A Simple Method for Spectrophotometric Determination
of Two-Components with Overlapped Spectra M. Blanco, H. Runiaga, S. Maspoch, and P. Tarfn Unlversitat Autonoma de Barcelona. 08193 Bellaterra, Spain The spectrophotometric determination of mixtures of components with overlapped spectra has lately been the subject matter of a number of chemometric studies (I, 2) dealing with various aspects of this major analytical problem. Quantitative instrumental analysis syllabuses usually include laboratory experiments involving the spectrophotometric analysis of mixtures of two components with partly overlapped spectra. As a rule, the mixture components are quantized by solving a system of two linear equations obtained by applying Beer's law a t two different wavelengths. Further imorovements of this method involve the selection of wavelengths providing optimum precision ( 3 . 4 ) or comoensatine for the matrix effect bv a simnlified version of the generalized standard-addition method (5). In order to introduce the chemistrv students to multicom~onentanalvsis. we have developed a graphical/numerical mkthod for quantitative analvsis of mixtures of two comDonents with o v e r l a ~ ~ e d linear regression spectra. he method (m~ltiwaveien~th analysis), allows easy handling of data obtained at several wavelengths, and the resultant accuracy and precision are comparable to that of rather more complex mathematical procedures (6,7). Multlwavelength Llnear Regression Analysls (MLRA) The absorbance of a mixture of two noninteracting species absorbing UV-visible radiation in the same spectra zone, A,, is given by: If c , ~and c.2 are the concentrations of standard solutions of each component, then A,, = qbe,~
A2 .
Apparatus Spectra were recorded and absorbances measured on a PerkinElmer model 554 spectrophotometer furnished with quartz cells of 10-mmlight path. Results and Dlscusslon The MLRA was applied to the determination of the components of three binary mixtures in which the spectra of the absorbing species are overlapped to a greater or lesser extent. The mixtures resolved were 1. Pemanganatedichromate. The spectra are only partly overlapped, and it is possible to choose a wavelength at which only
one of the components absorbs. 2. Titanium(1V)-vanadium(V). H202was used as chromogenic re-
agents. The spectra are quite overlapped and the reagent does not absorb. 3. Copper(I1)-zinc(I1). PAR was used as chromogenic reagent. The spectra are very overlapped and the absorbance of the reagent is not negligible. Permanganate-Dichrornate Mixture The permanganate-dichromate mixture is the commonest subject of multicomponent mixture resolution cited in the literature. M for each component Standard solutions of 1.00 X and a sample mixture, 1.77 X M dichromate and 0.8 X M permanganate, were prepared by appropriate dilution of stocks. The spectra of standards and the sample are shown in Figure 1. The absorbance of each solution was measured at five wavelengths in the 250-400-nm range, where the spectra of two species are widely overlapped.
=LZ~C,~
Substitution of the molar absorptivities into eq 1and rearrangement yields By plotting A,/A,l as a function of AS2/A,1 a t various wavelengths one obtains a straight line, the slope and intercept of which allow calculation c2 and cl, respectively.
.. .
Reagents 10-2 M KMn04solution 10WM solution of K2Cr207in 0.1 M H2S04 CuSO, solution containing 1g n Cu ZnClzsolution containing 1g L Zn 10V M Ti(1V)solution obtained by dissolutionofTiOxinconcentrated H2SOnand subsequent dilution with Hz0 2 x 10-2 M solution of V(V) in 0.1 M H2SOc obtained from NH,VO1 0.05% solution of 2-pyridyl-azo-resorcinol(PAR)ethanol
.
.
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Journal of Chemical Education
WAVELENGTH lml
Figure 1. UV-visible spechum of 1.0 10-'MMn04i (-- -), 1.0 10-'MCr20,2(- -), and their mixture (---). :
.
Table 1. Abwrbanws of Permanganate, Dlchromate, and meir Mlxture Used In the Llnaar Regression for the Resolution of the Mixture
A
Mn0,- standard
Absorbance Cr2072-standard
Mixtvre
266 288 320 350 360
0.042 0.082 0.168 0.125 0.056
0.410 0.283 0.158 0.318 0.181
0.766 0.671 0.422 0.672 0.365
Table 2. Absorbances of the H20, Complexes 01 TI(IV) and V(V) and Their Mlxture Used In the Linear Regression for the Resolution at the Mlrture
A
TI standard
Absorbance V standard
Mixture
-
Figure 2. Vlsible spechum of the H A complexes 01 83.1 ppm Ti (- -1. 96.4 ppm V(V) (- -I, and their mlxture (---I.
.
