Loi de Beer Lambert

What I’ll be talking about today in my presentation is the Beer-Lambert Law. I will begin by a brief history of the law, then I will explain it (units and constant) under two forms, you will understand after. And finally, I will give you an example of what we can do with this law.

The law was discovered at first by Pierre Bouguer in 1729, while looking at red wine, during a travel in Portugal. Johann Heinrich Lambert took up the idea of Bouguer and he said that the loss of light intensity when it propagates in a medium is directly proportional to intensity and path length. Much later, August Beer discovered another attenuation relation in 1852. Beer's law stated that the transmittance of a solution remains constant if the product of concentration and path length stays constant. The modern derivation of the Beer–Lambert law combines the two laws and correlates the absorbance, to both the concentrations of the attenuating species and the thickness of the material sample.

At first, this is the general form of the law. With A (absorbance) without unit, epsilon (molar attenuation coefficient) in L/mol/cm., l (optical path length) is in cm and c (concentration) in mol per liter.

Epsilon depends on the nature of the solute and the solvent and l is the same if you take the measure by the same device. So, in the same conditions of experimentation, epsilon and length are constant so, this is why we can also express the law as A = k x C.

We want to dose an S1 solution containing copper ions Cu2+ and sulphate ions SO42- . The S1 solution is obtain by dissolution. The equation of reaction: CuSO4 → Cu2+ + SO42- .The spectrophotometric determination of the solution S1 is carried out. For this, a set of calibrations solutions is prepared, then the absorbance A of each of the solutions is measured with a spectrophotometer. And, we obtain this graph. We prepared a S2 solution which is S1 dilute 5 times, the absorbance of this solution is A=1.7.

By graph reading, the concentration is 0.15…

It’s a really basic example, but this law is very powerful. You can also measure the absorbance to find the concentration on oxonium ions (H3O+) and after find the pH of the solution.

To prepare the spectrophotometer, you must find the lambda max of the solution to set up the device. The lambda max of copper sulfate is almost 800 nm. It absorbs the red so the complementary color is the blue. You must adjust the spectrophotometer at 800 nm for a solution of copper sulfate.

I hope you liked my presentation; I will now take any questions.

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