This is a calibration curve.Īccording to the Beer-Lambert Law, absorbance is proportional to concentration, and so you would expect a straight line. Then you plot a graph of that absorbance against concentration. You would just make up solutions of accurately known concentrations some of which are a bit lighter and some a bit darker in colour.įor each solution, you measure the absorbance at the wavelength of strongest absorption - using the same container for each one. With coloured solutions, this isn't a problem. Those concentrations should bracket the concentration you are trying to find - some less concentrated some more concentrated. What you do is make up a number of solutions of the compound you are investigating - each of accurately known concentration. It saves doing any calculations for one thing!įinding concentration by plotting a calibration curveĭoing it this way you don't have to rely on a value of molar absorptivity, the reliability of the Beer-Lambert Law, or even know the dimensions of the cell containing the solution. It is much better to measure the concentration by plotting a calibration curve. It also assumes that the Beer-Lambert Law works over the whole concentration range (not true!). This method, of course, depends on you having access to an accurate value of molar absorptivity. Notice what a very low concentration can be measured provided you are working with a substance with a very high molar absorptivity. You find a value for molar absorptivity in a table of 19400 for that wavelength. You measure the absorbance of the solution at a particular wavelength using a spectrometer. That's easy to measure and, in fact, the cell containing the solution may well have been manufactured with a known length of 1 cm.įor example, let's suppose you have a solution in a cell of length 1 cm. The only other variable in the expression above is the length of the solution. If you know the molar absorptivity of a solution at a particular wavelength, and you measure the absorbance of the solution at that wavelength, it is easy to calculate the concentration. Note: You will find this all explained in more detail on the page about the Beer-Lambert Law.įinding concentration using the molar absorptivity The symbol epsilon is the molar absorptivity of the solution. The equation is sometimes written in terms of that absorbance. The expression on the left of the equation is known as the absorbance of the solution and is measured by a spectrometer. You should remember the Beer-Lambert Law: Using UV-absorption spectra to find concentrations
Let's look at something a bit more complicated! If your spectrum showed a very large peak at 180, and an extremely small one at 290, that just adds to your certainty.Īny question set at this level is going to be so trivial, and so obvious, that it isn't worth spending any more time on this. For example (again using the simple carbon-oxygen double bond), data shows that the peak at 290 has a molar absorptivity of only 15, compared with the one at 180 of 10000. That might help you to be even more sure. Lists of known peaks often include molar absorptivity values as well.
In carefully chosen simple cases (which is all you will get at this level), if you compared the peaks on a given UV-visible absorption spectrum with a list of known peaks, it would be fairly easy to pick out some structural features of an unknown molecule. We also talked about the two peaks in the spectrum of ethanal (containing a simple carbon-oxygen double bond) at 180 and 290 nm.
The two conjugated double bonds in buta-1,3-diene have a maximum absorption at a longer wavelength of 217 nm. If you have worked through the rest of this section, you will know that the wavelength of maximum absorption (lambda-max) depends on the presence of particular chromophores (light-absorbing groups) in a molecule.įor example, on another page you will have come across the fact that a simple carbon-carbon double bond (for example in ethene) has a maximum absorption at 171 nm. Using UV-absorption spectra to help identify organic compounds Important: If you don't know about these things, this page is a waste of your time! Explore the rest of the UV-visible spectroscopy menu before you go on. You also need to be familiar with the Beer-Lambert Law. It assumes that you know how these spectra arise, and know what is meant by terms such as absorbance, molar absorptivity and lambda-max. This page takes a brief look at how UV-visible absorption spectra can be used to help identify compounds and to measure the concentrations of coloured solutions.