When is beers law not followed

Experimental measurements are usually made in terms of transmittance T , which is defined as:. The relation between A and T is:.

An unknown concentration of an analyte can be determined by measuring the amount of light that a sample absorbs and applying Beer's law. If the absorptivity coefficient is not known, the unknown concentration can be determined using a working curve of absorbance versus concentration derived from standards.

Most substances follow Beer's law at low to moderate concentrations of absorbing species. Beer's law may not be followed very well due to saturation effects in highly concentrated samples, changes in the refractive index of the sample, solute-solvent interactions, stray light effects, or the polychromaticity of the spectrometer light. The direct relationship between absorbance and concentration illustrated by Beer's law often makes absorbance a more useful mode for spectra than transmittance.

Sign In View Cart 0 Help. Citation: D. View SPIE terms of use. We look at the way in which the intensity of the light radiant power changes as it passes through the solution in a 1 cm cuvette. We will look at the reduction every 0. The Law says that the fraction of the light absorbed by each layer of solution is the same. For our illustration, we will suppose that this fraction is 0. If we plot absorbance against concentration, we get a straight line passing through the origin 0,0.

Question : What is the significance of the molar absorbtivity, e? In words, this relationship can be stated as " e is a measure of the amount of light absorbed per unit concentration". Molar absorbtivity is a constant for a particular substance, so if the concentration of the solution is halved so is the absorbance, which is exactly what you would expect. Let us take a compound with a very high value of molar absorbtivity, say , L mol -1 cm -1 , which is in a solution in a 1 cm pathlength cuvette and gives an absorbance of 1.

Now let us take a compound with a very low value of e , say 20 L mol -1 cm -1 which is in solution in a 1 cm pathlength cuvette and gives an absorbance of 1.

The answer is now obvious - a compound with a high molar absorbtivity is very effective at absorbing light of the appropriate wavelength , and hence low concentrations of a compound with a high molar absorbtivity can be easily detected.