Electrode Potential: Manganate(VII) vs Dichromate(VI) for Fe(II) Titration

[Kushal]

Homework Statement

Suggest why manganate(VII), rather than dichromate(VI) is used to titrate with Fe(II) in volumetric analysis.

The Attempt at a Solution

i have figured out 2 possible answers but both may not be correct...

1. the colour change is sharper with manganate (purple to mostly colourless) while for dichromate it is orange to green... I'm not quite sure about it though...

2. The emf of the reaction with manganate(+0.75V) is higher than that for dichromate(+0.56V). But that shouldn't mean that the rate of reaction is faster, right?


i would have preferred dichromate because then i could use HCl as acid?! which i guess is more available than H2SO4... i don't know :-S

 

[Borek]

What about solubility of chromates/dichromates?

 

[chemisttree]

If you use dichromate and HCl, what color would all of the products be?

 

[Kushal]

i think dichromates/chromates are quite soluble. i remember a practical we did at school to find its Ksp and the solubility in water was high if I'm not wrong.

the solution will become green!

 

[Borek]

Ag₂CrO₄ is soluble weakly enough to be used for endpoint detection in Mohr method. I don't have exact tables here, but intuition tels me that iron chromates will be weakly soluble and their precipitation may to some extent interfere with the redox reaction.

Cr³⁺ is green, that's correct. Fe³⁺ is sligthly yellow. Will you be able to see added excess of dichromate? Compare it with the situation in the permanganate case.

 

[Kushal]

yeah that makes sense for the colour when chromate is added in excess!