This article throws light upon the top five instrumental techniques used for monitoring industrial pollutants. The techniques are: 1. Atomic Absorption Spectroscopy 2. Nephelometry and Turbidimetry 3. Gas Chromatography 4. Polarography 5. Voltammetry and Chronopotentiometry.
Instrumental Technique # 1. Atomic Absorption Spectroscopy:
This technique was invented by Alan Walsh in 1950’s for the qualitative determination of trace metals in liquids. The superiority of the technique over other is based on the fact that by this technique 50-60 elements can be determined without any interference from trace to big quantities.
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All these elements can be detected here which fail to yield satisfactory results in flame photometry. Thus, it is a successful instrument for detection and estimation of metals and non-metals both type of pollution from factories.
The technique has also been proved very helpful to both aqueous and non-aqueous solutions.
Principle:
When a solution having a mixture of metallic species is introduced into the flame, the solvent evaporates and vapours of metallic species are obtained. Some of metal atoms can be raised to an energy level sufficiently high to emit characteristic radiation of metal—a phenomenon that is used in flame photometry.
Here a large amount of metal atoms remain in non-emitting ground state. These ground state atoms of a particular element are receptive of light radiation of their own specific resonance wavelength.
In this way, when a height of this wavelength passes through a flame, a part of light will be absorbed and this absorption will be proportional to the density of atoms in the flame. So in atomic absorption spectroscopy the amount of light absorbed is determined because the absorption is proportional to the concentration of the element.
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Advantages of Atomic Absorption over Flame Photometry:
(1) It does not suffer from spectral interferences which occur in flame emission spectroscopy.
(2) It is independent of flame temperature.
(3) By atomic absorption technique, traces of one element can easily be determined in presence of high concentration of other elements.
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(4) It has been proved very successful in the analysis of bronze and copper alloys and in the determination of metals like platinum, gold etc.
Disadvantages of Atomic Absorption Spectroscopy:
Some of the disadvantages can be summarised as follows:
(1) This technique has not been proved very successful for the estimation of elements like V, Si, Mo, Ti and Al because these elements give oxides in the flame.
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(2) In aqueous solution, the anion affects the signal to a noticeable degree.
(3) A separate lamp is needed for the determination of each element. Attempts are being made to overcome this difficulty by using a continuous source.
Instrument:
The apparatus consists of:
(1) Radiant source.
(2) Atomizer.
(3) Monochromatic.
(4) Lenses and Slits.
(5) Detectors.
The main compounds used in the instrument can be described as follows:
Radiation Source:
Generally a hydrogen lamp is used for a continuous source of radiation.
Atomizer:
Generally burners are used to break the liquid sample into droplets which are then allowed to enter into flame. These droplets are then evaporated and sample element is left in residue. The residue is then decomposed by flame. Thus in this process the sample is reduced to atoms.
Hollow Cathode Lamp:
For atomic absorption spectroscopy the radiation source is a hollow cathode lamp (shown in figure 2).
It consists of the following parts:
(i) Cathode; is made of the element to be determined or coated with it
(ii) Anode; is made of tungsten, zirconium or nickel.
(iii) Window; is made of Pyrex glass depending on wavelength of emitted radiation.
(iv) The lamp is filled with neon or argon gas.
These gases emit sharp line spectra.
Generally these lamps are constructed for individual elements but multi-element lamps have also been made for all purposes.
The hydrogen lamp is a hollow cathode lamp.
A hollow cathode lamp emit more than one composite line for each element but the required spectral line can be separated by means of a relatively low dispersion monochromator. Most of lines are non-absorbing lines because they involve transition other than from ground state.
The most intense absorption line is selected to provide maximum intensity. The inlet and exit slit widths of monochromator should be narrow to isolate the particular line being used; the requirements depend on:
(1) Focal length,
(2) Grating ruling of monochromator.
(4) Monochromator:
Generally the monochromators are gratings and prisms.
(5) Filters or Slits:
Filters or slits are used for slits isolation of required spectral line if element has a simple line spectrum. Filter photometers are used for determination of potassium, sodium, calcium, magnesium etc. in samples.
(6) Detectors:
Generally photomultipliers are used as detectors. In some instruments two filters and two detectors are used to compensate the fluctuations in the output of source.
Experimental Technique:
First of all, a meter is adjusted to read zero absorbance or 100% transmittance when a blank solution is sprayed into the, flame and light of hollow cathode lamp passes on to photompltiplier tube. Now the solution to be investigated is introduced, a certain part of light is absorbed resulting in decrease of light intensity falling on photomultiplier. This gives a deflection in the meter needle which is noted immediately.
As this is a comparative method hence standard solutions of elements are used to make a calibration curve from which the concentration of sample elements can be calculated.
Interferences:
(1) Chemical Interferences:
In practice, it has been found that phosphate ions interfere with determination of calcium and magnesium. This interference can be reduced by adding a salt of lanthanum.
(2) Ionisation Interferences:
Ionisation interference is caused due to alkali metals which need low energy for ionisation. For example, a loss in spectrochemical sensitivity results due to splitting of a free metal atom into a positive ion and an electron
(3) Role of Solvent:
Solvent plays an important part to interfere in the determination of concentration of metals. The results have shown that metals in aqueous solution yield lower absorbance readings than same concentration of such metals when present in the organic solvent.
