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Determine extinction coefficients in glass for Fe2+/Fe3+ 
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#1
Aug2813, 03:14 AM

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Hello everybody,
I want to determine the extinction coefficients of Fe2+ and Fe3+ in glass. There are literature data (e.g. Weyl's book "coloured glass"), so I know what kind of curves I should expect. As I am studying a slightly different sodalimesilicate system, I want to recalculate the curves. I have glasses with different Fe2+/Fe3+ ratio and total iron concentration [Fe]. I have measured the absorbance of two glasses and I have solved a simple two equations system starting from LambertBeer law: A = Ʃ εCd where A is absorption, ε is the extinction coefficient, C is the concentration and d is the thickness of my glass. Unfortunately, I get negative values in one of the two extinction coefficient curves. Obviously, this doesn't make sense. Anyone sees what is wrong in my reasoning? I can't figure it out. Thank you very much in advance, Mark PS For a close look at the system I have made, see the attachment (.pdf) 


#2
Aug2813, 11:24 PM

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Try solving for Fe+3 in terms of Fe+2 and total Fe.



#3
Aug2913, 01:50 AM

P: 9

Thank you chemisttree for replying.
However, I don't get what you mean. How can I solve in terms of total Fe? In the system I have the concentrations of the two absorbing species: Fe3+ (CFe3+) and Fe2+ (CFe2+) Where should total Fe appear in the equations? 


#4
Aug2913, 02:44 AM

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Determine extinction coefficients in glass for Fe2+/Fe3+
You said you know total Fe for the glass, so you can write an equation for the mass balance:
Fe^{2+} + Fe^{3+} = F_{total} 


#5
Aug2913, 05:41 AM

P: 9

You are right! I understand.
But even if I substitute C_{Fe3+} with (C_{Fetotal}  C_{Fe2+}), I don't see how it would solve the problem. At the end it is always the same value. 


#6
Aug2913, 06:58 AM

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the unknown variable are the ε for the two ionic species.
so adding the mass balance equation does not add any value. 


#7
Aug2913, 03:46 PM

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Sooo, you're not even going to try it my way?



#8
Aug3013, 02:12 AM

P: 9

I am sorry chemisttree, maybe I hadn't explain well myself. I didn't want to be disrespectful.
I have tried to use your advice. But I don't see how. Substituting ferric concentration with the subtraction of ferrous from total iron concentration, the final solution remain unchanged. I attached the spectra of the two glasses I am using. Also, I have plot quickly (read as "I haven't add units and axis names") the absorption coefficients I obtained for Fe^{2+} and Fe^{3+}. You see that ε_{Fe3+} is negative, which doesn't have any physical meaning. Below you find the concentration data for both GlassA and GlassB. GlassA: C_{Fe2+} = 0.056 wt% C_{Fe3+} = 0.125 wt% C_{Fetotal} = 0.181 wt% GlassB: C_{Fe2+} = 0.127 wt% C_{Fe3+} = 0.285 wt% C_{Fetotal} = 0.412 wt% If I substitute in GlassA C_{Fe3+} = C_{Fetotal}  C_{Fe2+}, I get the same value: 0.181  0.056 = 0.125 wt%. I would appreciate if you could keep helping me. Thank you. 


#9
Sep413, 01:45 AM

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Nobody has an idea on what's wrong?



#10
Sep413, 12:15 PM

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For the attached spectra, what value of extinction coefficient did you use for Fe+2 and Fe+3? Did you hold the extinction coefficient for Fe+2 constant? Or did it vary with wavelength?



#11
Sep413, 12:34 PM

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they both vary with wavelength. In the 2equations system I have attached to my first thread, the unknowns are the two extinction coefficients.



#12
Sep413, 12:45 PM

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So you have the spectra in the form of wavelength and absorbance? Can't help you without raw data.



#13
Sep513, 12:40 AM

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Yes, my data are absorbance as a function of wavelength. NOw I don't have access to the hard drive where data data are. I will post the raw data later on today so you can have a look.
thank you again! 


#14
Sep613, 03:26 AM

P: 9

sorry for the delay. here is the file with the absorbance for both glasses.



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