reaction between Iron(ii) and Vanadium(v)
I think I realize (a part of) the answer, concerning the conditions of a redox titration of vanadium(V), for which a metavanadate, such as $\ce{NH4VO3}$ is typically used as starting material. Under acidic conditions, i.e. in the presence of 85% $\ce{H3PO4}$, the colourless $\ce{VO2+}$ is formed.

$\ce{VO3- + 2 H+ -&gt; VO2+ + H2O} $ 

Again under acidic conditions, the latter species is reduced by Fe(II) to the blue $\ce{VO^{2+}}$.

$\ce{VO2+ + Fe^{2+} + 2 H+ -&gt; VO^{2+} + Fe^{3+} + H2O} $

Is is conceivable that the peroxodisulfate is used to oxidize the excess of Fe(II), which otherwise would be oxidized by the permanganate?