PART SPECTROPHOTOMETRIC STUDIES

Transcription

1 PART SPECTROPHOTOMETRIC STUDIES

2 CHAPTER General Principles of Spectrophotometry

3 123 In absorption spectroscopy, absorption measurements based upon ultraviolet light and visible radiation find application for the detection and quantitative determination of an absorbing species. Spectrophotometry is one such technique noted for its remarkable sensitivity and precision. A change in the intensity of the colour of the system with change in concentration of the system is termed as colorimetry. A substance appears coloured whenever it transmits or absorbs a part of a visible radiation. Absorption spectrum constitutes the optical activity of the substance. When a normal electronic structure of the substance is deformed there is either the production of the colour (or) change of the colour, Thus when molecules containing one or more chromophores and auxochromes, when subjected to irradiation undergo, variation in electronic energy. The presence of chromophores and auxochromes in organic molecules causes deepening of colour by displacing the absorption maximum towards lower wavelengths. This effect is termed as bathochromic shift. The reverse effect is hypsochromic shift. The wavelength range of nm in an electromagnetic spectrum is called the visible region. Beer and Bernard established the general law of absorption of radiation. In spectrophotometric technique an absorbing medium is placed between the source of a radiation and the spectroscope and the light absorbed is measured. The plot of light absorbed on ordinates versus wavelength is characteristic for an absorbing component, and forms the basis for qualitative analysis. The height of the ordinate in the plot due to the component under investigation at any particular

4 124 wavelength is a measure of the concentration of the component and is thus useful for quantitative work. Laws of photometry When a light falls upon a homogeneous medium, a portion of the incident light is reflected, a portion is absorbed within the medium and the remainder is transmitted. Thus the intensity of the incident light IQ is given as Io=I. + I. + Ir... (1) where Ia = intensity of the light absorbed It = intensity of the light transmitted Ir = intensity of the light reflected. Since the measurements are always made with reference to a reference solution in an identical cell, Ir is regarded as constant and hence it can be neglected. The equation (1) becomes lo=i. + It... (2) Fundamental laws of photometry Lambert s law The fraction of radiant energy absorbed increases exponentially with the linear increase in the thickness of the medium, i.e., - = k'.dt... (3) I I = I0e kdt (4)

5 125 k' is proportionality constant. Ie is intensity of incident radiation when V is zero. The ratio of 1/I0 is the fraction of the incident light transmitted by the medium and is termed transmission or transmittance. The reciprocal \J\ is opacity. Beer s law The exponential decrease of the intensity of monochromatic radiation depends upon the arithmetic increase in the absorption of the absorbing species. The law is represented as (5) (6) k" is proportionality constant. The combination of the two laws is known as Lambert-Beer law. The law states that the fraction of radiant energy absorbed increases exponentially with the linear increase in the thickness and concentration of the absorbing species. (7) where k is new proportionality constant. When expressed logarithmically equation (7) becomes log Io/I = a.c.t. (8) where a = k/ Since a.c.t. is a logarithmic quantity, it is a pure number, a in the equation is known as absorptivity. Its units are Litre/gm-cm when the thickness is expressed in centimetre and concentration in gm/litre. When the concentration is expressed in moles/litre the proportionality constant is known as s and is termed molar absorptivity or molar absorption index or molar extinction coefficient. Its units are

6 126 Litre mole'1 cm'1. Emperically molar absorptivities that range from zero upto a maximum of the order of 105 are observed in ultraviolet and visible absorption. For any particular peak the magnitude of e depends on the capture cross section of the species and the probability for any energy absorption transition to occur. The relation between s and these parameters is shown to be s = 3.7 x 1019PA where P = transition probability A = cross section target area in cm2. From electron diffraction studies the cross section target area for a typical organic molecule is estimated to be about 10'15 cm2. Values of P range from for quantum mechanically allowed transition. This value of P leads to strong absorption bands i.e., smax is equal to about 10s. The absorption peaks having e values less than about 103 show low intensity bands. Beer-Lambert law as expressed in equation (8) is also shown as A = log Io/I = set... (9) where A = log Iq/I is referred as absorbance or optical density or extinction. Beer s law - Deviations The absorbances of a series of solutions of known concentration at a fixed wavelength and cell path must bear a linear relationship to the concentration. The deviations from this behaviour is due to so many factors such as interionic forces, complex formation, time and temperature variations, solvent composition in blank and