(4) Dissociation of Metal Compounds:
Some metals like Al, Ti, etc. when subjected to flame give oxides in place of metal atoms and thus complete the system.
(5) Spectral Interferences:
Sometimes interference occurs due to overlapping of any radiation with that of characteristic radiation of sample element, e.g., potassium doublet (4044, 4047A) manganese triplet (4031, 4033 and 4035 A). This interference can be removed by working with AC amplifiers in the technique.
Applications:
(i) Quantitative Analysis:
As we know that each element has its own characteristic emission spectrum, hence the intensity of the lines is compared with standard and the concentration can be easily evaluated from the graph (Fig. 3).
Suppose the intensity of unknown element is C, then the concentration is evaluated by drawing a perpendicular on the line (calibration curve) and from the point it cuts the curve, a perpendicular is drawn on the x-axis. The value from (0 to 9) will give the concentration of unknown in moles per litre.
In atomic absorption spectroscopy, the same method is followed for determining the concentration of the element in an unknown solution.
(ii) Method of Standard Addition:
If calibration graph is linear, the sample concentration can be calculated by adding known amount of the test element to the sample. This gives selection of calibration graph above the unknown sample concentration and the resulting straight line can be extrapolated back to zero signal intensity.
The concentration scale is determined by standard additions and unknown concentration is given by the point at which extrapolated line crosses concentration axis.
(iii) Quantitative Analysis:
Generally first a curve is plotted between absorbance values vs. concentrations of standard samples of the element. A linear curve is obtained.
From this curve, the concentration of unknown is evaluated by working absorbance value only from the following equation:
A = S x C
or Absorbance = Slope x Concentration
It is very sensitive technique and hence it gives accurate results than many analytical methods.
2. Nephelometry and Turbidimetry:
Nephelometry and Turbidimetry are used for continuous monitoring of air and water pollution. In water, turbidity is monitored whereas in air, smoke and dust are monitored. The techniques are also used in food, beverages and in the determination of molecular weight of high polymers which are settled down on earth from factories. The CO2 can also be determined by this technique.
Nephelometric analysis (nephelometry) is based on measuring the intensity of a luminous flux scattered by solid particles suspended in solution. Turbidimetric analysis (turbidimetry) is based on measuring the weakening of intensity of a luminous flux when it passes through a solution containing particles in suspension. The intensity decreases owing to absorption and scattering of light.
Principles and Theory:
The principle of nephelometry and turbidimetry is based on the scattering or absorption of light by solid or colloidal particles suspended in solution.
When light is passed through the suspension, part of incident radiant energy is dissipated by absorption, refraction, and reflection while remainder is transmitted. In fact the measurement of the intensiy of transmitted light as a function of the concentration of dispersed phase is the basis of turbidimetric analysis.
This is given in the following diagram 1.
Nephelometry is somewhat different than turbidimetry. In nephelometry light is passed through the sample solution (suspended particles) directly and the amount of scattered radiation is measured generally at 90° angle (Fig. 2).
The measurement of intensity of scattered light as a function of concentration of dispersed phase is the basis of analysis of nephelometry. It is very important to note that in nephelometry incident and scattered light are of same wavelength whereas in fluorimeter (in fluorimetry) scattered light is of longer wavelength than incident light.
Theory:
For turbidimetric measurements the transmitted intensity I can be determined from the equation
where IO = incident intensity
It = transmitted intensity.
c = concentration of absorbing particles in the solution.
1 = thickness of absorbing layer of solution.
This equation (1) is known as the basic equation of turbidimetry and is similar to Bouguer-Lambert-Beer equation:
where k’ is molar turbidity coefficient of solution.
Operating Conditions:
In turbidimetric and nephelometric analysis a number of conditions must be satisfied for a successful working. As the amount of light scattered or absorbed depends on size of the particles in the solution, hence correct results will depend upon method of preparing the suspensions and on reproducibility of their optical properties.
There are following factors which influence optical properties of suspension and particle size:
(a) Ratio of concentrations of solutions mixed,
(b) Order of mixing of solutions,
(c) Concentration of ions forming the precipitate,
(d) Temperature,
(e) Rate of mixing,
(f) Presence of extraneous electrolytes,
(g) Stability of dispersion,
(h) Presence of non-electrolytes,
(i) Presence of protective colloids,
(j) Time required to attain maximum turbidity.
Instrumentation:
The instruments used in nephelometry and turbidimetry are similar as used in spectrophotometry.
We will describe here some special features:
(1) Sources:
Monochromatic radiation is used both in turbidimeter and ncphelometer. Generally a mercury arc or a laser with special filter combinations for isolating one of its emission lines is the most suitable source.
The tungsten lamp (which is polychromatic source) is used when one has to determine the concentration of a particular substance. It has been observed that even in such a case blue spectral region gives the best results.
(2) Cells:
Generally a cell with a rectangular cross section is selected for the study. We can also use cylindrical cells having flat faces where entering and existing beams are passed. The octagonal faces allow measurements to be made at 135°, 90°, 45° or 0° to primary beam. The walls through which light beam are not to pass, are coated black so as to absorb unwanted radiation.
