Precipitation and hydrolysis of metallic ions—IV: Studies on the solubility of aluminum hydroxide in aqueous solution

Abstract

The region of precipitation of aluminum hydroxide by mixing the aqueous solutions of aluminum chloride (0·03–200 mM) and sodium hydroxide (pH 3·5–11·5) was investigated tyndallometrically at 20°C, 24 hr after mixing, at varied ionic strengths. By means of graphical analysis of experimental solubility curve (pH vs. log total aluminum concentration) and trial and error method the charge p(3+, 1+, 0,1−) of aluminum hydrolytic species, which predominate in saturated solution, was determined, and the simplest polynomial for the solubility curve was found ([Al] tot=2·24×10 −13[H +] −1+1·24×10 −4+3·87×10 2[H +]+2·51×10 11[H +] 3). The problem of the polynuclearity of the complexes in saturated solution cannot be solved by that method. Under the assumption that the hydrolytic aluminum species are formally mononuclear, the apparent solubility constants were calculated (log K S0 =11·40 for ∗K S0= [ Al 3+]; log ∗K S2=2·59 for ∗K S2= [ Al(OH) 2 +] [ H +] ; log ∗K S3=−3·92 for ∗K S3=[ Al(OH) 3 0]; log ∗K S4=−12·62 for ∗K S4=[ Al(OH) 4 −]/ H +] −1) . Some experiments at constant ionic strength, 0·6 M NaCl, showed no significant difference. The zero point of charge, ZPC at pH 9·0–9·2, was determined electrophoretically for total aluminum concentration 16 mM. The results are compared with the data from the literature and discussed.