Precipitation of NH4UO2PO4·3H2O—Solubility and Structural Comparison with Alkali Uranyl(2 +) Phosphates

Precipitates formed in the system UO 2 (NO 3 ) 2 -NH 4 OH-H 3 PO 4 -H 2 O, aged for 30 days at 298 K, were studied. The precipitates were characterized by chemical and thermogravimetric analyses, x-ray powder diffraction, infrared spectroscopy, polarized light microscopy, and by their fluorescent properties. The precipitation boundary was established tindallometrically and microscopically. On the basis of these measurements, the stability conditions, structural parameters, and solubility of the tetragonal polymorph of NH 4 [UO 2 PO 4 ]·3H 2 O were determined. This compound shows a close structural relationship with H 3 O[UO 2 PO 4 ]·3H 2 O (space group P 4 / ncc ) and alkali uranyl(2+)phosphates polyhydrates M[UO 2 PO 4 ]· n H 2 O ( n =4 for M=Li; n =3 for M=Na, K, Rb and n =2.5 for M=Cs). The unit-cell dimensions determined for NH 4 UO 2 PO 4 ·3H 2 O are: a = b =7.02 Å, c =18.08 Å ( P 4 / ncc ). The thermodynamic solubility product constant, K s =a (NH 4 + )× a (UO 2 2+ )× a (PO 4 3− ), for NH 4 UO 2 PO 4 ·3H 2 O was determined: log K s = −26.50±0.09. The K s values of M[UO 2 PO 4 ]· n H 2 O (at ionic strength, I =0.23 mol dm −3 ) calculated from previously published experimental data by using correct stability constants of uranyl(2+)phosphate complexes are: log K s =−22.61±0.08 for M=Na; log K s = −23.92±0.12 for M=K; log K s = −24.13±0.19 for M=Rb; log K s = −23.80±0.20 for M=Cs; and log K s = −24.74±0.10 for M=NH 4 , showing that NH 4 UO 2 PO 4 ·3H 2 O is less soluble than corresponding alkali uranyl(2+)phosphates.