References 25 molecules 2 Herzberg G (1967)Mo D1066/C7.VNKo.Produktbereich Original Hanau, Postfach 1553,D-6450 Hanau 1 23.Osram GmbH,Bern- rochure"Licht fur Kinoprojektion,Technik und Wissen 988/F030 24.Hcracus,.Brochure D310218/1C,10,.85VNKo;D310531/1C10.85;D310311/1C10.85 25. Produktb chafe 26.Demtroder W(1988)Laser Spectroscopy.Springer,Berlin Heidelberg New York,3rd print- ing with corrections
References 25 20. Kiefer J (ed) (1977) Ultraviolette Strahlen. Walter de Gruyter, Berlin, p 96 21. Herzberg G (1967) Molecular spectra and molecular structure. I Diatomic molecules 22. Heraeus WC, Brochure D 310686/2C 7.86 VNKo. Produktbereich Original Hanau, Postfach 1553, D-6450 Hanau 1 23. Osram GmbH, Berlin-Munich, Brochure "Licht fUr Kinoprojektion, Thchnik und Wissenschaft". Issue August 1988IFO 302 24. Heraeus, Brochure D 310218/1C, 10.85 VNKo; D 31053111C 10.85; D 310311/1C 10.85. Produktbereich Original Hanau, see 22 25. Schafer V, Heinrich G (1977) In: Kiefer J (ed) Ultraviolette Strahlen, Chap 3. Walter de Gruyter, Berlin 26. DemtrOder W (1988) Laser Spectroscopy. Springer, Berlin Heidelberg New York, 3rd printing with corrections
Analytical Applications of UV-VIS Spectroscopy For about 130: vears,the Bouguer-Lambert-Beer law has been used as the athien mdo. quantitative bas This correlation was then formulated mathematically by Lambert in 1760 [1]and Beer discovered the dependence upon the concentration in 1852 [2]. Initially,the human eye was the detector for comparing different light inten- sities.In 1925,Pulfrich [3]introduced his photometer as "a photometer ap- propriately adapted to exceed the levels of sensitivity of the human eye,call- ed a step photometer.".Our eyes are capable of assessing the uniformity of two light densities with an accuracy of approximately 1%.The principle of colorimetry or visual photometry is based on this fact(see [4- 6]etc.) The Bo r-Lambe t-Reer law a dilute conta: one abs nce wavenum en equa m of abs orbance hweouldobiainseparateytog s whi ch solution had we measured th utions individually at the same wavenumber v,or wavelength A,and pathlength d: A1=A1+A12+A13+.=∑A (14) We can then write the general formula: Ai=6i'c'd++ec'd+.=d 9= A (14a Index i refers to wavelength A or wavenumber and the second index j to the components. Equation(4)is the basic equation for the photometric determination of a single substance and Eq.(14a)is the foundation for photometric multicomponent analysis. 4.1 Photometric Determination of a Single Substance Equation(4)shows the simple linear correlation between absorbance A and concentration c and the molar decadic extinction coefficient,. H.-H.Perkampus,UV-VIS Spectroscopy and Its Applications Springer-Verlag Berlin Heidelberg 1992
4 Analytical Applications of UV-VIS Spectroscopy For about 130 years, the Bouguer-Lambert-Beer law has been used as the quantitative basis of absorption spectroscopy. Bouguer established empirically a correlation between pathlength and light absorption in 1729. This correlation was then formulated mathematically by Lambert in 1760 [1] and Beer discovered the dependence upon the concentration in 1852 [2]. Initially, the human eye was the detector for comparing different light intensities. In 1925, Pulfrich [3] introduced his photometer as "a photometer appropriately adapted to exceed the levels of sensitivity of the human eye, called a step photometer . ". Our eyes are capable of assessing the uniformity of two light densities with an accuracy of approximately 1 0/0. The principle of colorimetry or visual photometry is based on this fact (see [4-6] etc.). The Bouguer-Lambert-Beer law applies to a dilute solution containing one component or equally to a dilute solution of several components. The measured absorbance, Ai> of muIticomponent solutions at wavenumber VI then equals the sum of absorbances which we would obtain separately for each solution had we measured the solutions individually at the same wavenumber VI or wavelength Al and pathlength d: Al = A11 + A12 + A13 + . = L Alj . j We can then write the general formula: (14) n n Aj=Bjl·cj·d+Bj2c2d+Bi3·c3·d+ . =d L Bjj"Cj= L Ajj . (14a) j=1 j=1 Index i refers to wavelength A or wavenumber V and the second index j to the components. Equation (4) is the basic equation for the photometric determination of a single substance and Eq. (14a) is the foundation for photometric muIticomponent analysis. Equation (4) shows the simple linear correlation between absorbance A and concentration c and the molar decadic extinction coefficient, Bji. H.-H. Perkampus, UV-VIS Spectroscopy and Its Applications © Springer-Verlag Berlin Heidelberg 1992