(3) Detectors:
In turbidimeters, phototubes are used as detectors. The photo-multiplier tubes are used as detectors in case of nephelometers because intensity of scattered radiation is generally very small.
Generally the detector is fixed at 90° to primary but for maximum sensitivity the detector angle should vary. There are some nephelometers where detector is mounted on a circular disc which allows measurements at many angles, i.e., at 0° and from 35° to 135°.
Experimental technique:
A powerful light from electric lamp passes through filter which is just put in place only when instrument is to be utilized for luminescence studies, and falls on glass plate. Some part of beam is reflected from this plate and falls on glass attenuator, while part of it enters cell filled with solution under study. Now the light beam passes through cell is extinguished in light trap.
The part of luminous flux reflected by particles in solution passes through lens, adjustable diaphragm, lens and then is directed by rhombic prism through filter into eye piece where it illuminates only one half of the optical field. The luminous flux from attenuator traverses a similar path through lens, adjustable diaphragm, lens, rhombic prism, filter and enters eye piece to illuminate second half of the optical field.
Now by varying slit width of adjustable diaphragms, luminous fluxes can be equalized, i.e., optical equilibrium attained. While working with this instrument solution under study should be placed in cell, dials of adjustable diaphragms and are set to zero and intensities of luminous fluxes are brought close by putting in interchangeable attenuators. Then luminous fluxes are equalized by means of adjustable diaphragms.
Such measurements are made for a series of solutions containing definite quantities of substance analysed and then a calibration curve is plotted relating the adjustable diaphragm reading to the concentration of solutions. After this, the unknown concentration can be determined from the diaphragm readings by using this calibration curve.
Calculation:
The concentration of metals or ions can be determined from the formula:
where C1 and C2 are concentrations of standard and unknown solutions respectively.
I1 and 12 are layer thickness in nephelometer cells.
Applications of Turbidimetry and Nephelometry:
Turbidimetry and nephelometry can be used in gaseous, liquid or even transparent solid samples.
The applications of both these techniques can be summarised as given below:
Air and Water Pollution:
Turbidimetry and nephelometry are used in the study of air and water pollution. In air, dust and smoke are measured whereas in water, turbidity is measured.
Inorganic Analysis:
The main uses of nephelometry and turbidimetry are the determination of sulphate as BaSO4, chloride as AgCl, fluoride as CaF2, cyanide as AgCN, calcium as oxalate, carbonate as BaC03 and Zinc as ferrocyanide. Out of all these, sulphate determination is of high importance and serves for determination of total sulphur in oils, coal, coke, plastics, rubbers, and other organic substances. For determination of sulphur, it is converted into sulphate and then shaken with sodium.
Chloride solution and excess of solid barium chloride are needed to get a suspension of barium sulphate. Finally this suspension is subjected to nephelometry or turbidimetry as the case may be and the concentration of suspension can be evaluated from calibration curve.
The CO2 can also be determined by the techniques:
The procedure includes the bubbling of gas through an alkaline solution of a barium salt and then analysing for BaCO3 suspension with nephelometry or turbidimetry. The analysis can give high accuracy in determining the amount of CO2 in the sample.
Instrumental Technique # 3. Gas Chromatography:
The word chromatography has been coined in 1906 by a Russian Botanist. Michael Tswett published a description of separation of chlorophylls and other pigments in a plat extract. The appearance of coloured bands proposed the name ‘Chromatography’ from the Greek words for ‘colour’ and ‘to write’. Besides Tswett, now Martin Gordon, Synge, Williams, Weil, Classidy can also be called pioneers in this field.
Chromatography is mostly employed for the purification and separation of organic and inorganic substances present in polluted air. It is the most successful technique used in the analysis of air pollution. It has been proved as a versatile tool for the fractionation of complex mixtures, separation of isomers, homologous etc. and also in the isolation of unstable substances.
The main advantage in this technique is that components are recovered without change or alteration. Thus it is a better technique over usual analytical procedures which either contaminate or destroy the component to be separated.
The technique of chromatography depends upon the differential distributions of the sample components between two phases. One phase is fixed in the system and is called stationary phase while the other is not fixed and called the mobile phase. This mobile phase causes a differential migration of the sample components.
Distribution Equilibria:
The partition coefficient or distribution coefficient K between two phases of a system can be written as
K = Cs/Cm
where Cs and Cm are the concentrations of stationary and mobile phases respectively.
This K determines the relative populations in two phases. If K is large, the population in the stationary phase is large than that in mobile phase and molecules of component will spend enough time in stationary phase.
The time utilized in mobile phase
= No. of molecules in mobile phase/Total no. of molecules
For the identification of the component, the rate flow, Rf is calculated from the following formula and is compared with the standard values given in the literature.
RF distance solute moved/distance solvent moved
All the distances are measured from the centre of the spot or band. The RF is also called the retardation factor.
(i) Development:
The process of allowing the solvent to move along the filter paper is called development.
(ii) Resolution:
It is defined as the degree of separation of components after development.
