Peroxomolybdates have shown to be efficient selective agents in the degradation of lignin in non-chlorine based bleach processes of kraft pulp. Furthermore, the process can be improved when anions such as phosphate are present. To clarify the chemistry in aqueous solution, fundamental speciation studies of relevant systems have been made under conditions similar to those in the bleaching step.
In this article the equilibrium speciation in the system pH+ + qMoO42− ⇆ (H+)p(MoO42−)q in 0.300 M Na2(SO4) medium at 25 °C has been studied using potentiometric data in the range 2.5 <- pH <- 6.0, 1.25 <- Motot/mM <- 20. The speciation was found to consist of the monomers MoO42− (0,1), HMoO4− (1,1), H2MoO4(2,1) and the heptamers Mo7O246−(8,7), HMo7O245−(9,7), H2Mo7O244−(10,7) and H3Mo7O243−(11,7) (numbers in parentheses refer to the values of p and q in the general reaction above). The following formation constants and 3σ were obtained: log β1,1 = 3.40 ± 0.10, log β2,1 = 7.79 ± 0.06 (pKa = 4.39), log β8,7 = 52.43 ± 0.04, log β9,7 = 57.42 ± 0.03 (pKa = 4.99), log β10,7 = 61.24 ± 0.04 (pKa = 3.82) and log β11,7 = 63.90 ± 0.10 (pKa = 2.66). The pKa value for HSO4− was determined to 1.27 ± 0.01. The effects of different ionic media on this system are discussed.
Finally, this article presents some preliminary results of the ongoing speciation studies in the H+ - MoO42− - H2O2 and H+ - MoO42− - H2O2 - H2PO4− systems.
Springer Netherlands , 2001. 161-73 p.