Analytical Chemistry PDF

Analytical chemistry Qualitative Quantitative Volumetric Instrumental Spectroscopy Electrochemical Chromatography 7  Instrumental Methods of Analysis (Phys...

Analytical chemistry Qualitative Quantitative Volumetric Instrumental Spectroscopy Electrochemical Chromatography 7  Instrumental Methods of Analysis (Physicochemical)  It depends on measuring some physical properties that is quantitatively related to the concentration of the constituent to be analyzed.  It requires certain instruments, that is why it is called instrumental analysis. Assoc. Prof. Dr. Hytham M. 10/4/2024 8 Ahmed  Spectroscopy Why we see color of solution or matter? 9 10/4/2024  From the earliest times, color has been used to identify chemical substances and spectrometry is an extensive of this primitive technique. Historically, spectroscopy referred to a branch of science in which visible light was used for theoretical studies on the structure of matter and for qualitative and quantitative analyses. Spectroscopy: is based on the absorption of light by analyte. Also, defined as the study of the interaction between light and matter. 10 10/4/2024 What is Light? Light is an electromagnetic radiation (EMR), composed of both electric and magnetic components. EMR is stream of discrete particles or wave packets of energy called photons or quanta. It displays the property of continuous waves and can be described by the characteristics of wave motion 11 10/4/2024 Wave Properties of Electromagnetic Radiation Light exhibits wave property during its propagation and energy particle during its interaction with matter. Light waves, propagate at the highest known velocity of 3 x 1010cm/sec. or 3 x 108 m/sec. Such wave motion may be classified according to the wavelength (λ lambda) 12 10/4/2024  Wavelength is the linear distance measured along the line of propagation, between the crest of one wave to that of the next wave. 13 10/4/2024 Different units of length are used to express wavelengths in different parts of the EM-spectrum. Metre (m) = 100 cm = 1000 mm = 1000000 (106 µm) = 10 9 nm = 1010 A˚ The following units are in common use: 1 angstrom =1 A˚ =10-10m = 10-8 cm 1 nanometre = 1 nm = 10 A˚ =10-7 cm 1 micrometer = 1 um =104 A˚ =10-4 cm 14 10/4/2024 15 10/4/2024 Some other terms are also used such as: Frequency (ν), which is the number of waves occurring per second and is expressed in cycles/sec(CPS) or Hertz(Hz) Wavelength inversely proportional to the frequency, Wavenumber(ύ) is number of waves per centimetre, which is expressed in cm-1. wavenumber (ύ ) =. 1/ λ cm-1 16 10/4/2024 Relations between λ, ν and ύ are given by the following equations: 1 = wave number (ύ) Frequency (ν ) = Wavelength (λ) velocity of light (C) Where C is the velocity of light in vacuum = 3 x 1010 cm/sec Note that, the longer the wavelength, the lower the frequency and the smaller the wave number and17 vice versa. 10/4/2024 Example:  Particle properties of electromagnetic radiation When a sample absorbs electromagnetic radiation it undergoes a change in energy, if we assume that electromagnetic radiation consists of a beam of energetic particles called photons. Photon energy is directly proportional to the frequency E=hν Where h is the Planck’s constant (6.63x10-34J.s) The energy of a photon is [in joules] We can also relate the energy of radiation to wavelength and wave- number: E = hc / λ = hc ύ Note that the wave-number in common with frequency is directly proportion to energy. 18 10/4/2024 This electromagnetic spectrum ranges from very short wavelengths (including gamma and x-rays) to very long wavelengths (including microwaves and broadcast radio waves). 19 10/4/2024 20 10/4/2024 21 10/4/2024  Light and radiation 22 10/4/2024  Wavelength/Energy Practice Problems 1. The yellow light given off by a sodium vapor lamp used for public lighting has a wavelength of 589 nm. What is the frequency of this radiation? 10/4/2024 23 2. A certain microwave has a wavelength of 0.032 meters. Calculate the frequency of this microwave. Solution: 10/4/2024 24 3. A radio station broadcasts at a frequency of 590 KHz. What is the wavelength of the radio waves? Solution: 10/4/2024 25 5. Calculate the energy of one photon of yellow light that has a wavelength of 589nm. Solution: 10/4/2024 26  Ultraviolet -Visible Spectrum  UV-visible absorption spectra can be used to help identify compounds and to measure the concentrations of solutions  Visible spectrum: The human eye is only sensitive to a tiny proportion of the total electromagnetic spectrum between approximately 400 and 800 nm and within this area we perceive the colors of the rainbow from violet through to red 10/4/2024 27 It comes in colors as seen when white light is passed through a prism and broken into a rainbow. 