(iii) Rate of Travel:
The rate of travel of average molecule depends on:
(a) Distribution coefficient
(b) Carrier velocity (same for all components)
(c) Ratio of volumes of stationary phase to mobile phase (same for all components).
Chromatogram:
In a column chromatography, each component comes out after a definite interval and is ‘sensed’ by a detector. The plot of detector response vs. time is called chromatogram.
Retention Time:
The time taken by a component to move the length of the column is called the retention time.
Retention time = length/rate
Retention Volume:
It is defined as the product of flow rate and retention time.
Retention volume = retention time x flow rate.
The most recent development in chromatography known as gas chromatography was introduced in 1952 by James and Martin. Their initial work was not given due importance but this technique is now one of the most widely used applications of chromatography. Gas chromatography is now-a-days used as an important analytical tool for the separation of gases and volatile substances.
(i) Gas Chromatography is defined as a method of separation in which gaseous or the vaporized components to be separated are distributed between two phases, a fixed stationary phase with large surface area and a moving gas phase.
(ii) Gas-Solid Chromatography is defined as a method of separation in which fixed phase is a solid adsorbent.
(iii) Gas-liquid Chromatography is defined that method of separation which has a fixed phase, a liquid distributed in an inert support. This is also known as Vapour phase chromatography.
Separation of constituents by gas solid chromatography and gas-liquid chromatography involve the following techniques:
(a) Elution analysis,
(b) Displacement analysis, and
(c) Frontal analysis.
(a) Elution Analysis:
In this technique the sample is introduced into the column and is carried through the column by inert gas like helium or nitrogen. In gas-solid chromatography the constituents of the sample are selectively retarded by solid adsorbent while in gas-liquid chromate-graph the constituents are retarded by liquid phase. In this analysis best separations are obtained more readily than by any two other techniques.
Apparatus:
The apparatus usually consists of the following parts:
(i) Carrier gas tank.
(ii) Sample injector.
(iii) Adsorbent column.
(iv) Detectors.
(v) Gas flow meters.
The experimental arrangement for gas chromatography is represented in figure 1 and 2 below.
(i) Carrier Gas Tank:
In this tank, helium is used as best carrier gas. Nitrogen, hydrogen and carbon dioxide are also used as carrier gases. Generally inert gas having a high coefficient of thermal conductivity is chosen as carrier gas.
(ii) Sample Injector:
This is a small receptacle covered with a stretched out rubber sheet. It can also be fitted with a rubber cap similar to the caps on injection capsule. The main aim is to make an efficient gas-tight self-sealing inlet. A ‘By pass’ sample injector has also been devised by Shell Development Co. R.E. Davis and J.M. Mc. crea has also devised a plugged mercury sealed orifice for gas chromatography.
(iii) Adsorbent Column:
This column is made up of metal or glass tube of diameter 0 to 5 m.m. and capable of holding 6 c.c. of material per foot length. The adsorbent material is always poured from the upper end and gently tapped down. The tube is then suitable bent in the form of u or uu. The u and uu models which are generally used are known as Perkin Elmer and Burrell models.
The appropriate bent and Packed adsorbent column is enclosed in a well adjusted thermostat chamber whose temperature can be maintained above the condensing point of the components of the vapour mixture. Generally a temperature gradient along a column is maintained to facilitate better resolution of the chromatogram.
(iii) Adsorbents:
Different partition liquids are used for different purposes depending on the nature of the work. In isolating saturated gases from unsaturated, a silver nitrate glycerine column is used. Tri-isobutylene and paraffin columns are used for chlorinated and lower hydrocarbons.
The utility of different adsorbent liquids can be summarised as follows:
(i) Up-to 225°C, silicon substrate is of generally utility being fast and inert.
(ii) Carbo wax is stable up-to 225°C and is used for analysis of polar substances.
(iii) Up-to 225°C, liquid paraffin is used for the analysis of hydrocarbons such as methane, ethane etc.
(iv) Silicon-Stearic acid substrate can also be used for fatty acid analysis up to 225°C.
(iv) Detectors:
Generally three types of detectors are used:
(1) Integral Detectors:
It works on a function of whole quantity. It gives record to the components in the mixture, and
(2) Differential Detectors:
It works on a function of vapour concentration. It gives peaks in elution analysis.
(3) Hydrogen Flame Detector:
Here hydrogen is used.
Signals for differential detectors are mostly integrated for qualitative analysis and signal from integral detectors are differentiated to interpret for qualitative analysis. Either signals (as shown in figures 3) can be obtained from the other.
Hydrogen Flame Detector:
In this detector hydrogen is used as a carrier gas. It is one of the simplest detectors. The diagram is given below (Figure 4).
Here the retention time is measured with stopwatch. The amount of component is roughly proportional to height and/or luminosity of flame. Since most organic compounds are ionized in flame, an ion current can be collected between two oppositely charged electrode. This is the principle of flame ionization detector.
This is now-a-days most popular and a sensitive detector. The figure is shown in Fig. 5. The ionization processes, occurring in flame are not completely understood. However, it is considered that ion current is nearly proportional to number of carbon atoms entering the flame.
