Instrumental methods of analysis

Instrumental Methods of Analysis 5.1 Introduction An exciting and fascinating part of chemical analysis is the use of instrumentation, which interacts with all the areas of chemistry and with many other fields of pure and applied sciences The instrumental methods of analysis come under the branch of chemistry known as Analytical Chemistry Analytical chemistry may be defined as the science and art of determining the composition of materials in terms of elements or compounds contained in them In analytical instrumentation, the term analytical technique refers to a fundamental scientific phenomenon that has proved useful for providing information on the composition of substances The instrumentation techniques can be classified in three principal areas: (a) Spectroscopy (b) Electrochemistry (c) Chromatography The analysis can be classified as: (1) Qualitative analysis (2) Quantitative analysis The qualitative analysis measures the property and merely indicates the presence of analyte in matrix or which reveals the identity of the compounds in a sample The quantitative analysis is a magnitude of measured property which is proportional to the concentration of analyte in matrix or which indicates the amount of each substances present in the sample These analyses can be performed by two ways namely (1) Classical Chemical Methods and (2) Instrumental Methods The various instrumental methods used to study different properties are tabulated in Table.5.1 Table-5.1 Types of instrumental Methods Property Radiation emission Method Emission spectroscopy – fluorescence, phosphorescence, Radiation absorption luminescence Absorption spectroscopy – spectrophotometry, photometry, Radiation scattering Radiation refraction Radiation diffraction Radiation rotation Electrical potential Electrical charge Electric current Electrical resistance Mass Mass-to-charge ratio Rate of reaction Thermal Radioactivity nuclear magnetic resonance, electron spin resonance Turbidity, Raman Refractometry, interferometry X- ray, electron Polarimetry, circular dichroism Potentiometry Coulometry Voltammetry – amperometry, polarography Conductometry Gravimetry Mass spectrometry Stopped flow, flow injection analysis Thermal gravimetry, calorimetry Activation, isotope dilution 5.2 The role of Analytical Instrumental Methods in the field of engineering: Analysis of a chemical property of a compound of interest varies from the field for which the chemical compound actually finds its application For example in the field of engineering the analysis of hydrocarbons, nitrogen oxides and carbon monoxide present in automobile exhaust gases are measured to asses the effectiveness of smog- control devices Assessment of percentage composition of an inorganic metal in the steel industry is needed to achieve the desired strength, hardness, corrosion resistance and ductility 5.3 Radiant energy and Electromagnetic Spectrum: The Radiant energy can be defined as the form of energy transmitted from one body to another in the form of radiation Radiation involves electromagnetic waves of lower wave lengths to higher wave lengths such as γ-rays, X-rays, UV rays, visible spectrum, infra -14 -12 -10 -8 -2 -6 -4 Wavelength 10 10 and radio 10 waves 10 The frequency 10 10 10 wavelength 10 10 red rays,(m) microwaves and of electromagnetic radiation vary over many orders of magnitude For convenience; electromagnetic radiation 20 18 16 10 14 12 Frequency (s-1) 10 22 10 regions 10 based on 10 the type10of 10 10 or molecular 10 is divided into different atomic transition that gives rise to the absorption or emission of photons The details of the electromagnetic radiations are given in the Fig 5.1 Type of Transition Nuclear Core – level electrons Valence electrons Spectral Region γ - ray X - ray UV Molecular vibrations Molecular Rotations Electron spin Nuclear spin Microwave Radio Wave IR Visible Wavelength (nm) 380 Violet 480 Blue Green 580 Yellow Orange 680 Fig 5.1 The electromagnetic Spectrum Red 780 5.4 Spectroscopy Spectroscopy mainly deals with the interaction of electromagnetic radiation with matter or any chemical substance When different regions of electromagnetic radiation interact with matter of chemical substance, they give rise to different kinds of spectroscopy Absorption of electromagnetic radiation by the matter in the radio frequency region can give rise to Nuclear Magnetic Resonance (NMR) or Electron Spin Resonance (ESR) spectroscopy based on the possibility of the resonance Absorption of electromagnetic radiation by the matter in micro wave region, different rotational levels of molecules give rise to rotational spectroscopy Absorption of infra red radiation by the matter in the infra red region can produce molecular vibrations and hence it is known as Vibrational spectroscopy Absorption of visible or ultra violet radiation by the matter in the visible or ultra violet region can produce electronic transitions of atoms or molecules and hence they are known as Electronic spectroscopy X-rays can be produced by the bombardment of metal targets with