STANDARD HEATS OF SOLUTION OF YTTRIUM AND RARE-EARTH-METAL NITRATE HYDRATES IN WATER

Heats of solution of yttrium and lanthanoid metal nitrate hydrates (Ln(NO3)3.n H2O: Ln = La Lu; n = 1 6) in water were measured at 298.15 K. A linear relationship was found between the heats of solution DELTAH(s) and water contents n per mole of nitrate for all the nitrates. When the heats of solution of anhydrous crystalline nitrate are plotted against the reciprocal of the ionic radii of trivalent lanthanoid ions, they tend to decrease from lanthanum to gadolinium and to increase from gadolinium to lutetium, with a few exceptions. The values of the slope of the line DELTAH(s)/n tend to increase from lanthanum to praseodymium, to be kept at an approximately constant value from praseodymium to thulium, and then to decrease from thulium to lutetium. These results were discussed on the basis of the lattice energy of crystalline nitrates and the hydration structure of trivalent lanthanoid ions in aqueous solution.