[rahuldg11]

I have a natural rock sample, in which Fe is present in the +2 state, Cr is present in +3 state. I want to oxidise Fe to +3, such that Cr does not get oxidised. Please suggest the procedure.

[Hunter2]

Sulfuric acid and hydrogenperoxide mixture should work. Iron will be oxidised Chromium not.

[Enthalpy]

But how to let a rock react? Grind it before? Dissolve it somehow?

"natural rock sample, in which Fe is present in the +2 state, Cr is present in +3 state" can well be some silicate where Fe and Cr are only colouring elements.

[rahuldg11]

ya, rocks are grinded and dissolved using HF, HNO3 and HCl mixture. For Sulphuric acid and peroxide mixture, should I use concentrated sulphuric acid?

[phth]

ya, rocks are grinded and dissolved using HF, HNO3 and HCl mixture. For Sulphuric acid and peroxide mixture, should I use concentrated sulphuric acid?

Follow a written procedure, and if you don't have one look for one.