high speed electrons and the study of absorption, emission or scattering of X-rays by the matter can be studied which is known as X-ray spectroscopy 5.5 Visible, UV and IR regions: The visible light is a form of electromagnetic radiation which is in the region 380 nm-750 nm i.e 3800 A°- 7500 A°, The region of 3800 A° and less than that belongs to the Ultraviolet region (U.V region), The wave length above the region 7600 A° constitute the infra – red region (IR- region).It is found that all kinds of electromagnetic radiation travel in the same speed i.e the velocity of the light The velocity is related to energy as E = hν = hc/ λ where ‘E’ is the energy, λ is the wave length and ‘c’ is the velocity of light From the above equation we can infer that lower the energy ‘E’ greater will be the wavelength ‘λ’ The order of energies of the electromagnetic radiations is given below γ -rays > X-rays > U.V rays > visible light > Infra red rays > Microwaves > Radio waves 5.6 Interaction of Electromagnetic Radiation with Matter Whenever electromagnetic radiation interacts with matter one of three things can happen The electromagnetic radiation may undergo surface reflection All electromagnetic reflections are governed by the same physical laws as reflections of visible light Optics describes the general laws of reflection and may be applied to all types of electromagnetic reflections ranging from radio waves to gamma rays The electromagnetic radiation may be transmitted completely through the substance it encounters If absolutely no energy is absorbed by the material, it is said to be transparent to the radiation The velocity of the radiation is usually slower in the transparent medium and as a result the radiation usually undergoes refraction Various materials are transparent at various wavelengths For example, lead glass is transparent to visible light but not X-rays, whereas several thicknesses of black paper sheets are transparent to X-rays, but not visible light No known material is perfectly transparent The electromagnetic radiation may be totally or partially absorbed by the substance In this process energy is transferred to the absorbing medium and this may cause significant changes to occur within the absorbing medium Because of the quantum nature of matter on atomic and molecular scales it has been discovered that energy can only be absorbed at the atomic or molecular level if the energy of the incident radiation exceeds a specific threshold value Based on the reaction of the compound to the radiant energy several instruments are designed to study their interaction and they can be classified as: Absorption methods: Absorption spectroscopy (a) UV spectrophotometer, (b) IR spectrophotometer, (c) AAS Emission methods: Flame photometry Dispersion and scattering methods 5.6.1 Absorption method This method deals with optical methods which are based on the response of a compound / element to radiant energy The response differs with the compounds i.e on exposure to radiant energy that they may absorb, emit or scatter radiation However all these interactions bring about changes in the electronic structure of the compound and the change can be subsequently evaluated Absorption spectrophotometry in the ultra violet and visible region is considered to be one of the oldest physical methods which are used for quantitative analysis and structural analysis It mainly deals with the interaction of radiation with matter Principle Absorption spectra arise from transition of an electron or electrons with in a molecule or an ion from a lower to a higher electronic energy level and the emission spectra due to the reverse type of transition (Fig 5.2) For radiation to cause electronic excitation, it must be in the respective region of the electro magnetic spectrum Absorption spectra Excited state EM Radiation Emission spectra Excited state Atom emits particular wavelengths Atom absorbs particular wavelengths EM Radiation Ground State Atom Ground State Atom 5.6 Visible and Ultra-violet (UV) spectroscopy When energy is absorbed by a molecule in the U.V region (100 nm-400 nm) or visible region (400 nm- 750 nm) it brings about some changes in the electronic energy of the molecule resulting on electronic transition of valence electrons When an electromagnetic radiation of UV region is made to pass through a compound having multiple bonds in its structure, it is observed that a part of the incident radiation is usually absorbed, and this results invariably in the transitions of valency electrons 5.7 ABSORABANCE, BEER- LAMBERT LAW The intensity of absorption at maximum value (λ max) is related to the number of impringing photons being absorbed by the molecules Usually, only some of the photons are absorbed by the molecules The fractions of photons being absorbed at a given frequency depends on (a) The nature of the absorbing molecules; (b) The concentration of the molecules The higher the concentration, the more molecules are present to absorb the photons; (c) The length of the path of the radiation through the material The longer the path, the larger