10/4/2024 28  Color Wheel 10/4/2024 29 The visible spectrum forms only a small part [very tiny fraction ] of the complete EMR radiation spectrum. It extends from 400 to 800 nm and is further subdivided into colors according to their wavelengths. Violet: 400 - 420 nm  Indigo: 420 - 440 nm  Blue: 440 - 490 nm  Green: 490 - 570 nm  Yellow: 570 - 585 nm  Orange: 585 - 620 nm  Red: 620 - 780 nm  10/4/2024 30 When polychromatic light (white light), which contains the whole spectrum of wavelength in the visible region is passed through an object, the object will absorb certain of the wavelengths, leaving the unabsorbed wavelengths to be transmitted. These residual transmitted wavelengths will be seen as color. 10/4/2024 31  Interactions of photons with matter 10-Oct-24 5  Interactions of photons with matter: There are three basic process by which a molecule can absorb radiation all involve raising the molecule to a higher internal energy level 1) By increasing the rotation of molecule about various axes so the molecule may absorb radiation and be raised to higher rotational energy level. (rotational transition). 2) Atoms or groups of atoms within molecule vibrate relative to each other (vibrational transition). 3)By raising an electron of molecule to a higher electron energy level (electronic transition). E total = E electronic + E vibrational + E rotational 10/10/2024 6  The molecule at room temperature is usually in its lowest electronic energy state called ground state When a molecule interacts with photons in the UV-VIS region, the absorption of the energy results in displacing an outer electron (valence electron) in the molecule. The molecule is said to undergo transition from the ground state of energy level (Eg) to an excited state of energy level (Es). Energy of transition is given by the equation: ∆E= Es - Eg = hν = h C/ λ Excited state Es ∆E ground state Eg 10/10/2024 7  Interactions of photons with matter: There are three basic process by which a molecule can absorb radiation all involve raising the molecule to a higher internal energy level 1) By increasing the rotation of molecule about various axes so the molecule may absorb radiation and be raised to higher rotational energy level. (rotational transition). 2) Atoms or groups of atoms within molecule vibrate relative to each other (vibrational transition). 3) By raising an electron of molecule to a higher electron energy level (electronic transition). E total = E electronic + E vibrational + E rotational 10-Oct-24 8  The molecule at room temperature is usually in its lowest electronic energy state called ground state When a molecule interacts with photons in the UV-VIS region, the absorption of the energy results in displacing an outer electron (valence electron) in the molecule. The molecule is said to undergo transition from the ground state of energy level (Eg) to an excited state of energy level (Es). Energy of transition is given by the equation: ∆E= Es - Eg = hν = h C/ λ Excited state Es ∆E ground state Eg 10-Oct-24 9   Electronic Spectra &Molecular Structure  The electronic transitions that take place in the UV- Vis regions of the spectrum are due to the absorption of radiation by specific types of groups (functional groups ) & bonds within the molecule.  When light passes through a compound, some of the energy in the light kicks an electron from one of the bonding or non-bonding orbitals into one of the anti-bonding ones.  The energy gaps between these levels determine the frequency (or wavelength) of the light absorbed, and those gaps will be different in different compounds.  Electrons in molecule can be classified into different types 10-Oct-24 10  Types of electrons:  (1) Closed shell – electrons that are not involved in bonding (very high excitation energies and don’t contribute to absorption in Vis or UV regions).  (2) Covalent single –bond electrons (σ or sigma, electrons) These also posses too high excitation energy to contribute to absorption of Vis or UV radiation (eg.-CH2-CH2- ).  (3) electrons in π ( pi) orbital for example in double or triple bonds.  (4) Paired non –bonding outer shell electrons (n electrons) such as those on N,O, P, S and halogen absorb UV radiation. 10-Oct-24 11  Accordingly, there are three types of Transitions: a) σ -electrons; They are bonding electrons, represent valence bonds and possess the lowest energy level ( the most stable) b) π -electrons; They are bonding electrons, forming the π-bonds (double bounds), and possess higher energy than σ - electrons. c) n -electrons; They are nonbonding electrons, present in atomic orbital of hetero atoms (N, O, P, S or halogens). They usually occupy the highest energy level of the ground state. A molecule also possesses normally unoccupied orbitals called antibonding orbitals. 