Path length and absorbance relationship help

Beer's Law - Theoretical Principles

path length and absorbance relationship help

You can derive this equation from Beer's law (Absorbance = e L c) that Beer's Law is valid by looking for a straight-line relationship for the data. Question: What is the concentration of a cm (path length) sample that has an absorbance of . Feb 10, The absorbance is directly proportional to the length of the light path Assumption one relates the absorbance to concentration and can be. absorbance is described in Beer's Law*. (1). [X] α A concentration, the absorbance of the solution increases a specific relationship is also called the Beer-Lambert Law or the Boulguer-. Beer Law. versions of this principle, and assume a constant light path length . helps to even out experimental error, and helps with.

However, since the units of molar absorptivity is always the above, it is customarily reported without units.

The Beer-Lambert Law

Guanosine Guanosine has a maximum absorbance of nm. What is the concentration of guanosine?

path length and absorbance relationship help

What is the extinction coefficient? Suppose you have got a strongly colored organic dye.

Chem - Experiment II

If it is in a reasonably concentrated solution, it will have a very high absorbance because there are lots of molecules to interact with the light. However, in an incredibly dilute solution, it may be very difficult to see that it is colored at all. The absorbance is going to be very low. Suppose then that you wanted to compare this dye with a different compound.

Emission and Absorption Spectra

Unless you took care to make allowance for the concentration, you couldn't make any sensible comparisons about which one absorbed the most light. The absorbance is not likely to be very high.

path length and absorbance relationship help

On the other hand, suppose you passed the light through a tube cm long containing the same solution. More light would be absorbed because it interacts with more molecules. Again, if you want to draw sensible comparisons between solutions, you have to allow for the length of the solution the light is passing through.

Both concentration and solution length are allowed for in the Beer-Lambert Law. Molar absorptivity compensates for this by dividing by both the concentration and the length of the solution that the light passes through.

path length and absorbance relationship help

Essentially, it works out a value for what the absorbance would be under a standard set of conditions - the light traveling 1 cm through a solution of 1 mol dm The calibration plot is like taking a vertical slice through the all the absorbance spectra at the specific wavelength nm. The wavelength nm was a very good choice for the calibration plot, but how do you know which wavelengt is the best wavelength, based on the absorbance spectrum?

Choosing Your Wavelength Look at the images above.

path length and absorbance relationship help

The left is an absorbance spectrum of 0. Is the slope of the calibration line at nm greater than, less than, or equal to the slope at nm? You can choose any wavelength to create a calibration plot, the only differerence will be the slope of the line.

When you actually choose your wavelength to create your calibration graph, you would generally like to choose a wavelength where there is room for the concentration to decrease.

Beer's Law Tutorial

Look at the spectrum above. Do you think nm would be a good wavelength to use for a calibration graph? You would not choose that wavelength because when you lower the concentration, you would not be able to see much of a difference in the absorbance, and the calculations would be inaccurate. You would most likely want to choose wavelengths like nm or nm where there is a lot of room for absorbance change. Now for the fun part! Using the calibration plot that YOU made from the data two pages ago.