The other types of differential detectors are:
a. infrared analyser,
b. gas density balance,
c. surface potential detector,
d. hydrogen flame detector, and
e. electron capture detector.
Generally all the differential detectors used, depend upon the following properties of the gas:
(i) Glow discharge,
(ii) Dielectric constant of the gas,
(iii) Photo ionization,
(iv) Flame ionization,
(v) Gas density,
(vi) P-ray ionization,
(vii) Thermal conductivity,
(viii) Flame temperature,
(ix) Concentration of the gas.
(v) Gas Flow Meters:
The number of devices for measuring the gas flow rate is as follows:
(a) The Mobile Bubble Flow Meter:
In this a concentrated solution of soap is used in flow meter. Generally oleate or ammonium stearate soap solution is used.
(b)Rota meters.
(c) U-tube flow-meter.
Experimental Technique:
First of all apparatus is set up as shown in fig. 1. Now the column is packed with the treated celite (take about a third as much of partition liquid as there is celite and dissolve the liquid in volatile solvent and mix gently with celite.
Evaporate the volatile liquid off and now the celite obtained is called treated celite). The gas pressure is adjusted according to the need and pressure is noted. Now the carrier gas is allowed to flow until the thermal conductivity cell becomes stable. Finally sample of definite quantity is introduced and time is noted.
The galvanometer or potentiometer readings are noted for every 20 seconds at first instance and then after some minutes. Now we run samples of pure materials from which the unknown are prepared to calibrate the apparatus. Plot response versus time. From the time at which components appear, substances present in the unknown are identified.
Applications:
(i) Qualitative Analysis:
Comparison of retention behaviour with a known sample is almost certain proof of identity. But generally the components are identified from the retention time.
The distribution coefficient K, which is a characteristic of component and liquid phase at a given temperature, can also serve as an identifying parameter. The compounds can be collected in different tubes and can be identified with the help of N.M.R., I.R. and mass spectrometry.
(ii) Quantitative analysis:
The quantitative analysis can be performed by measuring the area under peak.
For measurement of the area:
(1) Cut out the peak with a scissor and weigh the paper. Now calculate the weight of paper per unit area in a separate experiment.
(2) Estimate the area by triangulation. Generally tangents are drawn to the point of inflection on peak sides and area of triangle is calculated.
Measurement of peak area is done by triangulation (fig. 7).
X = Y + Z
Measuring peak width and peak height at half height then multiply these values.
Gas-Solid Chromatography:
Separation of GSC, is very similar to GLC. The main difference is that partition within the column is caused by partial and selective adsorption on a solid surface rather than solubility as liquid phase. The apparatus is identical to GLC except that for column packing which is a porous solid with a high specific surface. Some common adsorbents in GSC are Carbon, Calcium zeolites, Ammoniates etc.
Gas-liquid Chromatography:
The versatility of GLC is due to wide variety of liquid phase available. Generally liquids like methyl silicone, dinonyl phthalate, silicon oil, carbowax 20 M, polyamid resin, AgN03 in propylene glycol are used.
The liquid for GLC should have the following characteristics:
(1) It should be non-volatile;
(2) It must be thermally stable;
(3) It should be inert towards the solute;
(4) It should be available in reproducible form.
The procedure of analysis is the same as described in case of gas chromatography.
Applications:
(1) The technique is useful in the identification of 60—70 polluted gases like methyl isocyanate, phosgene, methane etc. at one time fuels and toxic gases.
(2) The GLC has been proved very helpful in the separation of complex organic mixtures and essential oils.
(3) Alcohols, aromatic amines, ketones, amino compounds can easily be separated by the technique.
(4) The GLC is a versatile tool for the separation of volatile in-organics which pollute the atmosphere.
(5) The high boding hydrocarbons, esters, ethers, alkaloids and pesticides can easily be separated and identified.
(6) The technique is used for the identification of natural products and other environmental pollutants.
(7) The technique is a boon in biomedical applications.
It is possible that sometimes when you walk into a medical lab, leave a sample of blood, urine and breath and within a few minutes a complete diagnosis of the state of your health can be noticed with these techniques. It can detect the elements present in blood etc. even to the extent of 1 ppm. Now-a-days this technique together with computer and mass unit is used to give the results within 1 minute.
Instrumental Technique # 4. Polarography:
Polarography is a technique concerned with electrode reactions at the indicator or microelectrode, i.e., with reactions involving transfer of electrons between the electrode and the components of the solutions. These components are called reductants when they can lose electrons and oxidants when they can accept electrons.
The electrode is cathode when reduction takes place at its surface and an anode when oxidation takes place at its surface.
Polarography deals with the relationship among current, electrode potential and solution composition in a cell of which one electrode is a dropping mercury electrode (cathode) and other is a pool of mercury (anode).
The current voltage curves can be interpreted to give both qualitative and quantitative composition of the solution. As the curve obtained with the instrument is a graphical representation of the polarization of dropping
electrode, the apparatus is called a polarograph. A polarogram is a plot of the current flowing through a polarographic cell against the potential of dropping electrode.
Polarographic methods are those methods of analysis in which advantage is taken of polarization processes at mercury or other cathode. In 1922 Heyrovsky the inventor of polarography suggested these methods of analysis for qualitative and quantitative determination of the substances present in environment.
