Anonymous asked in Science & MathematicsChemistry · 1 decade ago

Derive an expression for the concentration in terms of absorbance and molar extinction coefficient?

I think I need to get from Beer Lambert's law I = Io x 10^-e.c.l

to A=e.c.l

1 Answer

  • 1 decade ago
    Favourite answer

    The transmission of light through a sample is defined as I / Io:

    I / Io = 10^-e.c.l


    I (capital i) = intensity of transmitted light through the sample

    Io (capital i subscript zero) = intensity of transmitted light in the absence of the sample

    l (lower case L) = the path length of the sample through which the light passes

    c = the molar concentration of the absorbing species

    e = the molar absorption coefficient of the absorbing species

    Absorbance of light by the sample, A, is defined as:

    A = -log(I / Io)

    or A = log (Io / I)

    So taking logs of the first equation:

    log(I / Io) = -e.c.l

    -log(I / Io) = e.c.l

    Substituting for -log(I / Io) gives:

    A = e.c.l

    You need to keep track of your units throughout all your calculations, otherwise it's easy to come unstuck when you quote your final value for e.

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