-
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