Theoretical foundations for determining the optical density of a solution

2024 Author: Angel Austin | [email protected]. Last modified: 2024-01-02 17:39

Any particle, be it a molecule, an atom or an ion, as a result of the absorption of a light quantum goes to a higher level of energy state. Most often, the transition from the ground state to the excited state occurs. This causes certain absorption bands to appear in the spectra.

The absorption of radiation leads to the fact that when it passes through a substance, the intensity of this radiation decreases with an increase in the number of particles of a substance with a certain optical density. This research method was proposed by V. M. Severgin back in 1795.

This method is best suited for reactions where the analyte is able to transform into a colored compound, which causes a change in the color of the test solution. By measuring its light absorption or comparing the color with a solution of known concentration, it is easy to find the percentage of the substance in the solution.

Basic law of light absorption

The essence of photometric determination is two processes:

• transfer of the analyte toabsorbent compound;
• measuring the intensity of absorption of these same vibrations by a solution of the test substance.

Changes in the intensity of light passing through the light absorbing material will also be caused by light loss due to reflection and scattering. To make the result reliable, parallel studies are carried out to measure the parameters at the same layer thickness, in identical cuvettes, with the same solvent. So the decrease in light intensity depends mainly on the concentration of the solution.

The decrease in the intensity of light passed through the solution is characterized by the light transmission coefficient (also called its transmission) T:

Т=I / I_{0, where:}

• I - intensity of light passed through the substance;
• I_{0 - intensity of the incident light beam.}

Thus, transmission shows the proportion of unabsorbed light flux passing through the solution under study. The inverse transmission value algorithm is called the optical density of the solution (D): D=(-lgT)=(-lg)(I / I₀)=lg(I _{0 / I).}

This equation shows which parameters are the main ones for research. These include the wavelength of the light, the thickness of the cuvette, the concentration of the solution, and the optical density.

Bouguer-Lambert-Beer Law

It is a mathematical expression that displays the dependence of the decrease in the intensity of a monochromatic light flux from concentrationabsorbent and the thickness of the liquid layer through which it is passed:

I=I₀10^{-ε·С·ι, where:}

• ε - light absorption coefficient;
• С - concentration of a substance, mol/l;
• ι - layer thickness of the analyzed solution, see

After transforming, this formula can be written: I / I₀ =10^-ε·С·ι.

The essence of the law is as follows: different solutions of the same compound at equal concentration and layer thickness in the cuvette absorb the same part of the light falling on them.

By taking the logarithm of the last equation, you can get the formula: D=εCι.

Obviously, the optical density directly depends on the concentration of the solution and the thickness of its layer. The physical meaning of the molar absorption coefficient becomes clear. It is equal to D for a one-molar solution and with a layer thickness of 1 cm.

Restrictions on the application of the law

This section includes the following items:

1. It is valid only for monochromatic light.
2. The coefficient ε is related to the refractive index of the medium, especially strong deviations from the law can be observed when analyzing highly concentrated solutions.
3. The temperature when measuring optical density must be constant (within a few degrees).
4. The light beam must be parallel.
5. The pH of the medium must be constant.
6. The law applies to substanceswhose light-absorbing centers are particles of the same type.

Methods for determining concentration

It is worth considering the calibration curve method. To build it, prepare a series of solutions (5-10) with different concentrations of the test substance and measure their optical density. According to the obtained values, a plot of D versus concentration is plotted. The graph is a straight line from the origin. It allows you to easily determine the concentration of a substance from the results of measurements.

There is also a method of additions. It is used less frequently than the previous one, but it allows you to analyze solutions of complex composition, since it takes into account the influence of additional components. Its essence is to determine the optical density of the medium D_x, containing the analyte of unknown concentration С_x, with repeated analysis of the same solution, but with the addition of a certain amount of the test component (С_st). The value of C_{x is found using calculations or graphs.}

Research conditions

In order for photometric studies to give a reliable result, several conditions must be met:

• reaction must end quickly and completely, selectively and reproducibly;
• the color of the resulting substance must be stable over time and not change under the action of light;
• the test substance is taken in an amount sufficient to convert it into an analytical form;
• measurementsoptical density is carried out in the wavelength range at which the difference in the absorption of the initial reagents and the analyzed solution is greatest;
• light absorption of the reference solution is considered to be optical zero.