determined: log beta(1) = 7.87, log beta(2) = 14.79 , log beta(3) = 20.92, and log beta(4) = 25.98. The spectra of individual c hlorohydroxo complexes AuCliOH4-i- (i = 1-4, v = 45 000-28000 cm(-1)) were calculated. When the gold(III) concentration is lower than 0.01 mol/l, no p olynuclear complexes are present in alkaline chloride solutions. They are a lso absent in acidic solutions of HAuCl4, at least up to a concentration of 0.2 mol/l. The formation of polynuclear hydroxo complexes was displayed in the spectra at pH > 6.5 and at high gold(III) concentrations (0.1-0.2 mol/ l). The gold(III) hydroxide Au2O3. xH(2)O precipitates in alkaline solution s (C-Au = 0.15-0.2 mol/l, c(OH)/c(Au) > 3) The process of its formation is kinetically slow. The delay of the onset of precipitation depends on the re agent concentrations. The constant of the heterogeneous equilibrium Au(OH)( 4)(-) + 0.5(x - 3)H2O = 0.5Au(2)O(3). xH(2)O + OH- was determined, logK = - 3.4 +/- 0.2.