Determine extinction coefficients in glass for Fe2+/Fe3+

[ACvub]

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)

 

[chemisttree]

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

 

[ACvub]

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?

 

[Borek]

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

Fe²⁺ + Fe³⁺ = F_total

 

[ACvub]

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.

 

[ACvub]

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

 

[chemisttree]

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

 

[ACvub]

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 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.

 

[ACvub]

Nobody has an idea on what's wrong?

 

[chemisttree]

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?

 

[ACvub]

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

 

[chemisttree]

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

 

[ACvub]

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!

 

[ACvub]

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