[Giger4444]

Hello everyone!

I need your help in solving a problem. Let me start by saying that Im not well educated in chemistry, I know some very basic things, but not much more, so I really appreciate all of your help in advance.

The problem at hand:
I need to find out the chromium content of a solution (a tanning bath to be exact). The exact compound they dissolve in this bath is Cr2O3 (the solution has a greenish-bluish tint). I tried to find some books at a local library and also look around on the internet, but Id like to be confident about the method. This is what I could put together so far:
-take a sample of the solution, dilute it (I can make out the extent of the dilution later)
-oxidize the Cr(III) into Cr(VI) by adding hydrogen-peroxide and sodium-hydroxide in excess (I can work out the amounts later).
Following reaction takes place: Cr[3+] + H2O2 + OH[-] = Cr[6+] + H2O
-heat the solution up till it boils to remove excess peroxide
-add some hydrochloric acid to it (I dont know how much exactly but I think it needs to be in excess) and a measured amount of Lugol (?) solution. Following reaction takes place: Cr[6+] + I[-] + H

• = I2 + Cr[3+] + H2O

-titrate the solution with sodium-thiosulfate with a starch-solution indicator: I2 +2 S2O3[2-] = S4O6[2-] + 2 I[-]
at the end-point, the yellowish-brownish tint disappears (?).

What I need help with:
-the reaction equations. The first two I wrote are just the broad strokes, but I need to know what happens exactly (for example, I dont know whats the formula of the dissolved and hydrated Cr2O3...)
-do I have to use Lugol solution? Does the end-point indication look like how I wrote it?
-In general what are your suggestions? Is the method basically okay or I should use something completely different?

I can offer some more details if needed, thanks in advance for your help.

[Borek]

Cr(VI) is either in a form of chromate (CrO₄^2-, high pH) or dichromate (Cr₂O₇^2-, low pH).

Lugol solution is just iodine in dissolved in potassium iodide.

See if information here: http://www.titrations.info/iodometric-titration doesn't help.

[Giger4444]

Okay, been looking around for a bit.
Are these equations good?
oxidizing alkaline Cr2O3 with peroxide in alkaline solution:

2 Cr(OH)4{-} + 3 H2O2 + 2 OH{-} = 2 CrO4{2-} + 8 H2O

Adding excess acid and excess Lugol's solution (KI+I2)?:

2 CrO4{2-} + 6 I{-} + 16 H{+} = 2 Cr{3+} + 3 I2 + 8 H2O

Titrating produced I2 with Na2S2O3 (starch solution indicator):

I2 + 2 S2O3{2-} = 2 I{-} + S4O6{2-}

So one more question: I cant wrap my head around the iodine-iodide-iodate stuff... what solution should I use for reducing the Cr(VI) to Cr(III)? Simple KI solution, or Lugol's (KI+I2) solution? What indicator should I use at the thiosulfate titration?

Thanks again for the help.

[Borek]

Reactions look OK.

I cant wrap my head around the iodine-iodide-iodate stuff... what solution should I use for reducing the Cr(VI) to Cr(III)? Simple KI solution, or Lugol's (KI+I2) solution?

Just KI.

What indicator should I use at the thiosulfate titration?

Starch. But don't add it early during the titration.

[Giger4444]

Thank you very much for the support! I will try out this method and report back if it was successful if others are interested.