Photometric Determination of a Single Substance 27 The concentration c c=A, (15) eg'd can be determined if the extinction coefficient,of the substance to be determined is known. Equation (15)shows the concentation ranges that can be determined. At a maximum extinction coefficient of =10 1 mol-cm-, we obtain a detectable concentration at c=10-6moll-1. In modern equipment,the microcompute suresbase-ine stability of 10-3-10-4 absorbance units and a noise of the same or smaller ord 0 magnitude.Thus,absorbance values of 0,01 to 0. ).001 can be measured so that the limit of detection lies at c=10 'mol I and,in the most favor- able case,for an appropriately selected pathlength,at c=10mol1 However,extinction coefficients in the given range of 101 molcm are generally the exception.Most compounds absorbing in the UV-VIS region have considerably smaller extinction coefficients which lie in the region of 103≤e≤5×101mol-1cm1 This range of extinction coefficients applies to many organic compounds with chromophoric systems.However,typical dyes or dye-like chromo- phoric systems can have values of approximately 1000001 mol-cm In contrast,the visible colors of metal cations,which we are frequently ired to analyze,are characterized by small values of the extinction coef- ricents Thesii is more favorable with strongly colored compl The s HCIO. spectrum of hexaaquo-copper(II-perchlorate in 0.1 N 8 has orption maximum at=12400 cm-1 with 111m0 ed with 8-hydrox and 1-hydroxyacridine(Fi s.8b,c orption band occur with extinction coefficients[8]of 8=2.8x101 mol-cm for the 8-hydroxyquinoline complex in ethanol and 6=2.7x101 mol-i cm-for the 8-hydroxyacridine complex oform Since the extinction coefficients of such comple es are related to the mo lecular weights of the complexes,the sensitivity of the photometric method
Photometric Determination of a Single Substance 27 The concentration c (15) can be determined if the extinction coefficient, eji, of the substance to be determined is known. Equation (15) shows the concentation ranges that can be determined. At a maximum extinction coefficient of eji = 105 1 mol- 1 cm -1 , a pathlength of d = 1 cm and a lower absorbance value of Aji = 0.1 we obtain a detectable concentration at c = 10-6 moll-I. In modern equipment, the microcomputer ensures a base-line stability of 10-3 _10- 4 absorbance units and a noise of the same or smaller order of magnitude. Thus, absorbance values of 0.01 to 0.001 can be measured so that the limit of detection lies at c = 10-7 moll-I and, in the most favorable case, for an appropriately selected pathlength, at c = 10- 8 moll-I. However, extinction coefficients in the given range of 105 1 mol-I cm-I are generally the exception. Most compounds absorbing in the UV-VIS region have considerably smaller extinction coefficients which lie in the region of This range of extinction coefficients applies to many organic compounds with chromophoric systems. However, typical dyes or dye-like chromophoric systems can have e values of approximately 100000 1 mol-I cm -I . In contrast, the visible colors of metal cations, which we are frequently required to analyze, are characterized by small values of the extinction coefficients. The situation is more favorable with strongly colored complex compounds. The spectrum of hexaaquo-copper(II)-perchlorate in 0.1 N HCl04 (Fig. 8 a) has an absorption maximum at 17 = 12400 cm -I with e = 111 mol-I cm -I [7]. When complexed with 8-hydroxyquinoline and 1-hydroxyacridine (Figs. 8b,c) absorption bands occur in the visible region with extinction coefficients [8] of e = 2.8x 103 1 mol-I cm -I for the 8-hydroxyquinoline complex in ethanol and e = 2.7x 103 1 mol-I cm -I for the 8-hydroxyacridine complex in chloroform. Since the extinction coefficients of such complexes are related to the molecular weights of the complexes, the sensitivity of the photometric method