The metallic air pollutants of minute concentrations can easily be detected and estimated by this technique.
Types of Polarography:
There are different types of polarography:
(1) Simple Polarography:
In this case, polarographic curve is obtained as potential (E) versus current (i).
(2) Differential Polarography:
This is another technique used in polarographic analysis. Here polarographic curve is obtained as potential (E)
Versus ∆I/∆E (where Ai and AE are current and potential differences respectively)
Differential polarography can also be defined as that branch of polarography where the current increment with varying voltage is plotted along the ordinates instead of current intensity. It has a greater resolving power than simple polarographs.
(3) Oscillographic Polarography:
In this type of polarography, cathode ray oscillograph is used in polarography practice for obtaining various kinds of polarographic curves.
(4) Pulse polarography or Alternating Current Polarography:
Here an alternating current component of comparatively low voltage (about 20 mV), is applied to the electrodes in addition to the usual direct current. Pulse polarograms enable detection of peaks of compounds which refuse to be reduced at the electrode. This polarography has found extensive use in plant chemistry.
Factors Affecting the Current-Voltage Curve:
There are various factors which affect or may affect the current-voltage curve.
They are as follows:
(1) Non-Faradaic or condenser current
(2) Residual current
(3) Diffusion current
(4) Migration current
(5) Kinetic current
(6) Catalytic current
(7) Adsorption current
The most important among above is diffusion current.
Diffusion Current:
When a dropping mercury electrode is used, it is observed that if a solution of substance is taken then in the bulk of the solution the concentration of ions remains constant. Fresh amount of ions are transported to the electrode surface due to diffusion which is proportional to the difference of concentrations.
This difference of concentration has been found to be equal to the concentration of ions in the bulk of solution. Thus a limiting current, called diffusion current, is established. According to D. Ilkovic, the diffusion current is equal to
id = 607 nD1/2 Cm2/3t1‘6
where 607 = a combination of natural constants,
id = average diffusion current,
n = number of Faradays,
D = diffusion coefficient of oxidisable or reducible substances in the limit cm-2 sec-1,
C = concentration in the molecules per litre,
m = rate of flow of mercury from dropping electrode,
t = drop time in seconds.
The diffusion current id depends upon following factors:
(a) Temperature
(b) Viscosity of the medium
(c) The molecular or ionic state of electro-active species
(d) Composition of base electrolyte
(e) Composition of capillary of the dropping mercury electrode
(f) And pressure on dropping mercury.
Half Wave Potential:
A plot of current against applied voltage is called a polarogram depicted here in Fig. 1. A reducible substance is said to give a polarographic ‘wave’. The height of the curve which is known as wave height, is the diffusion current.
Half wave potential is defined as follows:
‘A half wave potential is the potential of the middle of a polarographic wave: It depends only on the nature of reducible ion and is independent of its concentration’.
Here in the diagram EF is the half wave potential. AB is the residual current and CD denotes limiting current. Half wave potential defines the characteristic of the reacting material and is the basis of quantitative and qualitative polarographic analysis.
Now consider oxidation-reduction system
Oxidant + n electrons 
Reductant.
Ox + ne 
Red ….(1)
The electrode potential E can be written as:
R = gas constant; T = Absolute temperature,
n = number of electrons involved in the reaction.
F = Faraday,
aax. and ared. = activities of oxidant and reductant respectively.
As polarographic measurements have accuracy about ±1 millivolt, hence substitution of concentration in place of activities will not introduce any error.
Here E1 = standard potential reaction against reference electrode used to measure potential of dropping electrode.
E = the potential which refers to the average value during life of mercury drop.
K = a constant
id = diffusion current
i = current at any point on the wave.
where E’ = a potential having some constants
and E1/2 = Half wave potential when id\2. This E1/2 is a characteristic constant for a reversible oxidation—reduction system.
At 25°C, equations (4) and (5) give
This equation (6) is known as equation of polarographic wave. The Half wave potential E1/2 serves for the qualitative identification of an unknown substance as it is independent of electrode characteristics. For a complex ion it can be shown that
where E(1/2)C = Half wave potential of complex ion at 25°C
E(l/2)s = Have wave potential of simple ions at 25°C
D = dissociation constant of the complex ion
L = coordination number
[X-R] = concentration of complexing agent X-R in the body of solution.
This equation (7) is used for the determination of the coordination number L of the complex ion and also to evaluate dissociation constant of the complex ion.
Construction of Polarographic Unit:
The simplified diagram of polarographic unit is shown in Fig. 2:
(i) Anode:
Anode is a pool of mercury. It has very large area so that it may lead to incapability of becoming polarised i.e., its potential remains constant in a medium containing anions capable of forming insoluble salts with mercury. Anode also acts as a reference electrode.
In anode compartment, inlet tube is provided to introduce inert gas (nitrogen etc.) for expelling dissolved oxygen from the testing solution placed in the compartment. The dissolved oxygen is removed from outlet tube. It is always necessary to remove dissolved oxygen from outlet tube.
It is always necessary to remove dissolved oxygen from the solution before performing the actual experiment as otherwise the polarogram of dissolved oxygen will appear in the current-voltage curve. V is voltmeter which records voltage applied to the cell.
