Abstract :
The equilibrium speciation in the pH+ + q MoO4 2- + rH2O2 + sH2P04- + tSOl- (rightwards harpoon over leftwards harpoon ) (H+)p(MoO42-)q(H2O2)r(H2PO4-)s(SO4 2-) (p-2q-s-2t , t) system in 0.300 M Na2(SO4) medium at 25 °C and [H2O2]tot/[Mo]tot > 2 has been determined from potentiometric data and 31P NMR integral and chemical shift data in the range 1.0 =< pH=< 9.0, 2.0 =<[MoO4 2 -]tot/mM =<293, 32=<[H2O2]tot/mM =< 640, 2=< [H2PO4-]tot/mM =< 60 and 200=< [SO4 2-]tot/mM =< 380. Species with the following compositions were obtained: MoX2P3-(0,1,2,1,0), MoX2P2-(1,1,2,1,0), MoX2P-(2,1,2,1,0), Mo2X4P3-(2,2,4,1,0), Mo2X4P2(3,2,4,1,0), Mo3X6P3-(4,3,6,1,0), Mo3X6P2-(5,3,6,1,0) and Mo4X8P3-(6,4,8,1,0). The numbers and charges of molybdenum (Mo), peroxide (X), phosphate (P) and sulfate (S) in each species are given in the abbreviated formula MoqX,P,S(p-2q-s-2t , t). The numbers in parentheses refer to the values of p, q, r, s and t in the formula above. The following formation constants with 3(sigma) were obtained; log(beta)0,1,2,1,0= 5.16 (plus-minus) 0.09, log(beta)1,1,2,1,0= 12.73 (plus-minus) 0.02 (pKa= 7.63), log(beta)2,1,2,1,0= 16.14 (plus-minus) 0.03 (pKa= 3.42), log(beta)2,2,4,1,0= 25.03 ((plus-minus) 0.04), log(beta)3,2,4,1,0= 29.54 (plus-minus) 0.02 (pKa= 4.51), log(beta)4,3,6,1,0= 42.30 (plus-minus) 0.03, log(beta)5,3,6,1,0= 44.06 (plus-minus) 0.08 (pKa= 1.76), log(beta)6,4,8,1,0= 57.30 (plusminus) 0.07. pKa values for H3PO4 and H2PO4- weredetected by 31P NMR 2D EXSY and selective magnetisation transfer experiments between the complexes. An averaged lifetime, (tau)(X2Mo-OP) ~30 ms, can be estimated for the MoX2P and Mo2X4P species at pH = 5.5 and 5 °C.
