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Re: Chemistry Question - Cr(VI)
Posted By: Bourne, on host
Date: Tuesday, October 9, 2001, at 01:16:53
In Reply To: Chemistry Question - Cr(VI) posted by Wolfspirit on Tuesday, October 9, 2001, at 00:21:06:

> 1. Take liquid chromium sample and alkalinize it to pH 12.5 to 13 using a sodium hydroxide base.
> 2. Add 1-2% (w/w) of 35% peroxide H2O2.
> 3. Then boil the solution 10 minutes to remove all traces of H2O2. Allow solution to cool to room temperature.
> 4. Acidify sample solution to between pH 2 and 3 using sulfuric acid.

I'd imagine all of this back and forth is to mince any organic ligands associated to the Chromium that are more stable than the developing agent/Chromium complex, or breaking down a Cr compound, thus freeing up the available Cr for a more accurate assessment (although you wont be able to tell what complexes were in there already and it can't just be Cr hexahydrate because that would have a charge of
-6 which is just mad). What kind of solutions are you testing (i.e. what industry/business are you in)?

> 5. Pipette small samples of solution into colorimetric vials, filling one with just plain solution to act as a zero "blank," and the other vial with solution + a color-developing agent. After 10 minutes, measure the colour absorption using the colorimeter to check for chromium concentration.

Which is pretty much standard for colorimetric or UV/visible spectroscopic analysis.
The Cr complexes to a ligand and the associated electronic changes cause a bright colour to form (c/f Potassium permanganate). Does it mention what the active agent is (can be instrumental in determining upper and lower limits of detection)?
This is also important for the actual wavelength that the complex concentration is measured at - certain organics/materials/solvents can absorb at different wavelengths and screw up a reading, so it needs to be distinct.

> Questions.
> 1) What are the mechanics, or oxidation states, of the oxidation/reduction mechanisms that are being involved (targeted) by this metholodogy?
2 things I need to know - whats in the solution at the start (ie Cr and what else? Or is it a Cr compound?), and...what is the ligand complexing to Cr?

> 2) What is the Limits of Detection (the range of validity) of this method? I'm *presuming* that it is a colorimetric technique.
UV/Vis spectroscopy can be extremely sensitive - as long as you form a very stable complex with the developing agent (i.e. the equilibrium between free Cr and complex is so far over to the complex side that there is almost no free Cr left) and that the complex itself has a distinct absorption wavelength that is free from interference due to other UV/Vis active chromophores present.

> 2) Is there a better (faster, cheaper) way of detecting Cr(VI) content?

Again, it depends a lot on what you're starting off with - I'll go into the library and look up some textbooks on it after I'm finished inthe lab today, but without much information, its hard to be decisive about it...
...spectroscopic methods are the mainstay of chemical analysis though, and as long as your preparation is done with care, then its one of the best ways to determine a concentration without seriously costly equipment.

It does worry me that your initial criteria for "better" are faster and cheaper - both are good, but not at the expense of accuracy. Be rigorous in your examinations and you'll minimise error.

Even us synthetic chemists with 400MHz NMR machines still do melting points!

> Cheers and Thanks for the help.
> Wolfspirit

If you can get more info on the process, maybe I can give you a hand.


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