Through the potentiometer in the circuit, any e.m.f. up to 3 volts can be applied to the cell. The shunt is used for increasing the sensitivity to the galvanometer.
If we increase voltage continuously, we increase potential on the electrode. Hence if current passing through cell is recorded depending on the voltage applied, we get polarization current voltage curves or polarograms.
Principle and Working:
Polarography is a special case of chronoamperometry in which dropping mercury electrode is employed. The principle is based on the fact that mass transport takes place by diffusion. In this technique, a current-voltage curve called a polarogram is recorded in practice. A limiting (diffusion) current is obtained which is proportional to the concentration of the electro-active species in the bulk of the solution.
A solution for polarographic analysis must contain an electrolyte which carries current through the solution. This electrolyte is called supporting electrolyte or indifferent electrolyte such as KC1 etc.
Suppose the solution under investigation is a cadmium salt at low concentration (0.001 M). It is mixed with supporting electrolyte (KC1 – 0.1 M) and placed in the cell. Nitrogen is passed in the cell to remove the dissolved oxygen, (as it gives its own polarogram and which interferes with the actual polarogram).
The solution is not stirred. The voltage applied to the cell is varied with the help of potentiometer (as calibrated wire) and current is read on galvanometer.
The applied voltage is sufficiently large to cause cadmium to be reduced at the cathode. As cathode is very small hence the current is correspondingly small, only a few microamperes. As cadmium is reduced at cathode, mercury reacts with chloride ions at anode to give mercurous chloride.
2Hg + 2C1- 
Hg2Cl2 + 2e, Ea = + 0.25V (sat.)
As only a small current flows hence the concentration of chloride ions is not changed appreciably, so the term Ea remains constant.
The current-voltage curve as shown above shows that along AB a small reduced current flows. At B the decomposition voltage of cell is reached and current increases very fastly with increase in voltage. The cadmium is dissolved in Hg as an amalgam.
The reaction is represented as:
Cd++ + 2Hg + 2C1- 
Cd++ + Hg2Cl2
Along the portion BC the concentration of cadmium ions at the electrode interface drops to a smaller valve than concentration in the body of the solution because solution is not stirred. Actually this difference in concentration (concentration gradient) causes cadmium ions to move toward the cathode by process of diffusion.
The rate of diffusion is proportional to concentration gradient,
Rate of diffusion α[Cd++]bulk — [Cd++]interface
As the point C is approached, the concentration of Cd ions at interface falls to a low value and between C and D the concentration is so that small that it can be considered as negligible. Hence
Rate of diffusion α [Cd++]bulk
= K1 [Cd++]interface
Where KI = proportionality constant.
As we know that diffusion (id) is proportional to rate of diffusion,
id = k [Cd++]
where [Cd++] = concentration in the body of the solution.
Hence concentration of cadmium ions can be determined from the value of diffusion current.
Experimental Technique for More Than One Species:
The test solution containing about 10 -3 to 10 -4 M concentration of reducible species Pb+2 and Zn+2 in 1 M KC1 (supporting electrolyte) is taken in a cell. The nitrogen is passed in the cell for some-time to remove the air otherwise dissolved oxygen begins to be reduced at dropping mercury electrode and interferes with the work of analysis by giving its own polarogram.
Pure mercury is allowed to fall by capillary at the rate of 20 to 30 drops per minute. Each drop which is held at the end of capillary tube for 2-3 seconds acts as the cathode. When one drop falls, second comes to its place and so on.
Thus there is a continuous renewal of mercury drops. The pool of mercury serves as cathode. The applied voltage can be varied by moving sliding contact along the potentiometer wire. The current strength is measured by galvanometer or micro-ammeter. The potential of cathode is determined by subtracting from applied E.M.F. the potential of calomel electrode.
The current-voltage curve known as polarogram will be of the shape shown in fig. 5. The limiting current, which is given by the height of the wave, is proportional to the concentration of electro-active species.
The potential at the centre of the rising part of the wave, referred as half wave potential is characteristic of species being discharged and is independent of its concentration. The half wave potential is used to identify the particular species. Thus from polarogram, the relative amounts of various cations in the mixture can be calculated.
The great advantage of polarographic method of analysis is that a mixture consisting of a large number of reducible substances can be estimated in one solution and in a single operation. Thus if a solution containing Cu++, Pb++, Cd++, Zn++, Mn++ and Ba++ is subjected to polarographic analysis, a composite polarogram is obtained.
Applications:
(1) Qualitative determinations:
As half wave potential is independent of the concentration of the electro-active species in solution hence can be used for identification of the unknown species. The half wave potentials are compared with the standard values and thus identification is made.
(2) Quantitative Determinations:
The polarographic method is employed to determine concentrations in the range of 10-4 to 10-2 M. Sometimes the concentration is as low as 10-6 M can be detected. Since concentration is proportional to diffusion current, the main problem is to measure this current very accurately.
There are various methods to obtain concentration values from diffusion currents. Inorganic species and organic compounds can be quantitatively determined. The important are C — CI, C — Br, C—I, N — N, N—O, S—O bond compounds.