the number of molecules exposed and hence, greater the probability that a given photon will be absorbed Absorbing molecules (pure liquid or solution) Incident light, I0 ℓ Transmitted light, It Light source The absorption of light in the visible and near UV regions by a solution is governed by a photophysical law, known as the Lambert-Beer law Lambert - Beer law: When a beam of monochromatic light of intensity I is passed through a solution of concentration, C molar and thickness, dx, then intensity of transmitted light changes (due to absorption) by dI Then, probability of absorption of radiation is given by: d I / I = - KC dx where K is the proportionality constant On integrating the above expression, between limits I = I0 at x = and I = I at x = ℓ, we get: I l I dI ln = − Kcl = − KC dx or ∫I I ∫0 I o o Or 2.303 log Io = KCl I or log I0 / I = K Cℓ = ∈ Cℓ = A 2.303 where ∈ = K/ 2.303 is called the molar absorptivity coefficient, and log I / I = A is called the absorbance A= ∈C ℓ ∴ which is Beer-Lambert’s Law, Thus’’ the absorbance (A) is directly proportional : (i) to the molar concentration (C), as well as (ii) to the path length (ℓ) Applications: From the above equation, we can determine the concentration of species absorbing in ultraviolet or visible region It is also useful to calculate the transmittance T 5.8 Colorimetry: This method is specially convenient for colored solution Colorimetry is a technique in which the intensity of a colour of the solution is measured to determine the amount of particular sample present The relationship between intensity of colour and concentration of a substance is governed by Beer- Lamberts Law 5.9 Instrumentation-(Colorimetry) There are five basic parts to a spectrophotometer The source provides radiation over the wavelength range of interest White light from the source is passed through a wavelength selector that provides a limited band of wavelengths The sample holder for analyte The radiation exiting the wavelength selector is focused on to a detector which converts the radiation into electrical signals Finally the selected signal is amplified and processed as either an analog or a digital signal (display) We will consider each of the components separately Source of radiation Monochromators Sample holder Detector Read out Fig.5.3 The general block diagram of a simple colorimeter Light Source: The source used in UV- spectroscopy should meet the following criteria a) Beam produced should be in the detectable and measurable range b) It should save as a continuous source of energy c) It should be stable Since incandescent tungsten filament lamp, is found to satisfy these needs , it is widely used The other source generally used is Tungsten filament incandescent lamp, hydrogen / deuterium discharge lamp and hydrogen gas lamps Tungsten filament incandescent lamps are used in the visible and adjacent parts of ultraviolet and infrared regions Hydrogen or deuterium lamps are used in the wavelength from 160 to 360nm Deuterium lamps provide maximum intensity Monochromators Filters and Monochromators filter the energy source in such a way that a limited portion is allowed to be incident in the sample Filters allow a wider bound of energy to pass through and they are used in filter photometers whereas, monochromators find their application in spectrophotometers Sample holder: The selection of material from which the cuvette is constructed is based on the selected range of measurement while its thickness depends on the read intensity of absorption Cuvetts with varied shapes are used (rectangular, cylindrical or cylindrical with flat ends) However, the main factor is that the windows of the Cuvetts should be normal to the beam direction Requirement of Cuvetts in terms of its make and thickness are as follows UV region – quartz Visible region – Glass absorption cells, silica cells and plastic containers Cell thickness – 1, and cm Photometer / Detector: The mechanism behind the photoelectric devices is the conversion of radiant energy to electrical signal Basically types of photometers are used: a) Photovoltaic cells in which we detect the radiant energy by the current generation between the semiconductor and metal b) Phototubes in which the energy absorption induces the solid surface to emit electrons and c) Photoconductive cells in which the absorbed energy changes the electrical resistance Signal Processing: The electrical signal generated by the transducer is sent to a signal processor where it is displayed in a more convenient form for the analyst Currently, most spectrometers come either with built-in processors or provision for interfacing to a personal computer 5.10.1 Single beam instruments and double beam instruments(UV-VIS) The instruments currently used for UV/Vis absorption is the filter photometer which are shown in the following Fig 5.4 The filter is placed between the source and sample to prevent the sample from decomposing when exposed to high energy radiation A filter photometer has a single optical path between the source and detector and is called a single –beam instrument