_Thorium nitrate_, Th(NO₃)₄,12H₂O, crystallises at ordinary temperatures in large hygroscopic tablets, very soluble in water and alcohol. The hydrates, Th(NO₃)₄,6H₂O and Th(NO₃)₄,5H₂O, have been obtained from hot solution and from nitric acid solution respectively. Thorium is employed in commerce almost entirely in the form of this salt, which is dehydrated until it contains about 48 per cent. ThO₂, which approximates to the formula Th(NO₃)₄,4H₂O; the commercial product, however, is not a definite hydrate. Kolbe[484] has described the additive product with antipyrine, 2Th(NO₃)₄,5C₁₁H₁₂ON₂, which melts at 168°. The extent to which thorium salts are hydrolysed in solution is very considerable, as is evident from the fact that the nitrate may be titrated with standard potash in presence of phenolphthalein as indicator; the solution first becomes alkaline to this reagent when 3·5 molecules of potash have been added for each molecule of thorium nitrate present.[485] Of the large number of _double nitrates_ which have been prepared, the general types R´₂Th(NO₃)₆, where R´ = NH₄,K,Rb,Cs, and R´´Th(NO₃)₆,8H₂O, where R´´ = Mg,Mn,Zn,Ni,Co, are the most important. [484] _Zeitsch. anorg. Chem._ 1913, ~83~, 143. [485] Halla, _loc. cit._ _Thorium phosphates._--The precipitates obtained by addition of phosphoric acid or alkali phosphates to solutions of thorium salts are gelatinous solids of doubtful composition; they dissolve in mineral acids and in alkali carbonates, and their behaviour is of great importance in the technical treatment of monazite. Various phosphates and _double phosphates_ are obtained by fusion methods, but none of these are important. The _phosphite_, Th(HPO₃)₂,3H₂O, and _hypophosphite_, Th(H₂PO₂)₄, are insoluble solids obtained by double decomposition. The _hypophosphate_ ThP₂O₆,11H₂O, is of great importance for purposes of detection and estimation, since it is thrown down quantitatively from strongly acid solutions; under these conditions the rare earths remain in solution.