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 prob- lem.

Quantitative instrumental analysis syllabuses usually in- clude laboratory experiments involving the spectrophoto- metric 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 ob- tained 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 com- oensatine for the matrix effect bv a simnlified version of the generalized standard-addition method (5). In order to intro- duce the chemistrv students to multicom~onent analvsis. we have developed a graphical/numerical mkthod for quantita- tive analvsis of mixtures of two comDonents with o v e r l a ~ ~ e d spectra. he method (m~ltiwaveien~th linear regression 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,~ A.2 = L Z ~ C , ~ Substitution of the molar absorptivities into eq 1 and rear- rangement yields

By plotting A,/A,l as a function of AS2/A,1 a t various wave- lengths one obtains a straight line, the slope and intercept of which allow calculation c2 and cl, respectively.

Reagents . 10-2 M KMn04 solution . 10WM solution of K2Cr207 in 0.1 M H2S04 . CuSO, solution containing 1 g n Cu

ZnClz solution containing 1 g L Zn . 10V M Ti(1V) solution obtained by dissolutionof TiOxinconcen-

trated H2SOn and 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

Apparatus Spectra were recorded and absorbances measured on a Perkin-

Elmer model 554 spectrophotometer furnished with quartz cells of 10-mm light path.

Results and Dlscusslon The MLRA was applied to the determination of the com-

ponents of three binary mixtures in which the spectra of the absorbing species are overlapped to a greater or lesser ex- tent.

The mixtures resolved were 1. Pemanganatedichromate. The spectra are only partly over-

lapped, and it is possible to choose a wavelength at which only one of the components absorbs.

2. Titanium(1V)-vanadium(V). H202 was 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.

Standard solutions of 1.00 X M for each component and a sample mixture, 1.77 X M dichromate and 0.8 X

M permanganate, were prepared by appropriate dilu- tion 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.

WAVELENGTH lml

Figure 1. UV-visible spechum of 1.0 10-'MMn04i (-- -), 1.0 10-'MCr20,2- (- . -), and their mixture (---). :

178 Journal of Chemical Education

Table 1. Abwrbanws of Permanganate, Dlchromate, and meir Mlxture Used In the Llnaar Regression for the Resolution of the

Mixture

Absorbance A Mn0,- standard Cr2072- standard Mixtvre

266 0.042 0.410 0.766 288 0.082 0.283 0.671 320 0.168 0.158 0.422 350 0.125 0.318 0.672 360 0.056 0.181 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

Absorbance A TI standard 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 ab- sorption coefficients was

y = 1.78~ + 0.81 (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, con- centration 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 0.84 X M (permanganate) and 1.77 X M (dichro- 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), respective- ly, 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 solu- tions 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 concernration. 7.0 X M throughout.

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

Absorbance A PAR CU-PAR Zn-PAR Mixture

480 0.211 0.698 0.971 0.556 496 0.137 0.732 1.018 0.668 510 0.100 0.732 0.891 0.627 526 0.072 0.602 0.672 0.498 540 0.056 0.387 0.306 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 179

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. 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 namely 0.26 PPm for Zn(II) and 0.51 PPm for Cu(II), consis- 3. P ~ I ~ ~ ~ ~ ~ ~ . A . T . : S ~ V ~ ~ ~ ~ ~ ~ ~ . L . I . ; M ~ ~ ~ ~ O A . N . z ~ . ~ ~ I . ~ ~ ~ ~ . 1985.40.232. tent with the concentrations added. 4. ~i ~ u s a , M. R.: schiit,~. A. J. them E ~ U C . IS~P,~Z,GI.

~h~ plot of absorbance ratios defined in MLRA is shown 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,inp- in Figure 4. 7. Blaneo, M.: Gene, J.; lturriaga. H.: Maspoeh, S. Anolysf 1987.112,619.

180 Journal of Chemical Education