28 Analytical Applications of UV-VIS Spectroscopy 250300 400500nm1000 Fig.8.Absorption spectrum of a hexaaquo 45×10cm1353025 20 for elements of a similar molecular weight can be compared for a given ally applicable expression and hence a compari- son of photometric methods.This absorption is given as a= atomic weight 10-3 (16 The molar decadic extinction coefficient is adjusted for the atomic weight of the metal and this numerical value is multiplied by 10.Conse- quently,the specific absorption "a"has the dimensions ml gcm and corresponds to the extinction of a solution which contains 1x10-6g of the metal to be determined in 1 cm3 at a 1 cm cuvette pathlength;this equals 1 Dpm. d othe spifi bsorption,Sandell sitivity index"S".It gives the number of micrograms of an ele which has absorbance A=0.001 at a pathler gth of 1 m.The dimension is 10-6gcm 2ndsseaiadohesneaeaborpto8Tb S10-3 (17
28 Analytical Applications of UV-VIS Spectroscopy A- 5 250 300 400 500 nm 1000 ~ ~/\ ~/ \ 4 ~\ ~/\ - \ i \ \ \c \ \ . '\ \ \ ! ' \ af \ \ I \ I Fig. 8. Absorption spectrum of a hexaaquo- \ o I copper(II) in H20, b copper(II)-8-hydroxy- 45 x 103cm-1 35 30 25 20 15 10 quinoline in ethanol, c copper(II)-4- _ 'ii hydroxyacridine in chloroform for elements of a similar molecular weight can be compared for a given complexing agent. Ayres and Narang [9] introduced the specific absorption "a" for arriving at a generally applicable expression and hence a comparison of photometric methods. This absorption is given as a = G,l. xlO- 3 atomic weight (16) The molar decadic extinction coefficient G,l. is adjusted for the atomic weight of the metal and this numerical value is mUltiplied by 10 - 3• Consequently, the specific absorption "a" has the dimensions ml g -1 cm -1 and corresponds to the extinction of a solution which contains 1 X 10-6 g of the metal to be determined in 1 cm3 at a 1 cm cuvette pathlength; this equals 1 ppm. In addition to the specific absorption, Sandell [10] introduced the sensitivity index "S". It gives the number of micrograms of an element per ml in a solution which has absorbance A = 0.001 at a pathlength of 1 cm. The dimension is 10-6 g cm -2 and S is related to the specific absorption "a" by 10- 3 S=- a (17)
Photometrie Determination of a Single Substance 29 4.1.1 Photometric Determination of Elements by Means of Complexing Agents Metal cations which absorb weakly or not at all in the visible spectral region can be changed into strongly colored compounds by means of complexing agents.Strongly colored here means that absorption bands characteristic of the complex have an extinction coefficient s>10I molcm.Some complexing agents have proved to be extremely suitable,and these include dithizone [11]and 1-(2-pyridylazo)-2-naphthol (PAN)[12-16],8-hydroxy- quinoline (8-oxine)[17-201,formaldoxime [21-25],1,10-phenanthroline vridyl [26-281,N-benzoylphenylhydroxylamine [29],morin [30, arbamate (DTC)[41-431 and the thiocyanate ion as an in- agent [32-35](see Fig.9). mplexi orm number of metals there is no great n parti ar no city. m means that other metals can interfere v with the met determin er o elements and specific if it undergoes the desired color reaction with only one element when we adhere to particular reaction conditions. Among other things,the selectivity of a color reaction depends upon the chosen reagent,the oxidation number of the element and,in many cases,also upon the pH-value of the solution and above all upon the stability constant of the complex.This is extremely important since weaker complexes can be converted into more stable ones by recomplexing and therefore,they can no longer react with the first reagent.Thus,it is rfer ng eleme ts.This masking technique has stood im and cor ved selectivity of the e or pho etr nc det ination.Mar zenko [36]and Umland