Untersuchungen über Isopolywolframate

Abstract

Evidence for the presence of OH-groups in solid alkaline meta-, para-B- and ψ-metatungstates is given by infrared and 1H-nuclear magnetic resonance spectra. The three tungstates have to be formulated as MI 6[W12O 38 (OH) 2] • x H2O, MI10 [ W12O36 (OH) 10] • x H2O and MI 12 [W24O72 (OH) 12 • x H2O respectively. The mole masses of the different polytungstates were calculated from the speed of sedimentation in the ultracentrifuge. In the concentration range covered, 10-2 > Cwo4 2Ⓡ > 10-4, the same ions occur as in the solid state. The effect of aging and heating on polytungstate solutions was studied by means of pH-measurements, by pH-titrations, by ultraviolet spectra, and by the T-jump relaxation method. The para· tungstate-A-ion [W6 20(OH)]5® formed upon acidification of a H®/WO4 2® = 1,167 ratio solution according to the equation 6 WO4 2® + 7 H® ⇄ [W6O20(OH)]5® [log K ∼ 57,5] is in equilibrium with the paratungstate-B-ion [W12O36(OH)10] 10®. In a solution of solid paratungstate-B the same equilibrium is attained. Upon aging and, more rapidly, by heating the para-A-/para-B-solution in equilibrium changes to -ψ-meta and finally to metatungstate. During this process an increase in the pH-value is observed due to the reduced acidity of the para-B-ion and to the enhanced condensation of para-(probably of para-B only) to ψ-meta and further to metatungstate. Upon acidification to the H®/WO4 2® ratio Ρ=1,50 the ψ-meta-ion is formed quantitatively which changes to the metaion slowly. For P=1,50 characteristic ultraviolet absorption bands indicate the formation of a new polytungstate species [ W12Ox(OH)y]72— (2x+y), called polytungstate-Y. It changes rapidly and quantitatively to ψ-meta-tungstate. In aged solutions with P ≧ 1,50 only the meta-ion is found. In solutions with Ρ < 1,50 WO4 2e, polytungstate-X-ions were found besides meta-ions. The relative concentration of these ions as a function of Ρ is given. The so-called X-species is probably the para-B-ion. By means of a scheme, the reactions upon acidification of a monotungstate solution are illustrated.