7 127 test solution and wavelength of radiation etc. The law is obeyed only when the radiation employed is monochromatic. Sensitivity in spectrophotometric method The slope of a calibration plot between concentration and absorbance refer to sensitivity in photometry. Numerically it is expressed as molar absorptivity at wavelength of maximum absorption. s = a/ct... (10) If the value of e given by a method is greater than 104 then the method is said to be sensitive in photometric terms. Specific absorptivity (a) The specific absorptivity (a) is given by equation a = e/atomic weight x (11) The units are ml gm'1 cm'1. This value corresponds to an absorbance of 1 gg/ml solution taken in a cuvette of pathlength 1 cm. Sandell s photometric sensitivity (S) S represents the number of micrograms of determined per ml of solution having an absorption of for a pathlength of 1 cm. S = 10'3/a... (12) Stoichiometry The various spectrophotometric methods employed for establishing the composition of the complex in solution are Job s continuous variation method, Molar ratio method, Slope ratio method, Asmus method and Equilibrium shift method.

8 128 In the present investigations, the author has employed the Job s continuous variation method and molar ratio methods. A brief description of these methods is given. 1. Job s continuous variation method Job1 described a method for determining the stoichiometry of the complex formed between metal ion, M and ligand, L. The formation reaction of the complex is represented as M + nl -» ML... (13) In this method, identical formal concentrations of metal ion and the ligand are used and are mixed in varying volume ratios, keeping the total volume constant. The absorbance of these mixtures is measured at a suitable wavelength against an appropriate blank solution. The absorbance values are then plotted against the volume fraction (same as the mole fraction) Vm/Vm+Vl of one of the reactants. Where Vm and V). represent respectively the volume of the metal ion and the ligand. A curve is obtained which has a maximum (or minimum if the complex absorbs less than the reactants). The composition of the complex is given by the volume fraction ratio. X : [1 - X]...(14) where X V, M... (15) (Vm+Vl) The composition thus corresponding to the point of maximum bears a simple relation to n and is independent of equilibrium constant of the equiformal solutions of M and L\ However the position of the maximum depends on the equilibrium constants

9 129 when the concentrations are not equal. The composition of the several complexes is satisfactorily identified by the Job s method. However, if more than one complex is formed Job s method is generally not applicable. Cooper et al2 reported that the Job s method is still be useful if it is ascertained whether or not one complex is formed in the system under study. In the Job s modified procedure the optical measurements are made at various wavelengths covering the entire range instead of at the wavelength corresponding to the maximum. If all the wavelengths give same conclusion, it can be assumed that a single complex is formed. Thus Job s method is employed for the elucidation of the complex composition. For a system containing only one complex, the value of n is given by n = X (1-X) (16) where X is the volume in litres of L added to the volume in litres of M. Molar ratio method In this method introduced by Yoe and Jones3, a series of solutions are prepared in which the formal concentrations of one of the reactants usually the metal ion is kept constant while that of the other (reagent) is varied. The absorbance values are plotted against mole ratio of the reagent. If the system forms the stable complex, the plot consists of two straight lines of different slopes intercepting at a sharp break. The mole ratio at this break corresponds to the combining ratio in the complex. In the case of weak complexes, the mole ratio plot is a smooth curve and that can be extrapolated to give the combining ratio. A mole ratio plot may reveal stepwise formation of two or more complexes provided the molar absorptivities and formation constants of these complexes are different.

10 130 Methods for evaluating the stability constants of the complexes The available spectrophotometric methods for the determination of stability constant are Job s method, Bent and French method, Edmund and Birn Baum s method and Asmus method. In the present investigations, Job s method is employed for the determination of the stability of the metal complex and is described. Job s method The stability constant, P is obtained by applying the data obtained in Job s method. For 1:1, 1:2 and 1:3 complexes, the stability constant (p) values are calculated using the following equations respectively. 0-q) a2c (17) (1-a) 4a2C2 (18) (1-a) 27a4C3 (19) where Am = absorbance corresponding to the point of intersection of the extrapolated lines A = observed absorbance at concentration C C = concentration corresponding to the point of intersection a = degree of dissociation a Lm J

11 Job, P. Voseburgh, W.C. and Cooper, G.R. REFERENCES Ann. Chim., 9(1928) 113. J. Am. Chem. Soc., 63 (1941)437, 3 Yoe, H.J. and Jones, A.L. Ind. Eng. Chem. Anal. Ed. 16 (1944) 111.