In Table 1 are listed the ahsorhances nrovided hv the mixture and the two standards at the waveiengths used. The eauation of the straieht line obtained from the absorption coefficients was
+ 0.81
y = 1.78~
(r2= 0.9999)
so that the concentration of the two components in the mixture was 0.81 X 10-4 M (MnO;) and 1.78 X 10-4 M (C&-). Similar results were ohtained a t several other wavelengths in the same range. The concentrations found by measuring the MnO, concentration a t 545 nm (where (2-0;- does not absorb a t all) and that of CrzO;- by difference in the region where both species absorh-the difference is greatest a t 350 nm-were M (permanganate) and 1.77 X M (dichro0.84 X mate), respectively, i.e., very similar to those ohtained with the MLRA. TI(1V)-V(V) Mixture Five milliliters of 10% Hz02 solution was added to 10 mL of stock solution and diluted to 100 mL. These solutions containing 96.4 pprn V(V) and 63.1 pprn Ti(IV), respectively, were used as standards and a mixture of 57.8 pprn V(V) and 31.5 pprn Ti(1V) as sample. The spectra of these solutions are shown in Figure 2. The mixture was resolved hy using wavelengths in the range 350-550 nm. The absorhances of the standards and the mixture a t the wavelengths used are listed in Table 2. The equation of the straight line ohtained. yielded a concentration of 58.4 ppm for V and 31.5 pprn for Ti. Cu(I1)-Zn(l1) Mixture The standard solutions were prepared by taking 1 mL of 100 pprn Cu(I1) or Zn(I1) stock solutions, adding 3 mL PAR stock solution and 10 mL of acetate buffer pH 7, and finally dilutingto 100 mL. The sample solution wasprepared by the same procedure and contained 0.25 pprn Zn and 0.50 pprn Cu. The spectra of PAR, Cu-PAR, Zn-PAR and mixture are shown in Figure 3.
.
..
-
Flgure 3. Visible specbvm of PAR (- -1 the CU-PAR (- -) and Zn-PAR (- -) wmplexes formed by l-ppm Cu and Zn, and their mixture (---). PAR M throughout. concernration. 7.0 X
Table 5. Abrorbances of the PAR Solutions, the CU-PAR and Zn-PAR Complexes, and Thelr Mlxture Used In the Linear Regression for the Resolution of the Mlxture
A
PAR
480 496 510 526 540
0.211 0.137 0.100 0.072 0.056
Absorbance CU-PAR Zn-PAR 0.698 0.732 0.732 0.602 0.387
0.971 1.018 0.891 0.672 0.306
Mixture 0.556 0.668 0.627 0.498 0.290
The wavelengths used in resolving the mixture were in the range of450-560 nm. In Table 3 are listed the ahsorhances of the Cu(I1) and Zn(I1) complexes, their mixture, and PAR a t the wavelengths used. The absorbance of the standards and the mixture were corrected for the reagent absorbance. From the equation of the straight line ohtained, were calculated the concentrations of the components, Volume 66 Number 2 February 1989
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Conclusions The MLRA is a straightforward procedure allowing the accurate resolution of binary mixtures of compounds with highly overlapped spectra. The reliabilitv of the straight lines used and hence that of the results increases with the number of wavelengths, yet rather satisfactory results can be obtained with onlv four to six wavelengths. The best wavelength region for application of the method is that of maximal spectral overlap, i.e., that where both components absorb significantly and where the errors in the absorbance ratios are minimal.
Acknowledgment The authors are grateful to the CAICyT (Project no. 8211 84) for financial support. Figure 4.Plot of absorbance ratios for Cu(ll~Zn(l1)mixture resolution.
namely 0.26 PPm for Zn(II) and 0.51 PPm for Cu(II), consistent with the concentrations added. ~h~ plot of absorbance ratios defined in MLRA is shown in Figure 4.
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Journal of Chemical Education
Llterature Clted 1. Howell, J. A,: Haqlo, L. G. A d . Chem. 1986,58,108R. 2. Ramos,L.S.:Beebe.K.R.;Carey.W.P.;Sbchez,E.M.;Wiison,B.E.M.;Wangen,L. E.;Kouslski, B.R.Ano1. Chsm., 1986.58.294R 3. P ~ I ~ ~ ~ ~ ~ ~ . A . T . : S ~ V ~ N~ ~. ~z~ ~~ ~. .~L . ~1985.40.232. II . .; ~M ~~ ~ ~~ ~~ O .A . 4. ~i ~ u s aM. , R.:schiit,~. A. J. them E ~ U CIS~P,~Z,GI. . 5. Raymond M.: Jochum. C.; Kowakki. B.R.J. Chrm.Educ. 1983,60,1072. 6. Blenco,M.;Gene, J.:ltwriaga,H.;Maamh,S.;Riba,J. Tolanfo,inp7. Blaneo, M.: Gene, J.; lturriaga. H.: Maspoeh, S. Anolysf 1987.112,619.
