The molar absorptivity it is a chemical property that indicates how much light a species can absorb in solution. This concept is very important within the spectroscopic analysis of absorption of photon radiation with energies in the ultraviolet and visible range (UV-vis)..
As light is composed of photons with its own energies (or wavelengths), depending on the species or mixture analyzed, one photon can be absorbed to a greater degree than another; that is, light is absorbed at certain wavelengths characteristic of the substance.
Thus, the value of molar absorptivity is directly proportional to the degree of absorption of light at a given wavelength. If the species absorbs little red light, its absorptivity value will be low; whereas if there is a pronounced absorption of red light, the absorptivity will have a high value.
A species that absorbs red light will reflect a green color. If the green color is very intense and dark, it means that there is strong absorption of red light.
However, some shades of green may be due to the reflections of different ranges of yellows and blues, which are mixed and perceived as turquoise, emerald, glass, etc..
Article index
Molar absorptivity is also known by the following designations: specific extinction, molar attenuation coefficient, specific absorption, or Bunsen coefficient; It has even been named in other ways, which is why it has been a source of confusion..
But what exactly is molar absorptivity? It is a constant that is defined in the mathematical expression of the Lamber-Beer law, and it simply indicates how much the chemical species or mixture absorbs light. Such an equation is:
A = εbc
Where A is the absorbance of the solution at a selected wavelength λ; b is the length of the cell where the sample to be analyzed is contained, and therefore, is the distance that the light crosses within the solution; c is the concentration of the absorbent species; and ε, the molar absorptivity.
Given λ, expressed in nanometers, the value of ε remains constant; but when changing the values of λ, that is, when measuring absorbances with lights of other energies, ε changes, reaching either a minimum or maximum value.
If its maximum value is known, εmax, is determined at the same time λmax; that is, the light that the species absorbs the most:
What are the units of ε? To find them, it must be known that absorbances are dimensionless values; and therefore, the multiplication of the units of b and c must cancel.
The concentration of the absorbing species can be expressed either in g / L or mol / L, and b is usually expressed in cm or m (because it is the length of the cell that the light beam passes through). Molarity is equal to mol / L, so c is also expressed as M.
Thus, multiplying the units of b and c, we obtain: M ∙ cm. What units then must ε have to make the value of A dimensionless? Those that multiplying M ∙ cm gives a value of 1 (M ∙ cm x U = 1). Solving for U, we simply obtain M-1∙ cm-1, which can also be written as: L ∙ mol-1∙ cm-1.
In fact, use the M units-1∙ cm-1 or L ∙ mol-1∙ cm-1 expedite calculations to determine molar absorptivity. However, it is also often expressed in units of mtwo/ mol or cmtwo/ mol.
When expressed with these units, some conversion factors must be used to modify the units of b and c.
Molar absorptivity can be calculated directly by solving for it in the above equation:
ε = A / bc
If the concentration of the absorbing species, the cell length, and the absorbance obtained at a wavelength are known, ε can be calculated. However, this way of calculating it returns an inaccurate and unreliable value.
If you look closely at the Lambert-Beer law equation, you will notice that it looks like the equation of a line (Y = aX + b). This means that if the values of A are plotted on the Y axis, and those of c on the X axis, a straight line must be obtained that passes through the origin (0,0). Thus, A would become Y, X would be c, and a would equal εb.
Therefore, once the line is graphed, it is enough to take any two points to determine the slope, that is, a. Once this is done, and the length of the cell, b, is known, it is easy to solve for the value of ε.
Unlike direct clearance, plotting A vs c allows the absorbance measurements to be averaged and to reduce the experimental error; and in addition, infinite lines can pass through a single point, so direct clearance is not practical.
Likewise, experimental errors can cause a line not to pass through two, three or more points, so the line obtained after applying the least squares method is actually used (a function that is already incorporated in calculators). All this assuming high linearity, and therefore, compliance with the Lamber-Beer law..
It is known that a solution of an organic compound with a concentration of 0.008739 M presented an absorbance of 0.6346, measured at λ = 500 nm and with a cell length 0.5 cm. Calculate the molar absorptivity of the complex at this wavelength.
From these data, ε can be solved directly:
ε = 0.6346 / (0.5cm) (0.008739M)
145.23 M-1∙ cm-1
The following absorbances are measured at different concentrations of a metal complex at a wavelength of 460 nm, and with a cell of 1 cm in length:
A: 0.03010 0.1033 0.1584 0.3961 0.8093
c: 1.8 ∙ 10-5 6 ∙ 10-5 9.2 ∙ 10-5 2.3 ∙ 10-4 5.6 ∙ 10-4
Calculate the molar absorptivity of the complex.
There are a total of five points. To calculate ε it is necessary to graph them by placing the values of A on the Y axis, and the concentrations c on the X axis. Once this is done, the least squares line is determined, and with its equation we can determine ε.
In this case, plotting the points and drawing the line with a coefficient of determination Rtwo 0.9905, the slope equals 7 ∙ 10-4; that is, εb = 7 ∙ 10-4. Therefore, with b = 1cm, ε will be 1428.57 M-1.cm-1 (1/7 ∙ 10-4).
Yet No Comments