Beer Lambert law: Overview, Questions, Preparation

According to Beer-Lambert the absorbance of a solution is directly proportional to the absorbing substance's concentration in the solution and the path length.

Explanation

When the radiation travels through a solution, the quantity of light absorbed or transmitted is an exponential function of the minor component's molecular concentration or the solute and a function of the length of the path of radiation through the sample. Therefore,

Log I₀ / I = ε c l

Where I₀= Intensity of the incident light 

I = Intensity of light transmitted through the sample solution

c = concentration of the solute in mol l-1

l = path length of the sample in cm

ε = molar absorptivity constant

The ratio I /I₀ is known as transmittance T and the logarithm of the inverse ratio I₀ / I is known as the absorbance A.

- Log I /I₀ = - log T = ε c l

  Log I₀ / I = A = ε c l

  A = ε c l

Real deviation

This deviation is due to the concentration of the analyte in the solution is high(>10mM).

At higher concentrations

• The analyte behaves differently due to hydrogen bond interactions with the solvent and other solute molecules.
• Solute molecules can cause different charge distribution on their neighbouring species in solution. So, they would result in a shift in the absorption wavelength.

Spectral deviation

We use monochromatic radiation with a narrow range of wavelengths to measure the absorbance of the solution. If we irradiate the sample with polychromatic radiation, a deviation occurs.

Chemical deviation

The third type of the analyte is in consequence of the chemical changes like association, dissociation or solvent interactions in the analyte. Sometimes a variation in the pH of the solution affects the absorbance.

Beer law was named after August Beer which is defined as the relationship between concentration and absorbance. They are directly proportional to each other.

Mathematically, it is written as

A = log10 (l0/l) ~ C

Here, A is equal to εC

And A is absorbance,

L is the length of the dummy solution

ε is the coefficient of molar attenuation

The Lambert law was named after Johann Heinrich Lambert. It defines the relation between absorbance and path length. They both are directly proportional to each other. 

Mathematically, it is written as

A = log10 (l0/l) ~ l

Here, A is equal to εl

Ans A is absorbance,

L is the length of the dummy solution

ε is the coefficient of molar attenuation

the formula of beer-lambert law is written as:

I – I0 e- μ (x)

here, the intensity is denoted as I

the initial intensity is denoted as I0

depth is denoted as x

coefficient of absorption denoted as μ.

Beer-Lambert Law for Class 12

It is the most repeated question in both physics and chemistry fields. At higher levels, the application of this law is more theoretical physics.

1. A solution of thickness 2cm transmits 40% incident light. Calculate the concentration of the solution given that ε =6000 dm³/mol/cm.

 A = 2log10 %T = -2log 10 40 = 2 1.6020 =0.398

 A = ε c l

c = A/ ε l =0.398/6000*2

A =3.316× 10⁻⁵ mol/dm³

2. Calculate the molar absorptivity of a 1×10⁻⁴ M solution which has an absorbance of 0.20, when the path length is 2.5 cm.

A = ε c l

 ε = A/c l = 0.20/1×10⁻⁴×2.5 = 800 dm³/mol/cm.

3. A solution with a concentration of 0.18 M is measured to have an absorbance of 0.5. Another solution measured under the same conditions has an absorbance of 0.35. What is the concentration?

A = ε c l

Absorbance is directly proportional to concentration. 

So C1/C2 =A1/A2.

C1 = (A1/A2)*C2 =0.35/0.5*0.18 =0.126M

4. State the formula of Beer-Lambert law

The formula of Beer-Lambert law is I – I0 e- μ (x)

5. Write the equation of Lambert’s Law.

The equation of Lambert’s Law can be written as A = log10 (l0/l) ~ l

6. Illustrate the equation of coefficient of molar extinction.

The equation of coefficient of the molar equation is ε = A/Lc.

A: 

• Improper calibration of a spectrophotometer.

A: 

Q: What are the applications of Beer lambert Law?

Q: How are concentration and absorbance related?

Q: What causes the errors in calibration plot?

Q: What does molar absorptivity mean?

Q: How to avoid deviations?

Q: What is Beer’s Law?

Q: What is the reason behind the usage of Lambert’s Law?

Q: What is the unit of Beer’s law?

Q: What do you mean by Lambert’s Law?

Q: What is the formula used to calculate the absorbance?