Theme 5- DETERMINATION OF THE CONCENTRATIONS OF SOLUTIONS WITH THE APPLICATION OF ABSORPTION SPECTROPHOTOMETER Flashcards
What is spectrophotometry/spectroscopy
Investigation of different types of spectra
How does spectra analysis work
It makes it possible to identify elements or chemical compounds in the examined sample by making use of characteristic lines (bands) in the emission (absorption) spectrum
What is the spectrum
It is defined as a distribution of the electromagnetic radiation according to its energy, frequency or wavelength.
The principle of spectra analysis
-The basis of spectrum analysis is the fact that atoms of each element have a system of
permissible energy states or levels (making it possible to distinguish one element from
another and also determine the energy state of an atom or molecule).
-The Bohr Model of the hydrogen atom here should be referred to.
-Electrons in both atoms and molecules possess energy connected with their interaction with atomic nuclei.
-This energy is not unrestricted but can assume only some definite values (one says that electron energy is “quantised”).
-An electron can’t give away less energy than the energy difference between the permissible energy levels for the electron.
-The spectrum created in this way, the so-called
line spectrum or band spectrum is the information carrier of the chemical composition of the sample under investigation
What is the electromagnetic spectrum (EM spectrum)
The EM spectrum gives information not only about the source of given radiation (i.e. emission spectrum), but very often about the medium it went through (i.e. absorption spectrum)
How is the absorption spectrum formed
-When a continuous spectrum passes through matter.
-A considerable discontinuity of energy is observed in the absorption spectrum, this is typical of the given substance in the direction of the incident electromagnetic wave.
-Making it possible to investigate the chemical composition of the absorbent.
The 3 types of emission spectra
-Line spectra (for gases and vapours in atomic states)
-Band spectra (for gases and vapours in molecular states)
-Continuous spectra (for fluids
and solids).
How is spectrometry classified
-According to the type and the range of the radiation investigated, the type of sample investigated and the methods of obtaining the spectra.
-In spectroscopy, the electromagnetic radiation of 10-12 to 103 m in wavelength is investigated.
According to the wavelength ranges what is spectrometry divided into
-Gamma-ray spectroscopy (investigation of intranuclear processes)
-X-ray spectroscopy (investigation of internal electron shells of atoms)
-UV and visible light spectroscopy (determination of structure and distribution of atomic and molecular energy levels)
-IR spectroscopy (investigation of absorption spectra and using the investigations to analyse the structure of molecules and their oscillation and rotation).
According to the types of the investigated objects whose spectra are analysed, what can spectroscopy be categorised into
-Atomic spectroscopy including structure investigations of atom and ion energy levels.
These investigations are carried out based on the analysis of emission and absorption
spectra which are created during transition between energy levels of particular atoms. The
atomic spectrum has a line characteristic.
- Molecular spectroscopy involves the investigation of molecules, their structure and
physicochemical properties. The molecular spectrum has band characteristics. This is because molecules, apart from motions in their atoms, are characterized by oscillatory motions in their atomic nuclei and also rotational motions of molecules as a whole. The transition process of molecules between the energy levels is accompanied by the emission or absorption of a photon whose energy is as follows: ΔE = ΔEelectr + ΔEoscyl + ΔErot
where: ΔEelectr – change of molecule’s electron energy, ΔEoscyl – change of energy associated
with molecule’s oscillatory motion, ΔErot – change of energy associated with molecule’s
rotational motion. Since ΔEelectr»_space; ΔEoscyl»_space; ΔErot of each spectrum line which corresponds to the change in electron shell is accompanied by the change of a series of lines connected with oscillatory and rotational levels.
What can we distinguish in the molecular spectrum
-Rotational line spectrum (ΔEelektr = 0, ΔEoscyl = 0, ΔErot ≠ 0), spectrum lines lying in infrared
and microwave region
-Oscillatory-rotational band spectrum (ΔEelektr = 0, ΔEoscyl ≠ 0, ΔErot ≠ 0) lying in near
infrared region
-Electronic oscillatory-rotational band spectrum (ΔEelektr ≠ 0, ΔEoscyl ≠ 0, ΔErot ≠ 0) lying in the visible and ultraviolet regions.
What is light
-An electromagnetic wave
-Visible light includes wavelengths ranging from about 380 nm (which corresponds to violet) to about 720 nm (red colour).
What does the sensitivity of the human eye depend on
It varies depending on the particular length of the light wave
Bouger-Lambert’s law
The quantitative relation between the light intensity I0 hitting the body and the amount of
light leaving the body is described by Bouger-Lambert’s law :
I = I0 e^-μd
where: μ – absorption coefficient typical of a specific substance, d – thickness of the layer
penetrated by the light.
Beer’s law
For solutions which are composed of two kinds of molecules: a molecule of solvent and a
molecule of a dissolved substance and also of solutions of low concentration we can apply
Beer’s law, according to which the absorption coefficient μ is directly proportional to the
solution concentration c, namely:
μ = k · c
where: k is the factor of proportionality independent of concentration and is called molar extinction coefficient (absorbance coefficient).