Fig.5.4 shows the optical diagram of a single beam instrument Radiation from a source passes through the slit into the monochromator A reflection grating diffracts the radiation, and the selected wavelength band pass through the slit into the sample chamber A solid-state detector converts the intensity into a related electrical signal that is amplified on a digital read out This type of instruments has the limitations with respect to the bandwidth which is relatively fairly large Hence this instrument is more appropriate for a quantitative analysis than for a qualitative analysis In addition the accuracy of a single beam spectrophotometer is limited by the stability of its source and detector over time 5.10.2 Double beam Instruments: Many modern photometers and spectrophotometers are based on a double-beam design; fig-b illustrates a double- beam in-time spectrophotometer in which two beams are formed by a V shaped mirror called a beam splitter One beam passes through the reference solution to a photo detector and the second simultaneously passes through the sample to a second The outputs are amplified, and their ratio, or the log of their ratio, is obtained electronically or computed and displayed on the out put device Double beam Instruments offer the advantage that they compensate for all but the most short-term fluctuations in the radiant output of the source They also compensate the wide variations of source intensity with wavelength Furthermore the double-beam design is well suited for continuous recording of absorption species 5.11 Applications: a).Qualitative Analysis Spectrophotometric measurements with ultraviolet radiation are useful for detecting chromophoric groups, such as those shown in Table.5.2 Table 5.2 Absorption Characteristics of Chromophores: Example C6H13CH=CH2 C5H11C≡C.CH3 λmax 177 178 196 225 186 280 180 293 240 214 339 280 300 665 270 CH3-CO-CH3 CH3-CHO CH3-COOH CH3-CO-NH2 CH3N=NCH3 CH3NO2 C4H9NO C2H5NO2 Transition π → π* π → π* n → σ* n → π* n → σ* n → π* n → π* n → π* n → π* n → π* → n π* → n π* Because large parts of even the most complex organic molecules are transparent to radiation longer than 180nm, the appearance of one or more parts in the region from 200 to 400nm is clear indication of the presence of unsaturated groups or of atoms such as sulfur or halogens The identification of the absorbing groups is done by comparing the spectrum of an analyte with those of simple molecules, containing various chromophoric groups b).Quantitative Analysis Absorption spectroscopy based on ultraviolet and visible radiation is one of the most useful tools available to the analyst for quantitative analysis The determination of an analyte’s concentration based on its absorption of UV or visible radiation is one of the most frequently encountered quantitative analytical methods (i) Environmental Chemistry (ii) Clinical Chemistry : : To analyse metals in water and waste water Determination of total serum protein, serum (iii).Industrial Chemistry: Cholesterol, etc industry pharmaceuticals, food, paint, glass and metals c).Other Applications: It is used for the determination various factor like (i) Rate determination (ii) Determination of Pka values (dissociation constants) of weak acids or bases (iii) Complex ion determination (iv) Determination of percentages of keto and enol forms (v) Determination of ozone level in atmosphere (λ260nm) (vi) Study of cis & trans isomers (vii) Study of H+ ion concentration 5.12 Estimation of Fe2+ by using colorimeter Principle A complex of iron(II) is formed with 1,10-phenanthroline, Fe(C12H8N2)32+, and the absorbance of this colored solution is measured with a spectrophotometer The spectrum is plotted to determine the absorption maximum Hydroxylamine (as the hydrochloride salt to increase solubility) is added to reduce any Fe3+ to Fe2+ and to maintain it in that state Solutions and chemicals required 1) Standard iron(II) solution – Preparation of a standard iron solution 2) 1,10-phenanthroline solution 3) Hydroxylammonium chloride solution 4) Sodium acetate solution Procedure Into a series of 100ml volumetric flasks, add with pipettes 1.00, 2.00, 5.00, 10.00 and 25.00 ml of the standard iron solution Into another 100ml volumetric flask, place 50ml distilled water for a blank volumetric flask The unknown sample will be furnished in another 100ml To each of the flasks (including the unknown) add 1.0ml of the hydroxylammonium chloride solution and 5.0ml of the 1,10-phenanthroline [The iron(II)phenanthroline complex forms at pH to The sodium acetate neutralizes the acid present and adjusts the pH to a value at which the complex forms.] After adding the reagents the solutions is kept at least 15 minutes before making absorbance measurements so that the color of the complex can fully develop Once developed, the color is stable for hours Each solution is diluted to exactly 100ml The standards will correspond to 0.1, 0.2, 0.5, and 2.5 ppm iron, respectively Obtain the absorption spectrum of the iron solution by measuring the absorbance from about 400 to 700nm The blank solution