The Beer-Lambert LawWhat the Law looks likeYou will find that various different symbols are given for some of the terms in the equation - particularly for the concentration and the solution length. I'm going to use the obvious form where the concentration of the solution is "c" and the length is "l".
| Note: That's obviously "l" for length. The font I'm using won't distinguish between "l" for length and a capital letter "I" (for Intensity). That problem disappears in the equation below - where it is obvious which is which.
You should recognise the expression on the left of this equation as what we have just defined as the absorbance, A. You might also find the equation written in terms of A:
That's obviously easier to remember than the first one, but you would still have to learn the equation for absorbance. It might be useful to learn it in the form:
The Greek letter epsilon in these equations is called the molar absorptivity - or sometimes the molar absorption coefficient.
Molar absorptivityIf you rearrange the simplest of the equations above to give an expression for epsilon (the molar absorptivity), you get:
Remember that the absorbance of a solution will vary as the concentration or the size of the container varies. Molar absorptivity compensates for this by dividing by both the concentration and the length of the solution that the light passes through. Essentially, it works out a value for what the absorbance would be under a standard set of conditions - the light travelling 1 cm through a solution of 1 mol dm-3.That means that you can then make comparisons between one compound and another without having to worry about the concentration or solution length.Values for molar absorptivity can vary hugely. For example, ethanal has two absorption peaks in its UV-visible spectrum - both in the ultra-violet. One of these corresponds to...
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