At low pH vanadium(V) is present as VO2+, while in neutral and alkaline solution it is present as tetrahedral HnVO4(3−n)−. The dominating vanadium(V) species in weak acidic solution, 2 ≤ pH ≤ 6, is the decamer [V10O28]6−. The acidic constants of vanadic acid, H3VO4, pKa1 = 3.5, pKa2 = 7.8, and pKa3 = 12.5, respectively, resemble closely those of phosphoric acid, H3PO4, pKa1 = 2.1, pKa2 = 7.2, and pKa3= 12.7.
Vanadium(IV) starts to hydrolyze and dimerize at pH ≈ 4; the pKa value of the [VO(H2O)5]2+ ion has been reported to be in the range 5.3–6.0. At pH = 5 insoluble {VO(OH)2}n starts to form, which converts to water soluble [(VO)2(OH)5]− and [VO(OH)3]2− complexes in strong alkaline aqueous solution. (taken from literature)
If you are interested in metal ion hydrolysis i suggest to look for the book of Charles F. Baes, Robert E. Mesmer "The hydrolysis of cations" Wiley, 1976 - 489 pages where you will find very detailed information about the hydrolysis all cations including the V(V) and V(IV).
@Gyula Tircso can you provide the pdf or scan copy of these pages of book?
@Michael Päch can you explain how to prepare Pourbaix diagram in detail ?
I have the copy for you, i need your e-mail address, so that i can send it over.
For your possible interest, you may also want to check my post at another discussion of this forum, addressing vanadium(V) trisisopropoxide oxide (VTIP) hydrolysis: https://www.researchgate.net/post/How_can_one_obtain_pure_V2O5_films_by_the_sol-gel_method_using_vanadium_tri-isopropoxide
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