should be used as the reference solution By plotting the absorbance against the wavelength a calibration curve is prepared Measure the unknown in the same way Prepare a calibration curve by plotting the absorbance of the standards against concentration in ppm From this plot and the unknown's absorbance, the concentration of the unknown solution will be determined 5.13 Problems based on Lambert’s Beer Law Example:1 A monochromatic radiation is incident on a solution of 0.05 molar concentration of an absorbing substance The intensity of the radiation is reduced to one-fourth of the initial value after passing through 10cm length of the solution Calculate the molar extinction coefficient of the substance Solution: According to the Lambert-Beer law log Io/I = ∈Cl In this case i.e, I/Io Io/I = 0.25 = 25% = 100/25 log 100/25 = ∈ x 10 cm x0.05 mol.dm-3 ∴ Molar extinction coefficient, ∈ = 1.204 dm3 mol-1 cm-1 Example : The molar extinction coefficient of phenonthroline complex of irons(II) is 12,000 dm3 mol-1 cm-1 and the minimum detectable absorbance is 0.01 Calculate the minimum concentration of the complex that can be detected in a Lambert-Beer law cellm of path length 1.00 cm Solution : A = ∈Cl 0.01 C = A / ∈l =12,000 × = 1.20 x 10-6 mol dm-3 = 1.20 x 10-6 M 5.14 Atomic absorption spectroscopy: 5.14.1 Introduction: Atomic absorption spectroscopy (AAS) is a spectroanalytical procedure for the quantitative determination of chemical elements employing the absorption of optical radiation (light) by free atoms in the gaseous state Atomic absorption spectroscopy has proved itself to be the most powerful technique for the quantitative determination of trace metals in liquids The method was introduced by Alan Walsh in the mid-1950 Atomic absorption spectrophotometer is more popular due to its versatility in measuring about 50-70 elements, including most of the common rare earth elements By this technique, the determination can be made in presence of many other elements It means that it becomes unnecessary to separate the test element from the other element present in the sample and thus it saves a great deal of time and in the process eliminates several sources of error As atomic absorption spectroscopy does not demand sample preparation it is an ideal tool for non-chemist also e.g., the engineers, biologists or clinician are interested only in the significance of the results Atomic absorption spectroscopy (AAS) determines the presence of metals in liquid samples Metals include Fe, Cu, Al, Pb, Ca, Zn, Cd and many more It also measures the concentrations of metals in the samples Typical concentrations range in the low mg/L range In their elemental form, metals will absorb ultraviolet light when they are excited by heat Each metal has a characteristic wavelength that will be absorbed The AAS instrument looks for a particular metal by focusing a beam of uv light at a specific wavelength through a flame and into a detector The sample of interest is aspirated into the flame If that metal is present in the sample, it will absorb some of the light, thus reducing its intensity The instrument measures the change in intensity A computer data system converts the change in intensity into an absorbance As concentration goes up, absorbance goes up The researcher can construct a calibration curve by running standards of various concentrations on the AAS and observing the absorbances 5.14.2 Principle: The absorption of energy by ground state atoms in the gaseous state forms the basis of atomic absorption spectroscopy When a sample containing metallic species is introduced in to a flame, the vapours of metallic species will be obtained Some of the metals atoms may be raised to an energy level sufficiently high to emit the characteristic radiation of the metal This is known as flame emission spectrophotometry But a large percentage of the metal atoms will remain in the non-emitting ground state These ground state atoms of a particular element are receptive of light radiation of their own specific resonance wavelength Thus when a light of this wavelength is allowed to pass through a flame containing atoms of the metallic species, part of the light will be absorbed and the absorption is proportional to the density of the atoms in the flame Thus in AAS it is possible to determine the amount of light absorbed and once this value is known, the concentration of the metallic element can be determined because the absorption is proportional to the density of the atoms in the flame 5.14.3 Instrumentation: The general process that is involved in AAS can be explained as follows Fig.5.5 The components of AAS are: Radiation source Monochromator/ prism for dispersion and isolation of emission Sample container Detector Amplifier The schematic diagram of AAS is shown in Fig.5.6 MX Solution Nebulization Solvent Evaporation MX Solution Aerosol Excitation (AAS) MX* Association M (g) Emission Fig.5.6 (FES,AFS) MX Solid Aerosol MX (g) Dissociation (a) Radiation source: The radiation source for atomic absorption spectrophotometer should emit stable, intense radiation