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To calculate absorption coefficient

  1. May 11, 2007 #1
    want to calculate absorption coefficient or absorption crosssection for 1% C2H2 gas

    Path length = 1meter

    Absorbene = 0.32

    concentration = 8500ppm

    pressure = 1 atm

    Temperature = 25C

    I need to express this concentration to mol/cm3.

    Pls let me know if u have anyidea
    Last edited: May 11, 2007
  2. jcsd
  3. May 11, 2007 #2
    Question is not clear.
    Which absroption coefficient, microwave, UV, visible light, fast electrons, neutrons ... ?
    What do you mean by Absorbene ?
    Concentration 8500 ppm of what?
    1% of C2H2 whitihn what, air, N2, CO2, ... ?
    What do you mean by "this concentrations", what are you reffereing to?

    Making the question clearer could help us, and may you too.
  4. May 11, 2007 #3
    I want to calculate absorption cross-section by using Beer-Lamberts law for Acetylene gas at NIR region.I. The calibration done at 1531.6nm.

    Absorbance= log to base10(intensity of incident light/intensity after passing through gas cell) = 0.32 ;;;;gas cell filled with Acetylene gas

    1% C2H2 gas(diluted with N2)

    When I used concentration of 8500ppm of 1% Acetylene gas , I got Absorbence of = 0.32. So, I need to calculate absorption cross section from Beer`s Law.
    Absorbance = alpha * concentration * length , http://en.wikipedia.org/wiki/Beer-Lambert_law

    here alpha = absorption cross section

    I think, I need to express absorption cross section in cm2/mol. So If i can able to express the concentration of acetylen gas which is in parts per million to mol/cm3 then I can able to get absorption cross sectin in cm2/mol.If u have any other idea pls let me know.

    IF u have any other questions pls let me know


    waiting for ur reply.
    Last edited: May 11, 2007
  5. May 12, 2007 #4
    I begin to guess what you are asking for.
    However, your question is still very badly formulated.

    I think you should tell us what you have measured in very precise and clear words.
    Then you should explain us what you want to calculate exactly.
    If you want you can refer to the wiki page your mentioned.

    You should also explain the background of this question.
    Shall we assume that only Acetylene is responsible for absorption at 1531.6nm?
    Does nitrogen play no role in absorption?

    Please, think that the physics is the imprtant think to understand.
    Applying a formula is really too simple.
  6. May 12, 2007 #5

    Thanks for ur reply

    We assume only Acetylene is responsible for absorption.No need to worry about N2.

    expt details

    I used HOllow core Photonic crystal fiber(HC-PCF) as path length,NIR Laser diode(NIR-1531.6nm ) as light source and photo detector for detection of light.
    At 1531.6nm Acetylene has strong absorption.Initially filled the HC-PCF with Nitrogen and measured the intensity(I0), then replaced HC-PCF with Acetylene gas(concentration-8500ppm) and measured change in the intensity(I). From that calculated the Absorbance and we got A= 0.32.
    Now we need to
    calculate absorption cross section
    for this A= 0.32,
    with length =100cm,
    and concentratrion of ACetylene gas (8500ppm)
    at pressure = 1atm, T = 25C,

    Pls let me know any other details required


    waiting for ur reply.
  7. May 12, 2007 #6
    I don't see where the problem is. You have from the definition

    alpha = A/l/c

    The concentration c cannot be entered in this formula as ppm, of course.
    I assume that by ppm you meam ppm in volume of gas.
    You need therefore to specifify the pressure and the temperature.

    For a standard pressure (1 atm) and temperature, you know that 1 mole occupies 0.022414 m³.
    If the pressure or the temperature are not the standard one, you need to use the perfect gas law to calculate the volume of a mole.
    In standard conditions, a concentration c=8500ppm means that there is 0.008500 mole of acetylene in a volume of 0.022414 m³. So the concentration is 0.0085/0.022414 mole/m³.
    From this you can calculate alpha.

    Note that you may need to correct what I did here above ...
    because I did not understand what you meant by "... 8500ppm of 1% Acetylene gas ...".
    What did you do actually? Can't you explain that clearly?
    Should I guess that the absorber gas had a real concentration of 85ppm acetylene ????
    As I did not participate in your experiment, you should understand that I need a clear description of what you did.

    My guess is that the initial gas in the device was air.
    By some unknown mean you manage to introduce 8500 ppm of a gas mixture GM.
    This gas mixture GM has a concentration of 1% acetylene (1% in volume, I guess).
    If this guess is correct then the concentration of acetylene in the device is 85 ppm, and this value should be used to calculate alpha, instead of 8500 ppm.

    99% of a scientific job is good communication (bidirectional)
    Last edited: May 13, 2007
  8. May 17, 2007 #7
    Hi. Thanks a lot for ur reply. I calculated the absorption cross section.Pls let me know if it right

    pls forget about that 1% just assume concentration is 8500ppm

    A = 0.32, P = 1 atm, R = 0.082 liter * (atm/mol) * * deg k

    length = 100cm, T = let say (294K ) --- assuming 21C

    concentration = 8500ppm;

    By using PV = nRT ;;; 1 mole = 24.15 liter

    1 liter of gas = 1/(24.15moles)

    Total no. of moles in 1 liter

    = 8500/(10power6 total moles) = 0.0085

    Amount of moles of C2H2 in 1 liter of (1/(24.15) moles per liter * 0.0085 = 0.00035moles

    Molecular weight = 26

    = 0.00035moles/liter * 26 gms/mol = 9.1mg/cm3

    therefore alpha = A/(length * concentration)

    ===== (0.32)/(100cm * 9.1mg *cm-3)

    ======== 3.9 * 10 power -4 cm2/mg
  9. May 18, 2007 #8

    Is the calculation part is o.k or I need to change the units
  10. May 18, 2007 #9
    or also I can express in absorption coefficient(cm-1)
  11. Aug 25, 2009 #10
    how to calculate absorption coefficient for GaN and which measurement what I need?
  12. May 20, 2010 #11
    I also want to know .that I want to calculate absoption coefficient of silve.inface I have hollow core with silver coating and I want to calculate the losses using beer´law for 5 nm wave lenght through 5cm long fiber.please also letz me know how can I calculate.
  13. Aug 15, 2011 #12
    Re: Calculation of extinction coefficient

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  14. Apr 3, 2012 #13


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    Am a mechanical student. Currently working on Nanofluids. I wanted to know what will be the refractive index of these oxides mixed with distilled water.
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