10-Oct-24 12  σ-Electrons: Compounds containing only σ-electrons are the saturated hydrocarbons which absorb below 170nm (in the far UV region). They are transparent in the near UV region (200 - 400 nm) and this make them ideal solvents for other compounds studied in this range. They characterized by σ - σ* transition only. 10-Oct-24 13  n-Electrons : Saturated organic compounds containing hetero-atoms, possess n-electrons in addition to σ -electrons. So they characterized by the σ - σ * and n - σ * transitions. The majority of these compounds show no absorption in the near UV region.  They are useful also as common solvents in the near UV region. However, their intense absorption usually extends to the edge of the near UV producing what is called end absorption (cut-off wavelength) mostly in the 200 – 250 nm region. UV cut off is defined as the wave length where solvent also absorbs light (UV or Visible). In that region, the measurement should be avoided. It is difficult to determine the absorbance comes from your analyte or your solvent. 10-Oct-24 14  Cut-off wavelengths of some common solvents Solvent , nm Solvent , nm Solvent , nm Water 190 Ethanol 207 Benzene 280 Ether 205 Methanol 210 Acetone 331  Types of Electronic Transitions   Anti-bonding  Anti-bonding  -  n- -  -  n -  - n Non-bonding  Bonding  Bonding 150 200 250 300 Wavelength, nm  Look again at the possible jumps. This time, the important jumps are shown in black, and a less important one in grey. The grey dotted arrows show jumps which absorb light outside the region of the spectrum we are working in. 10-Oct-24 18  Absorption spectrum  The spectrum itself is a plot of absorbance vs wavelength and is characterized by the wavelength (λmax) at which the absorbance is the greatest. a) Line spectrum: Occur with atomic spectra such as sodium metal which has a sharp line of λ at 589nm b) Band spectrum: Occurs with molecules due to the presence of different vibration and rotation sub-levels which the molecules may occupy on transition to excited state. That is, rotational and vibration modes will be found combined with electronic transition result in band rather than line spectra Absorption band spectrum for some molecules Amax max Absorption spectrum: it is characteristic to substance, and the wavelength at which the maximum absorption is recorded and used to trace the substance strength to enhance the sensitivity.  Absorption Spectrum Is a plot of absorption intensity versus the wavelength of the absorbed light Line spectrum (for atoms) and band spectrum (for molecules) Line Spectra for some elements K Na 450 400 500 600 800 Wavelength, nm  According to the electronic transition that occur in each organic molecule, absorption spectrum is obtained by plotting A as a function of λ.  It has characteristic shape which show λ of maximum absorbance [λmax].  λ max characteristic for each molecule according to its structure [Number & arrangement of electrons] and consequently types of transitions. Therefore it is used for; - identification of a chemical substance - Used for quantitative measurement in order to increase sensitivity and to minimize error of analytical method. There are two parameters which define an absorption band: 1) Its position (λmax) on the wavelength scale. 2) Its intensity on the absorbance scale. An excited molecule return to the ground state in about 10-8 sec. Energy must be released in the form of: 1- Heat: When excited electron returns directly to the ground state. 2- fluorescence or phosphorescence ‫وميض‬ 3- Molecular collisions. Spectral changes SPECTRAL CHANGES & CONJUGATED SYSTEM Bathochromic (Red) shift Shift of absorption to longer wavelength due to substitution and solvent effects. Hypsochromic (Blue) shift It is shift of absorption to shorter wavelength. Hyperchromic & hypochromic effects: It is the increase and decrease in absorption intensity respectively.  Some Important Terms Hyperchromic Hypsochromic Bathochromic Absorbance APEX Hyporchromic Wavelength, nm Terminology for Absorption Shifts Nature of Shift Descriptive Term To Longer Wavelength Bathochromic To Shorter Wavelength Hypsochromic To Greater Absorbance Hyperchromic To Lower Absorbance Hypochromic Relationship between the number of fused rings and wavelength  Some important terms spectra-structure correlation The absorbance of EMR depends primarily upon the number and arrangement of electrons in organic molecule of absorbing substance. Chromophore: Auxochromes: 10-Oct-24 31 Chromophores: (Chrom = colour, phore = carrier). They are functional groups which confer colour on substances capable of absorbing UV