Precipitation · and Hydrolysis of Thorium(IV) in Aqueous Solution: Thorium Nitrate - Potassium Hydroxide. I. Determination of Solubility Constants of Th(OH)4

Sa？etak The precipitation and h ydrolysis 0f thorium (IV) in aqueous solutions of thorium nitrate - potassium hydroxide was investigated at 20oc with a tyndallometel'.' a nd a pH-meter. A three-dimensional precipitation diagram for a wide r ange of concentrations of both precipitating components was constructed. The concentration region of thorium hydroxide, the fast precipitation, the formation of sols and of stable clear solutions were observed. The decreased precipitation of thorium h ydroxide at pH between 9 - 10.4 is considered to be the effect of the carbonate ions present. By a very simple graphical tyndallometric method the solubility products of thorium hydroxide were directly determined as: log K so = - 45.7; log Ks2 = - 24.3 and the cumulative hydrolytic constant */3 2 = 1.029 X 10-1 was calculated. The predominant hydrolytic species of thorium-ion in aqueous solution in equilibrium with the solid phase of Th(OH) 4 were estimated. The predominant species is the free ion Th4+ for pH 3.4 and [Th(OH)22+]n for the pH between 3.5 and 4.L At values lower pH 4.1 ionic species of thorium are formed with more than two OH- ions per one thorium ion