of the element to be determined, usually a resonance line of the element Preferably, the resonance spectral lines should be narrow as compared with the width of the absorption lines to be measured These lines should not be interfered form other spectral lines which are not revolved by spectrophotometer For this reason hollow cathode lamps are generally used The spectral lines produced by the hollow cathode lamp are narrow that they are completely absorbed by the atoms By this method, one can easily detect and measure the atomic absorption Fig.5.10 Each hollow cathode lamp emits the spectrum of that metal which is used in the cathode For example, copper cathode emits the copper spectrum; zinc cathode emits the zinc spectrum and so on At the same time, the narrow spectral lines emitted by copper cathode are only absorbed by the copper atoms present in the sample to be analyzed by atomic absorption spectroscopy Similarly, zinc atoms will absorb spectral lines emitted by zinc cathode For this reason a different hollow cathode lamp has to be used for each element to be analyzed by atomic absorption spectroscopy This is not very convenient In atomic absorption spectrophotometer, gaseous discharge lamps are also used These lamps are called arc lamps which contain an inert gas at low pressure and a metal or metal salt These lamps are useful for the alkali metals, zinc, cadmium and mercury (b) Chopper: A rotating wheel is interposed between the hollow cathode lamp and the flame This rotating wheel is known as chopper and is interposed to break the steady light from the lamp into an intermittent or pulsating light This gives a pulsating current in the photocell ( c ) Burner The most common way is to use a flame which is used for converting the liquid sample into the gaseous state and also for conversion of the molecular entities into an atomic vapour There are two types of burners in common use, (a) the total consumption burner and (b) the premixed burner (d) Monochromators: In atomic absorption measurements the most common monochromators are prisms and gratings commercially packaged atomic absorption instrumentation commonly includes a monochromator of about ½ m focal length with a linear reciprocal dispersion in the range 16-35 Å /mm (c) Detectors: For atomic absorption spectroscopy, the photomultiplier tube is most suitable It has good stability if it is used with a suitable power supply It works satisfactorily and enables to compare intense lines in a satisfactory manner (d) Amplifier: The electric current form the photomultiplier detector is fed to the amplifier which amplifies the electric current many times Generally, ‘Lock-in’ amplifiers are preferred which provide a very narrow frequency band pass and help to achieve an excellent signal – to- noise ratio 5.17 Applications of Atomic absorption spectroscopy: Atomic absorption spectroscopy finds valid applications in every branch of chemical analysis The technique is already a firmly established procedure in analytical chemistry, ceramics, mineralogy, and biochemistry, water supplies, metallurgy and soil analysis 5.15.1 Flame Emission spectroscopy: The principle involved in flame photometry or emission spectroscopy is that when a solution containing a metallic compound is aspirated into a flame, a vapour containing metal atoms will be formed Some of the metal atoms in the gaseous state absorbs thermal energy and gets excited to the higher energy level The excited atoms, which are unstable, quickly emit photons of different wavelength and return to the lower energy level The emitted radiation is passed through an optical filter, which permits the characteristic wavelength of the metal under examination It is then passed into the detector and finally recorded 5.15.2 Instrumentation; The various components of flame photometer are: Pressure regulator and flow meter: These are used for the proper adjustments of pressure and flow of gases Atomiser: It is used to introduce the liquid sample into the flame 3 Burners: These include a total combustion burner a premix burner and an atomizer burner Reflector: The radiation from the flame is emitted in all directions in space In order to increase the amount of radiation reaching the monochromator and the detector, a concave mirror is set behind the burner Filters: It allows only the light of the required wavelength to pass through it Detectors: Produces an electrical signal from the radiation falling on them Fuel Flame excitation unit Air Sample Filter Detector Amplifier Recorder Fig.5.7 Block diagram of flame photometer 5.15.3 Applications: Elements like Na,K,Li,Ca can be easily detected 2.The measurements of these elements is very useful in food ,agriculture,biomedical investigations and in pollution monitoring 3.It is extensively used in determination of alkali and alkaline earth metals in soil,glass,ceramic,cement etc., 5.15.4 DETERMINATION OF CONCENTRATION OF SODIUM AND POTASSIUM IN A GIVEN SAMPLE BY FLAME PHOTOMETRY Principle The estimation of sodium and potassium is based on the emission spectroscopy, which deals with the