An equilibrium analysis of the aqueous H⁺–MoO₄²⁻–(HP)O₃²⁻and H⁺–MoO₄²⁻–(HP)O₃²⁻–HPO₄²⁻systems

Abstract

The speciation in the phosphitomolybdate system, H+-MoO4(2-)-(HP)O(3)2-, has been determined from combined potentiometric and 31P NMR measurements in 0.600 M Na(Cl) medium at 298(1) K. Potentiometric titration data were collected in the ranges 2.5<-log[H+]<6.2, 40.0<or=[Mo]tot<or=240.0 mM, 10.0<or=[P]tot<or=40.0 mM and 1<or=[Mo]tot/[P]tot<or=10. 31P NMR data were collected in the same ranges with the exception that -log[H+] was decreased to 0.8. The pKa values of phosphite were determined by means of potentiometric (1.5<-log[H+]<8.6) and 31P NMR chemical shift data (0.0<-log[H+]<12.0). The following log beta (3sigma) values were deduced: H(HP)O3-, 6.09(2); H2(HP)O3, 7.23(4) (pKa=1.14). In the phosphitomolybdate system, the following complexes were found along with their formation constants ((HP) denotes phosphite): Mo5(HP)2(4-), 65.31(4); Mo6(HP)(2)5-, 71.94(5); Mo5(HP)4-, 51.25(8); Mo5(HP)3-, 54.9(3); Mo6(HP)2-, 68.99(9); Mo7(HP)5-, 71.25(3); Mo7(HP)4-, 74.2(2). Because hydrolysis and oxidation of phosphite to phosphate were observed by NMR, with a mixed-hetero species of low [Mo]tot/[P]tot ratio being detected, the four-component H+-MoO(4)2--(HP)O3(2-)-HPO(4)2- system was also studied in the same way over the range 0.8<-log[H+]<6.2 and [Mo]tot/[P]tot fixed at 1.5. The following mixed heteropolyanion species have been found (P denotes the phosphate group): Mo5(HP)P5-, 62.27(6); Mo5(HP)P4-, 66.76(4). The equilibrium speciation in the two systems are illustrated by distribution diagrams. In addition, the aqueous structures are proposed for all the species formed in the systems.

Polyoxometalates in solution: speciation under spotlight

“…173 The confusion in the literature about the existence of mono-{PMo 11 } and trilacunary {A-PMo 9 } complexes is understandable, since their spectroscopic characteristics ( 31 P NMR shifts and Raman spectra ( , has been determined from both potentiometric and 31 P NMR measurements. 174,175 In the system with phosphite, phenyl-and methylphosphonate anions structures similar to the H x [P V 2 Mo VI 5 O 23 ] (6Àx)À (x = 0-2) anion 169 (Fig. 14A) dominate.…”

“…14A) dominate. 174 It was shown that partial oxidation of phosphite to phosphate occurs, especially in the thin-walled NMR sample tubes when exposed to fluorescent light. In {X 2 M 5 }-type HPO-Mos the phosphite anion HP III .…”

Vanadium(V)-Substitution Reactions of Wells–Dawson-Type Polyoxometalates: From [X2M18O62]6− (X = P, As; M = Mo, W) to [X2VM17O62]7−

“…Currently, the synthesis of metal-substituted or metal-incorporated POMs can be controlled to some extent through the use of the lacunary POMs. In addition, details of the formation of some of POMs have been investigated using NMR and Raman spectroscopy and cyclic voltammetry [33][34][35][36][37][38][39][40][41][42][43]. However, many aspects of the formation of POMs in solution remain ambiguous.…”

Synthesis of molybdo-methylphosphate and -ethylphosphate with inverted-Keggin structure

“…Many polyoxometalates containing XO 4 -type hetero anions such as PO 3À 4 have structures based on Keggin or Dawson-type structures. On the contrary, heteropolymolybdates containing RXO 3 -type hetero anions such as PHO 2À 3 and organophosphates, which were prepared in an aqueous solution, have been bounded to complexes of compositions of Mo/X = 5/2, 6/1, and 7/1 [1,[3][4][5][6][7][8]. Recently, novel RXO 3 -type hetero-ions, such as PHO 2À 3 and SeO 2À 3 , containing heteropolymolybdates of compositions of Mo/P(Se) = 15/2, 12/2, and 16/4 were isolated from acidic aqueous-organic solutions, and characterized with IR and Raman spectroscopy and cyclic voltammetry [9,10].…”