[ganesh2gig]

Hi all

I am doing a Chromium Sulfate Estimation that is by oxidation with Persulfate and by addition of ammonium ferrous sulfate and again titrating it with Potassium Dichromate .

The procedure is given in Vogel fifth edition .

I am stuck with the calculation though , can somebody please help me with it .


The procedure is posted here

http://imageshack.us/photo/my-images/225/chromium.gif/

[Borek]

Show what you did so far.

[ganesh2gig]

The Primary Equation in the original post is all about converting Cr3 to Cr6 . How ever it is not important part in calculation

I have done this

Cr207 + 6Fe+14 H= 2Cr+6Fe+7h2o.

Here Cr6 is converted to Cr3 , and Fe2 becomes Fe3 .

On calculation part

If I use 0.02 M Feso4 for the volumetric analysis  and 30 ml is consumed
It would be 30*0.02 which would be 0.7 moles of chromate .

As per the Equation one mole of chromate will oxidize 6 moles of Fe2,so 6*0.7 =4.5 moles of Fe2 .

I have done till here .

[ganesh2gig]

Just let me know , If it is in the right direction .

[Borek]

If I use 0.02 M Feso4 for the volumetric analysis  and 30 ml is consumed
It would be 30*0.02 which would be 0.7 moles of chromate .

You are on the right track, but things you do to math and units are horrible. 30*0.02 is not 7, and these are not moles.

[ganesh2gig]

Any how we are calculating the amount of dichromate required to convert fe2 to fe3 .

I can convert cr3 to cr6 and directly titrate against known volume of Fe2so4 .

Then I can calculate the molarity  of the original chromium solution

M Fe2so4 * vol Fe2so4 /Vol of dichromate  .


Can this be done ?

[AWK]

What does mean Fe2so4

[ganesh2gig]

Ferrous sulfate

where Fe is +2 form , after it is oxidized it becomes Fe3+

I should have mentioned it earlier .

[Borek]

Ferrous sulfate formula is not the one you wrote.

Any how we are calculating the amount of dichromate required to convert fe2 to fe3

From the stoichiometry of the reaction.

Basically it is a back titration. You should also take a look at the titration result calculation page.

[ganesh2gig]

Can this approach work ?

The whole idea of the estimation of Chromium is based on the ability of Cr6 to oxidise Fe2 to Fe3 .
 
I will convert the Cr3 to Cr 6 and directly titrate against Ferrous Ammonium Sulfate .

Then calculate the Normality of the Unknown Chromium Solution .

From this point I can know the amount of Chromium present in the sample .