November 28, 2021, 07:18:35 PM
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Topic: Why would current decrease with standard addition of Fe on AS voltammetry?  (Read 4302 times)

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Offline bboorriinnggg

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So I'm trying to identify a metal in 25 mLs of raspberry juice (hand-crushed) + a bit of KCl, and after running it on CV ultimately yielded unreadable results, I ran it on ASV.  There are two peaks.  Score!

I concluded that one of these peaks were iron (nevermind how I came to that conclusion).  So I spiked it with a certain concentration of Fe(III).  The signal reduced.  Okay, maybe it was a bit of bad luck, I thought.  I spiked it again, running it on the same conditions, deposition time, quiet time, etc.  Again, the signal reduced.  I up the concentration spiked.  Signal still reduces.  After several trials, I concluded that there is a definite correlative trend that the signal is being decreased with each iron spiked.

I'm pretty sure that the signal is supposed to INCREASE, but that's not happening in this case.  There is a possibility that the peak I chose is NOT actually iron, but this same peak is being lowered with each successive spike.  Could there be some sort of reaction that is taking place that I'm not aware of such that the existence of more irons could actually decrease the signal?  Maybe it's Fe2O3 that is somehow decreasing the current, but I would imagine successive increases of ferric oxide would still increase the signal in some way.  Could it have been that I introduced some iron-sucker with my standard additions?  I am completely flabbergasted.

I'm using a gold working electrode with a platinum counter electrode, and am using Ag/AgCl as my reference electrode.  Any ideas on why this would be happening would be really, really cool.

Offline Borek

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So I'm trying to identify a metal in 25 mLs of raspberry juice (hand-crushed) + a bit of KCl, and after running it on CV ultimately yielded unreadable results

Does it mean solution is full of organic compounds?

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I concluded that one of these peaks were iron (nevermind how I came to that conclusion).

It matters - data behind your conclusion can point to the real substance.

Not that I have any ideas.
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Offline bboorriinnggg

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Indeed, there's a lot of random components in the matrix from carbohydrates to pigments--but the voltammogram is definitely showing a peak.

As far as why I concluded it to be iron, I checked out redox potentials from the back of my textbook and ran another ASV with all the metals that added up--namely copper and iron.  Ran it in a more dilute form of raspberry juice to try to mimic the matrix while lowering the naturally occurring iron in raspberry, and of course, spiked it with KCl.  Then spiked it like crazy with copper and iron.  The potential at which this iron peaked was very similar, if not equal, to the solution that I want quantified.  Long story short, it's most likely iron.

That aside, and assuming this peak represents iron, I just can't figure out why the presence of more iron would lower response?  In what circumstances would a metal decrease signal?  Does this have anything to do with the raspberry matrix?  There is only one other noticeable peak in the voltammogram, and that's remained constant throughout all the spiking. 

It's been rattling my brain all week.  Haha

Offline Borek

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The only (more or less) obvious thing I can think of is that you are observing not iron, but something from the matrix - something that reacts with iron, hence iron addition lowers the signal.

It just occured to me - if that's the case, try to extrapolate how much iron you have to add to lower the current to zero. Try that and then add some excess. If the peak reappears, you know what is going on.
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Offline bboorriinnggg

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I did think, at one point, that there was some kind of organic 'iron absorber' within the matrix.  But then I said it was improbable that some organic species of significant concentrations would ionize itself and respond to exactly the same potential as Fe(III).  It's not something I would rule out... but because this signal is the representation of an oxidized species, it's hard to imagine any cationic organic structure that would bind itself to Fe(III) and still respond to anodic stripping.  I'm terrible at electrochemistry, so feel free to correct any errors I've made.

From that I did conclude that it was ferric oxide that was, perhaps, impeding the current somehow.  But even then, I would think that the abundance of more ferric oxide ions would be offset by Fe(III) ions, and so signal should still increase. 

In any case, the voltammetry equipment is out of my hands now.  I would literally do the whole thing over if I had the time, but with all the budget cuts going on in the state, I probably will not touch a voltammeter again in a long, long while. 

But I can't rule out what you said, and I'll probably spend a lot of time on Google looking for any compounds that would bind to these ions.   :-\

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