The phosphate complexation of Np(V) is investigated in the pH range fr
om 6.4 to 10.0 in 0.1 M NaClO4 at 22+/-2 degrees C by absorption spect
roscopy. The formation of two phosphate complexes is confirmed by spec
troscopic speciation: NpO2HPO4- at 988.8 nm and NpO2PO42- at 993.6 nm
with molar extinction coefficients of 251+/-8 and 201+/-11 I/mol cm, r
espectively. The complexation constants are determined to be log beta
= 2.54+/-0.02 for NpO2HPO4- and log beta = 6.33+/-0.08 for NpO2PO42-.
The competition of the phosphate complexation with the carbonate compl
exation is demonstrated by speciation calculation under different geoc
hemical conditions.