There are three methods for the determination of concentration:
Wave Height Concentration Plots:
Here a calibration curve is prepared by measuring the diffusion currents of standard solutions. The curve is a straight line. The diffusion current of an unknown solution is measured and concentration of the solution can then be read from the calibrated graph.
(3) Polarographic analysis is of great importance in cation and anion analysis:
For example, lithium having a negative half wave potential, can be estimated in the presence of other alkali metals, provided the latter metals are not of high concentration. A suitable supporting electrolyte used in this analysis is quaternary ammonium hydroxide.
(4) Analysis of Manganese, Aluminium, Iron, Vanadium, Cadmium etc. can be done by polarography.
(5) (i) Polarography is used in the analysis of steel and ferro alloys for minor constituents (Cu, Ni, Mn, Cr, Mo, W, Ti, Sn, Pb).
(ii) Analysis of manganese alloys (Al, Zn, Pb, Mn).
(iii) Analysis of copper base alloys (Pb, Sn, Ni, Zn, Cu).
(iv) Analysis of nickel alloys and nickel compounds (Cu, Pb, Fe, Ni).
(v) Analysis of cobalt and its salts (Cu, Pb, Ni, Co).
(vi) Analysis of high purity zinc and zinc alloys (Cu, Pb, Cd, Sn, Mn, Al, Zn).
(6) Polarographic analysis is also applicable to anions. Chlorates and bromates have been reduced polarographically in presence of a supporting electrolyte ammonium thiocyanate dissolved in anhydrous ammonia.
Instrumental Technique # 5. Voltammetry and Chronopotentiometry:
Voltammetry:
The voltammetry involves an indicator electrode and a reference electrode. A potential difference is created between these two electrodes and the current flows because of electro-chemical reactions.
Here the current versus voltage curves are recorded when a gradually changing voltage is applied to a cell. Generally the voltage is increased linearly with time. Such curves are called voltammograms. The concentration of unknown metals in solution is determined by knowing the diffusion current as in case of polarography.
Tensammetry:
In this technique, the capacity current is derived from the process of adsorption/desorption or the oscillatory movements of dipoles and the ions at the electrode surface, the technique is known as tensammetry. The circuit is given below.
The technique is similar to that of alternating current polarography and is shown in figure 8. It was derived by Breyer and Hacobian in 1957.
Here, P = Potentiometer, C = condenser, R = reference electrode, r1& r2 = resistances, A = microphone amplifier, V = voltmeter.
The current passing through the cell can be measured by calibrating the meter against a cell assembly of known resistance. The adsorption current can be directly measured from the meter and value obtained is plotted against applied d-c voltage. Capacitance peaks have been noticed which denote the adsorption of surface active agents at mercury electrode/solution interface.
Chronopotentiometry:
Chronopotentiometry is a technique based on the principle of polarography and involves measurement of potential-time pattern at a working electrode during a short period of exhaustive electrolysis.
In this technique, the electrode is polarized with constant current and the potential of the working electrode is observed at a function of time. In chronopotentiometry, the main defect of polarography that its charging current is exceedingly large has been removed.
In 1951 Sand gave the following relationship:
where r = transition time i.e., the time at which electrolyzed species concentration becomes zero at electrode surface,
E = potential,
n = number of electrons,
f = Faraday,
C = Bulk concentration in moles/lit.
I = current density in amp./sq cm.
Thus, from above equation r1/2 α C, the concentration of the substance analysed.
In 1963, Morris and Lingane modified the equation of Sand in the following way:
where t+ = transport number of ion species.
This equation can be applied for the determination of transport number. The same equation was applied by Simulin and Emelyanenko in 1964 for measurement of transport number in non aq. media.
Instrumentation and Experimental Technique:
The apparatus consists of:
(1) A source of constant current,
(2) A device for recording potential as a function of time,
(3) A suitable cell and
(4) Electrode system.
A potentiometer and standard resistor arc inserted in a separate standardization circuit for accurate measurement of current. A high resistance d.c. voltmeter has been used for following potential-time relation. For shorter transition time, oscilloscope recording is required and a variety of d.c. oscilloscopes can be used.
The cell is made up of glass vessel in which electrodes are arranged.
(1) The microelectrode:
May be of platinum, gold or other inert metal.
(2) The other electrode:
Is a pool of mercury or a large platinum electrode.
(3) Third reference electrode:
Silver electrode or a calomel electrode. For the analysis of sample solution say thallous ion in 1.0M KC1 is prepared in distilled water. The solution is taken in a cell and nitrogen is passed to remove the oxygen. Several drops of mercury are collected from a dropping mercury electrode and transferred to end of platinum or gold electrode.
Now switch S2 is closed for a fraction of a second before S1 to give a zero base line and if sweep rate or chart speed is correct a transition time is recorded.
The following type of graph is estimated between potential and time from which different valves are calculated.
(1) It is useful in the study of the mechanism of complex ions present in environment.
(2) It is useful in the study of irreversible electrode process.
(3) The technique can also be applied in adsorption studies from polluted air.
(4) It has been proved helpful in the quantitative analysis of air pollutants of the order of 10-12 M concentration.