excitation of electrons from ground state to higher energy state and coming back to its original state with the emission of light Trace amount of sodium and potassium can be determined by flame emission photometry at a wavelength of 589 nm and 766.5 nm respectively The sample is sprayed into gas flame and excitation is carried out under carefully controlled and reproducible conditions The desired spectral line is isolated by the use of interference filters or by a suitable slit arrangement in light-dispersing devices such as prison or grating, intensity of light is measured by a photo tube potentiometer The intensity of light at 589 nm and 766.5 nm is approximately proportional to the comentration of element After careful calibration of photometer with solution of known composition, it is possible to correlate the intensity of a spectral line of unknown solution with the amount of an element present that emits the particular radiation Unknown Absorbance Unknown Absorbance Concentration Na/K Concentration of Na/K 5.15.5 Differences between atomic absorption spectroscopy and flame emission emission spectroscopy: The main differences between atomic absorption spectroscopy and flame emission emission spectroscopy are as follows: (a)In flame emission spectroscopy, the atoms, when put in a flame, become excited Fig.5.8toCalibration graph state, atom which is unstable, quickly emits a photon of light and returns a lower energy eventually reaching the unexcited state The measurement of this emitted radiation forms the basis of flame emission spectroscopy Analytical signal in flame emission is the sum of all energies emitted as excited atoms drop to the ground state The signal comes entirely from the emitting atoms In atomic absorption spectroscopy, the signal is obtained from difference between the intensity of the source in the absence of metallic elements present in the liquid and the decreased intensity obtained when metallic elements are present in the optical path (b)In flame emission spectroscopy, the emission intensity is dependent upon the number of exciting atoms and is, therefore, greatly influenced by temperature variations In atomic absorption spectroscopy, atomic absorption depends upon the number of unexcited atoms and the absorption intensity does not depend upon the temperature of the flame directly (c)In atomic absorption spectroscopy, the relation between absorbance and concentration is nearly linear, that is Beer’s law is obeyed over a wide concentration range This is not true in case of flame emission spectroscopy The Effects of Flame Temperature: Both emission and absorption spectra are affected in a complex way by variations in flame temperature Higher temperatures tend to increase the total atom population of the flame and sensitivity With certain elements, such as the alkali metals, however, this ii-increase in atom population is more than offset by the loss of atoms by ionization, Also, flame temperature determines the relative number of excited and unexcited atoms in a flame In an air/acetylene flame, for example, the ratio of excited to unexcited magnesium atoms can be computed to be about 10E-8, whereas in oxygen acetylene flame, which is about 700C hotter, this ratio is about 10E-6 Control of temperature is thus of prime importance in flame emission methods For example, with a 2500 'C flame, a temperature increase of IO 'C causes the number of sodium atoms in the excited 3p state to increase by about 3% In contrast, the corresponding decrease in number of ground state atoms is only about 0.002% So, emission methods based as they are on the population of excited atoms Requires much closer control of flame temperature than absorption procedures, in which the analytical signal depends upon the number of unexcited atoms The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 10E3 to 10E10 or more And this fact suggests that absorption methods should be significantly more sensitive than emission methods EFFECTS OF TEMPERATURE ON ATOMIC SPECTRA Temperatures can have a dramatic effect on the ratio of excited atomic particles and unexcited atomic particles Emission spectra where quantitative values are being measured would be temperature sensitive (example: a 10oC increase in temperature would increase the number of excited atoms by 4%) Absorption spectra would not be as effected since it measures unexcited atoms Since the proportion of excited to unexcited atoms are say only 0.017% exact, a 4% change in that ratio (to 0.018%) would be inconsequential But temperature fluctuations indirectly affect atomic absorption Increased temperature increases the efficiency of the atomization process (total number of atoms in the vapor) Temperature variations influence the degree of ionization of the sample thus, the concentration of the non-ionized sample on which the analysis is based Electrons can also be excited by the energy in a flame These excited electrons are in equilibrium to the ground state electrons in the flame The equilibrium between the excited electrons and the ground state electrons is not dramatically effected by the temperature of the flame The number of excited electrons increase with a temperature increase but remains constant at a specific temperature Atomic State Number of Atoms 3000 K 3500 K Excited 30 Ground 999,999,999 999,999,970 The change in temperature does not significantly change the number of ground state electrons Therefore, the system is stable in regard to temperature variations [...]