have summa e mas commonly used today.Table s the e ents an for thei photometric determination for which detailed operating instructions are available. In practice,we must proceed according to exactly defined conditions This applies particularly if trace elements are to be analyzed.The operating instructions specific for each element and reagent have been summarized in excellent monographs by Sandell and Onishi [10],Iwantscheff [1],Schilt [28],Marczenko [36],Umland [37],Koch and Koch-Dedic [38],Lange and Veidelek [39],Fries and Getrost [401. The entries in Table 5 include mostly chelate complexes with large extinc tion dyes are representative of organic compounds which met trace the effectiv eness of dve tothe formati of an ion as plex between etal i already complexed and one or everal dy on com This comp can the solvent s and racted om agucoubasic dye to be uscd for neasu photometrically.Rhodamine B was the first bas or extraction spectrophotometry.For example,this dye forms an ion pair with
Photometric Determination of a Single Substance 29 4.1.1 Photometric Determination of Elements by Means of Complexing Agents Metal cations which absorb weakly or not at all in the visible spectral region can be changed into strongly colored compounds by means of complexing agents. Strongly colored here means that absorption bands characteristic of the complex have an extinction coefficient e> 104 I mol-I cm -I. Some complexing agents have proved to be extremely suitable, and these include dithizone [11] and 1-(2-pyridylazo)-2-naphthol (PAN) [12-16], 8-hydroxyquinoline (8-oxine) [17 - 20], formaldoxime [21- 25], 1,10-phenanthroline and 2-2'dipyridyl [26-28], N-benzoylphenylhydroxylamine [29], morin [30, 31], Na-dithiocarbamate (DTC) [41-43] and the thiocyanate ion as an inorganic complexing agent [32-35] (see Fig. 9). Since these complexing agents can form complexes with a large number of metals there is no great selectivity and in particular no specificity. This means that other metals can interfere with the metal we want to determine. A reagent is referred to as selective if it reacts with a limited number of elements and specific if it undergoes the desired color reaction with only one element when we adhere to particular reaction conditions. Among other things, the selectivity of a color reaction depends upon the chosen reagent, the oxidation number of the element and, in many cases, also upon the pH-value of the solution and above all upon the stability constant of the complex. This is extremely important since weaker complexes can be converted into more stable ones by recomplexing and therefore, they can no longer react with the first reagent. Thus, it is possible to mask interfering elements. This masking technique has stood the test of time and contributed to an extraordinarily improved selectivity and specificity of the method used for photometric determination. Marczenko [36] and Umland [37] have summarized the masking agents most commonly used today. Thble 5 shows the elements and methods for their photometric determination for which detailed operating instructions are available. In practice, we must proceed according to exactly defined conditions. This applies particularly if trace elements are to be analyzed. The operating instructions specific for each element and reagent have been summarized in excellent monographs by Sandell and Onishi [10], Iwantscheff [1], Schilt [28], Marczenko [36], Umland [37], Koch and Koch-Dedic [38], Lange and Vejdelek [39], Fries and Getrost [40]. The entries in Thble 5 include mostly chelate complexes with large extinction coefficients. Azo dyes are representative of organic compounds which chelate with metal ions. Frequently, we can trace the effectiveness of dyes back to the formation of an ion association complex between a metal ion already complexed and one or several dye molecules. This complex can then be extracted from aqueous solutions with organic solvents and measured photometrically. Rhodamine B was the first basic dye to be used for this extraction spectrophotometry. For example, this dye forms an ion pair with