Module 5Electromagnetic spectrum

EMR interaction with matter - absorption and emission of radiation.

UV- visible spectroscopy Principle, instrumentation – Quantitative applications of colorimetric analysis – estimation of concentration of a typical metal ion (Iron)

Atomic absorption and Atomic emission spectroscopy

Principles and instrumentation – flame photometry – Estimation of sodium and potassium ions

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

Instrumental Methods of Analysis

5

(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 MethodRadiation emission Emission spectroscopy – fluorescence, phosphorescence,

luminescenceRadiation absorption Absorption spectroscopy – spectrophotometry, photometry,

nuclear magnetic resonance, electron spin resonanceRadiation scattering Turbidity, RamanRadiation refraction Refractometry, interferometryRadiation diffraction X- ray, electronRadiation rotation Polarimetry, circular dichroismElectrical potential PotentiometryElectrical charge CoulometryElectric current Voltammetry – amperometry, polarographyElectrical resistance ConductometryMass GravimetryMass-to-charge ratio Mass spectrometryRate of reaction Stopped flow, flow injection analysisThermal Thermal gravimetry, calorimetryRadioactivity 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 -rays, X-rays, UV rays, visible spectrum, infra red rays, microwaves and radio waves. The frequency and wavelength of electromagnetic radiation vary over many orders of magnitude. For convenience; electromagnetic radiation is divided into different regions based on the type of atomic or molecular transition that gives rise to the absorption or emission of photons. The details of the electromagnetic radiations are given in the Fig. 5.1

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

10-14 10-12 10-10 10-8 10-6 10-4 10-2 10 0 10 2

10 22 10 20 10 18 10 16 10 14 10 12 10 10 10 8

NuclearCore – level

electronsValence electrons

Molecular vibrations

MolecularRotations

Electron spin

Nuclearspin

- ray X - ray UV IR Microwave Radio Wave

Visible

380 480 580 680 780Violet Blue Green Yellow Orange Red

Wavelength (nm)

Spectral Region

Type ofTransition

Frequency (s-1)

Wavelength (m)

Fig. 5.1. The electromagnetic Spectrum

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 Ultra-violet 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/

where ‘E’ is the energy, 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 ‘’. The order of energies of the electromagnetic radiations is given below.

-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.

1. 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. 2. 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

Emission spectra

Ground State

Excited state

Atom

EM Radiation

Atom emits particular

wavelengths

Absorption spectra

Ground State

Excited state

Atom

EM Radiation

Atom absorbs particular

wavelengths

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.

3. 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:1. Absorption methods: Absorption spectroscopy

(a) UV spectrophotometer,(b) IR spectrophotometer, (c). AAS

2. Emission methods: Flame photometry3. 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.

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 LAWThe intensity of absorption at maximum value ( 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 = 0 and I = I at x = ℓ, we get:

or

Or

or

where = k/ 2.303 is called the molar absorptivity coefficient, and log I0 / I = A is called the absorbance.

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: 1. From the above equation, we can determine the concentration of species absorbing in ultraviolet or visible region.

2. 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

A= C ℓ

log I0 / I = K Cℓ = Cℓ = A 2.303

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.

Fig.5.3. The general block diagram of a simple colorimeter.

1. 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.

2. 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.

3. 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 – quartzVisible region – Glass absorption cells, silica cells and plastic containers.Cell thickness – 1, 2 and 5 cm.

Source of radiation

Monochro-mators Sample

holderDetector

Read out

4. Photometer / Detector:The mechanism behind the photoelectric devices is the conversion of radiant energy

to electrical signal. Basically 3 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.

5. 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).Quanlitative 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 λmax TransitionC6H13CH=CH2 177 π π*

C5H11C≡C.CH3

178 π π*

196 -225 -

CH3-CO-CH3186 n *

280 n π*

CH3-CHO 180 n *

293 n π*

CH3-COOH 240 n π*

CH3-CO-NH2 214 n π*

CH3N=NCH3 339 n π*

CH3NO2 280 n π*

C4H9NO 300 -

665 n π*

C2H5NO2 270 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 : To analyse metals in water and waste water (ii) Clinical Chemistry : Determination of total serum protein, serum

Cholesterol, etc.(iii).Industrial Chemistry: 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

PrincipleA 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 required1) Standard iron(II) solution – Preparation of a standard iron solution 2) 1,10-phenanthroline solution - 3) Hydroxylammonium chloride solution

4) Sodium acetate solution

ProcedureInto 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. The unknown sample will be furnished in another 100ml volumetric flask. 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 2 to 9. 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, 1 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/Io = 0.25 = 25%

i.e, Io/I = 100/25

log 100/25 = x 10 cm x0.05 mol.dm-3

Molar extinction coefficient, = 1.204 dm3 mol-1 cm-1

Example : 2. 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

C = A / l = = 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 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.

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.

The components of AAS are:

1. Radiation source2. Monochromator/ prism for dispersion and isolation of emission.3. Sample container4. Detector5. Amplifier

The schematic diagram of AAS is shown in Fig.5.6.

MX SolutionAerosol

MX SolidAerosol

Nebulization

MX (g)

SolventEvaporation

Dissociation

M (g)

AssociationExcitation

(AAS)

Emission (FES,AFS)

MX*

MX Solution

Fig.5.5

Fig.5.6

(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.

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) Monochramotors:

In atomic absorption measurements the most common monochromators are prisms and gratings commercially packaged atomic absorption instrumentation commonly includes

Fig.5.10

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 preffered 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:1. Pressure regulator and flow meter: These are used for the proper adjustments of pressure and flow of gases.2. 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.

FilterFlame excitation unit

Detector Amplifier

Recorder

Fuel

Air Sample

4. 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.5. Filters: It allows only the light of the required wavelength to pass through it.6. Detectors: Produces an electrical signal from the radiation falling on them.

Fig.5.7 Block diagram of flame photometer

5.15.3 Applications:1. 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

PrincipleThe 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.

UnknownAbsorbance

Concentration of Na/K

UnknownAbsorbance

Concentration 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 atom which is unstable, quickly emits a photon of light and returns to a lower energy state, 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.