Chem - Experiment II
Your trusted laboratory spectrophotometer can fall into both categories. relationship between transmittance, absorbance and concentration becomes the foundation With the optical path length and molar absorptivity. Despite this fact, absorbance can still be used to determine concentrations (of known as the molar absorptivity or extinction coefficient. as a universal relationship for the concentration and absorption of all readings are to be trusted. How are concentration and absorbance related? How can you . The only difference to change the absorbance, is the Molar Absorptivity Constant. Colors. png.
Generating and Using a Calibration Graph How to Work with Plots You have two different plots, your absorbance spectrum on the left, and your calibration plot on the right.
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.
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.
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! 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.
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 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.
Table 1 gives values for the molar absorptivity of a solution of ethanal in hexane.
Beer's Law - Theoretical Principles
Notice that there are no units given for absorptivity. That's quite common since it assumes the length is in cm and the concentration is mol dm-3, the units are mol-1 dm3 cm Although, in fact, the nm absorption peak is outside the range of most spectrometers. You may come across diagrams of absorption spectra plotting absorptivity on the vertical axis rather than absorbance.
However, if you look at the figures above and the scales that are going to be involved, you aren't really going to be able to spot the absorption at nm. It will be a tiny little peak compared to the one at nm.
The Beer-Lambert Law
To get around this, you may also come across diagrams in which the vertical axis is plotted as log10 molar absorptivity. If you take the logs of the two numbers in the table, 15 becomes 1. That makes it possible to plot both values easily, but produces strangely squashed-looking spectra!