how do u calculate concentration in Beer lambert law?

and can someone give an example

10 Answers

• Absorbance = molar absorptivity x path length x concentration

A = epsilon x L x c

A = Elc

absorbance is unitless and usually a fraction like 0.2 or 0.1. molar absorptivity is in units of L/mol/cm. path length is then in cm. concentration in this case would be mol/L.

Find concentration for a solution with absorbance of 0.16, a molar absorptivity of 960L/mol/cm, in a cuvette with path length of 1cm.

A = Elc

0.16 = 960*1*c

c = 0.16/960/1 = 1.67×10^-4 mol/L = 1.67×10^-4 M

• Beer Lambert Law Equation

• Absorbance Vs Concentration

• This page takes a brief look at the Beer-Lambert Law and explains the use of the terms absorbance and molar absorptivity relating to UV-visible absorption spectrometry.

Absorbance

Measuring the absorbance of a solution

If you have read the page about how an absorption spectrometer works, you will know that it passes a whole series of wavelengths of light through a solution of a substance (the sample cell) and also through an identical container (the reference cell) which only has solvent in it.

For each wavelength of light passing through the spectrometer, the intensity of the light passing through the reference cell is measured. This is usually referred to as Io – that’s I for Intensity.

The intensity of the light passing through the sample cell is also measured for that wavelength – given the symbol, I.

If I is less than Io, then obviously the sample has absorbed some of the light. A simple bit of maths is then done in the computer to convert this into something called the absorbance of the sample – given the symbol, A.

For reasons to do with the form of the Beer-Lambert Law (below), the relationship between A (the absorbance) and the two intensities is given by:

On most of the diagrams you will come across, the absorbance ranges from 0 to 1, but it can go higher than that.

An absorbance of 0 at some wavelength means that no light of that particular wavelength has been absorbed. The intensities of the sample and reference beam are both the same, so the ratio Io/I is 1. Log10 of 1 is zero.

An absorbance of 1 happens when 90% of the light at that wavelength has been absorbed – which means that the intensity is 10% of what it would otherwise be.

In that case, Io/I is 100/I0 (=10) and log10 of 10 is 1.

Note: If you don’t feel comfortable with logarithms, don’t worry about it. Just accept that an absorbance scale often runs from zero to 1, but could go higher than that in extreme cases (in other words where more than 90% of a wavelength of light is absorbed).

Absorbance isn’t very good for making comparisons

The importance of concentration

The proportion of the light absorbed will depend on how many molecules it interacts with. Suppose you have got a strongly coloured organic dye. If it is in a reasonably concentrated solution, it will have a very high absorbance because there are lots of molecules to interact with the light.

However, in an incredibly dilute solution, it may be very difficult to see that it is coloured at all. The absorbance is going to be very low.

Suppose then that you wanted to compare this dye with a different compound. Unless you took care to make allowance for the concentration, you couldn’t make any sensible comparisons about which one absorbed the most light.

The importance of the container shape

Suppose this time that you had a very dilute solution of the dye in a cube-shaped container so that the light travelled 1 cm through it. The absorbance isn’t likely to be very high. On the other hand, suppose you passed the light through a tube 100 cm long containing the same solution. More light would be absorbed because it interacts with more molecules.

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.

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RE:

how do u calculate concentration in Beer lambert law?

and can someone give an example

• calculate concentration beer lambert law

• If you make a graph of absorbance versus concentration, if the absorbing species obey’s Beers lambert’s Law..then you will get a straight line. You can then have software of various types ( even excel ) can give the equation representing that straight line in the form

y = mx + b if you plot the concentration on the Y axis and absorbance on the x axis .then the above equation would be

Y ( concentration in whatever units the graph was derived from ) =

slope x ( absorbance value ) + intercept. So if the predictive equation were y = 0.93 X + 0.1 for a Beer’s Law line plotting absorbance vs concentration i mg/mLand the absorbance value of your unknown sample were 0.5 then the concentration would be

0.93 ( .5 ) + 0.1 = 0.565 mg/mL

Alternatively plot the straight line ..determine the absorbance of your sample ..then draw a perpendicular from the straight line to the concentration based on the absorbance value for your sample

• using beer-lambert’s equation to calculate the transmission T(nm) and absorbance A when the concentration of the test sample was 200mg/100ml, while the thickness of the spectrophotometer used was 15mm. constant k = 0.06

• Hi, A = e x C x L C = n / V = m / (M x V) = Cm / M C = A / (e x L) . . . then : Cm = M x C Cm = M x A / (e x L)

• The concentration of alcohole in Beer is usually 5.2 %.