# Determine extinction coefficients in glass for Fe2+/Fe3+

P: 9
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 soda-lime-silicate 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 Lambert-Beer 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)
Attached Files
 System_Solution.pdf (81.5 KB, 17 views)
 Sci Advisor HW Helper PF Gold P: 3,724 Try solving for Fe+3 in terms of Fe+2 and total Fe.
 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?
 Admin P: 23,533 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: Fe2+ + Fe3+ = Ftotal
 P: 9 You are right! I understand. But even if I substitute CFe3+ with (CFetotal - CFe2+), I don't see how it would solve the problem. At the end it is always the same value.
 P: 9 the unknown variable are the ε for the two ionic species. so adding the mass balance equation does not add any value.
 Sci Advisor HW Helper PF Gold P: 3,724 Sooo, you're not even going to try it my way?
 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 Fe2+ and Fe3+. You see that εFe3+ is negative, which doesn't have any physical meaning. Below you find the concentration data for both GlassA and GlassB. GlassA: CFe2+ = 0.056 wt% CFe3+ = 0.125 wt% CFetotal = 0.181 wt% GlassB: CFe2+ = 0.127 wt% CFe3+ = 0.285 wt% CFetotal = 0.412 wt% If I substitute in GlassA CFe3+ = CFetotal - CFe2+, I get the same value: 0.181 - 0.056 = 0.125 wt%. I would appreciate if you could keep helping me. Thank you. Attached Thumbnails
 P: 9 Nobody has an idea on what's wrong?
 Sci Advisor HW Helper PF Gold P: 3,724 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?
 P: 9 they both vary with wavelength. In the 2-equations system I have attached to my first thread, the unknowns are the two extinction coefficients.
 Sci Advisor HW Helper PF Gold P: 3,724 So you have the spectra in the form of wavelength and absorbance? Can't help you without raw data.
 P: 9 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!
P: 9
sorry for the delay. here is the file with the absorbance for both glasses.
Attached Files
 AbsExtCoeff.txt (37.2 KB, 4 views)

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