... determination (ii) Determination of Pka values (dissociation constants) of weak acids or bases (iii) Complex ion determination (iv) Determination of percentages of keto and enol forms (v) Determination of ozone level in atmosphere (λ260nm) (vi) Study of cis & trans isomers (vii) Study of H+ ion concentration 5.12 Estimation of Fe2+ by using colorimeter Principle A complex of iron(II) is formed with 1,10-phenanthroline,... state atoms of a particular element are receptive of light radiation of their own specific resonance wavelength Thus when a light of this wavelength is allowed to pass through a flame containing atoms of the metallic species, part of the light will be absorbed and the absorption is proportional to the density of the atoms in the flame Thus in AAS it is possible to determine the amount of light absorbed... about 10E-6 Control of temperature is thus of prime importance in flame emission methods For example, with a 2500 'C flame, a temperature increase of IO 'C causes the number of sodium atoms in the excited 3p state to increase by about 3% In contrast, the corresponding decrease in number of ground state atoms is only about 0.002% So, emission methods based as they are on the population of excited atoms... much closer control of flame temperature than do absorption procedures, in which the analytical signal depends upon the number of unexcited atoms The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 10E3 to 10E10 or more And this fact suggests that absorption methods should be significantly more sensitive than emission methods EFFECTS OF TEMPERATURE ON ATOMIC... use of interference filters or by a suitable slit arrangement in light-dispersing devices such as prison or grating, intensity of light is measured by a photo tube potentiometer The intensity of light at 589 nm and 766.5 nm is approximately proportional to the comentration of element After careful calibration of photometer with solution of known composition, it is possible to correlate the intensity of. .. plotting the absorbance of the standards against concentration in ppm From this plot and the unknown's absorbance, the concentration of the unknown solution will be determined 5.13 Problems based on Lambert’s Beer Law Example:1 A monochromatic radiation is incident on a solution of 0.05 molar concentration of an absorbing substance The intensity of the radiation is reduced to one-fourth of the initial value... temperature fluctuations indirectly affect atomic absorption Increased temperature increases the efficiency of the atomization process (total number of atoms in the vapor) Temperature variations influence the degree of ionization of the sample thus, the concentration of the non-ionized sample on which the analysis is based Electrons can also be excited by the energy in a flame These excited electrons are... gaseous state forms the basis of atomic absorption spectroscopy When a sample containing metallic species is introduced in to a flame, the vapours of metallic species will be obtained Some of the metals atoms may be raised to an energy level sufficiently high to emit the characteristic radiation of the metal This is known as flame emission spectrophotometry But a large percentage of the metal atoms will... to increase the total atom population of the flame and sensitivity With certain elements, such as the alkali metals, however, this ii-increase in atom population is more than offset by the loss of atoms by ionization, Also, flame temperature determines the relative number of excited and unexcited atoms in a flame In an air/acetylene flame, for example, the ratio of excited to unexcited magnesium atoms... Procedure Into a series of 100ml volumetric flasks, add with pipettes 1.00, 2.00, 5.00, 10.00 and 25.00 ml of the standard iron solution Into another 100ml volumetric flask, place 50ml distilled water for a blank volumetric flask The unknown sample will be furnished in another 100ml To each of the flasks (including the unknown) add 1.0ml of the hydroxylammonium chloride solution and 5.0ml of the 1,10-phenanthroline ... injection analysis Thermal gravimetry, calorimetry Activation, isotope dilution 5.2 The role of Analytical Instrumental Methods in the field of engineering: Analysis of a chemical property of a compound... comentration of element After careful calibration of photometer with solution of known composition, it is possible to correlate the intensity of a spectral line of unknown solution with the amount of an... of the presence of unsaturated groups or of atoms such as sulfur or halogens The identification of the absorbing groups is done by comparing the spectrum of an analyte with those of simple molecules,