and/or visible light (200 - 800 nm). They have unsaturated bonds (double or triple bonds) such as -C=C;-C=O,N=N , -C=N and etc... Example Excitation λmax, nm ε Solvent Chromophore C=C Ethene π __> π* 171 15,000 hexane C≡C 1-Hexyne π __> π* 180 10,000 hexane n __> π* 290 15 hexane C=O Ethanal π __> π* 180 10,000 hexane n __> π* 275 17 ethanol N=O Nitromethane π __> π* 200 5,000 ethanol C-X X=Br Methyl bromide n __> σ* 205 200 hexane X=I Methyl Iodide n __> σ* 255 360 hexane The molecule containing a chromophore is called chromogen 10-Oct-24 32 A chromophore (literally color-bearing) group is a functional group, not conjugated with another group, which exhibits a characteristic absorption spectrum in the ultraviolet or visible region. Some of the more important chromophoric groups are: However, although the next molecule contains two double bonds, they aren't conjugated. They are separated by two single bonds. If any of the simple chromophores is conjugated with another (of the same type or different type) a multiple chromophore is formed having a new absorption band which is more intense and at a longer wavelength that the strong bands of the chromophores.10-Oct-24 33  Auxochromes:  An auxochrome doesn’t itself absorb radiation, but if present in a molecule , it can enhance the absorption by chromophore or shift the wavelength of absorption when attached directly to the chromophore. Examples:  hydroxyl groups [ OH ]  Amino groups [ NH2 ]  Halogens [ Cl, Br, I ] 10-Oct-24 34 Some important terms: spectra-structure correlation THE ABSORBANCE OF EMR depends primarily upon the number and arrangement of electrons in organic molecule of absorbing substance. Chromophores: (Chrom = colour, phore = carrier). They are functional groups which confer colour on substances capable of absorbing UV and/or visible light (200 - 800 nm). They have unsaturated bonds (double or triple bonds) such as -C=C;-C=O,N=N , -C=N and etc... Example Excitation λmax, nm ε Solvent Chromophore C=C Ethene π __> π* 171 15,000 hexane C≡C 1-Hexyne π __> π* 180 10,000 hexane n __> π* 290 15 hexane C=O Ethanal π __> π* 180 10,000 hexane n __> π* 275 17 ethanol N=O Nitromethane π __> π* 200 5,000 ethanol C-X X=Br Methyl bromide n __> σ* 205 200 hexane X=I Methyl Iodide n __> σ* 255 360 hexane The molecule containing a chromophore is called chromogen A chromophore (literally color-bearing) group is a functional group, not conjugated with another group, which exhibits a characteristic absorption spectrum in the ultraviolet or visible region. Some of the more important chromophoric groups are: However, although the next molecule contains two double bonds, they aren't conjugated. They are separated by two single bonds. If any of the simple chromophores is conjugated with another (of the same type or different type) a multiple chromophore is formed having a new absorption band which is more intense and at a longer wavelength that the strong bands of the chromophores.  Auxochromes: An auxochrome doesn’t itself absorb radiation , but if present in amolecule , it can enhance the absorption by chromophore or shift the wave length of absorption when attached directly to the chromophore. Examples are hydroxyl groups [OH], Amino groups [NH2]and halogens[ Cl, Br, I]. Effect of pH on absorption spectra: The spectra of compounds containing acidic or basic groups are dependent on the pH of the medium (e.g.) phenols and amines. The UV spectrum of phenol in acid medium (where the molecular form predominates) is completely different 'from its spectrum in alkaline medium (where the phenolate anion predominates). The spectrum in alkaline medium exhibits bathochromic shift with hyperchromic effect. The red shift is due to the participation of the pair of electrons in resonance with the π-electrons of the benzene ring, thus increasing the delocalization of the π-electrons. OH O O OH- H+ - in acid medium  Another example about ph.ph. indicator The rearrangement now lets the delocalization extend over the entire ion. This greater delocalization lowers the energy gap between the highest occupied molecular orbital and the lowest unoccupied pi anti-bonding orbital. It needs less energy to make the jump and so a longer wavelength of light is absorbed. Remember: Increasing the amount of delocalisation shifts the absorption peak to a longer wavelength On the other hand, UV spectrum of aniline in acid medium shows hypsochromic (blue) shift with hypochromic effect (decrease in absorption intensity). This blue shift is due to the protonation of the amino group, hence the pair of electrons is no longer available and the spectrum in this case is similar to that of benzene (thus called benzenoid spectrum).. NH2 + NH 3 in acid medium Blue shift