Determine extinction coefficients in glass for Fe2+/Fe3+

  1. 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:

  2. jcsd
  3. chemisttree

    chemisttree 3,723
    Science Advisor
    Homework Helper
    Gold Member

    Try solving for Fe+3 in terms of Fe+2 and total Fe.
     
  4. 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?
     
  5. Borek

    Staff: Mentor

    You said you know total Fe for the glass, so you can write an equation for the mass balance:

    Fe2+ + Fe3+ = Ftotal
     
  6. 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.
     
  7. the unknown variable are the ε for the two ionic species.
    so adding the mass balance equation does not add any value.
     
  8. chemisttree

    chemisttree 3,723
    Science Advisor
    Homework Helper
    Gold Member

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

  10. Nobody has an idea on what's wrong?
     
  11. chemisttree

    chemisttree 3,723
    Science Advisor
    Homework Helper
    Gold Member

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

    chemisttree 3,723
    Science Advisor
    Homework Helper
    Gold Member

    So you have the spectra in the form of wavelength and absorbance? Can't help you without raw data.
     
  14. 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!
     
  15. sorry for the delay. here is the file with the absorbance for both glasses.
     

    Attached Files:

Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

0
Draft saved Draft deleted