ELSEVIER Journal of Electroanalytical Chemistry 432 (iq97) 223-227

Jotl~l~u.

Solving the interference due to coupled reactions in the polarographic determination of benzaldehyde with soft modelling

Ana Herrero *, M. Cruz Ortiz I)pto. Quhuica, Foc. de C~ TA y (: Qubnicas. Unil'ersidad de Burgos, Pza Misael B.huelos s / n, 09001 Burgos, Spai~z

Received 27 Jmmary 1997; revi,;ed 13 March 1997

Abstract

The polamgral~hic determination of benzaldehyde was cmtied out in a wide nmge ot' pH values using partial legist squ~res (PLS) regression to analyse signals obtained as a result of complex processes. In the electrochemical reduction of benzaldehyde at mercury electrodes, several chemical (proton transfer and dimerization) and electrochemical reactions take place, depending both on the concentration of benzaldehyde and pH. This interferes seriously in the quality of the recorded analytical signals, causing the shift and overlapping of the polarographic peaks. The multivariate technique used models these interferences and allows the successful determination of this organic compound independently of the shape of the signals and the pH of the medium in the concentration range analysed, the mean absolute relative errors being equal to 2.10%. © 1997 Elsevier Science S.A.

IG.ywol~L~': Multivariate calibration; Partial least squares regression; Benz~ddchyde; Coupled rent|ions; Dimerization: Polarography

1. Introduction

Polarographic techniques have developed significantly during the past few decades. It is now possible to apply this electroanalytical methodology in a routine way to deternline several organic and inorganic compounds by means of simple and experimentally easy procedures. Among these techniques, differential pulse polarography (DPP) [!,2] is usually used to analyse organic compounds.

Frequently, in the course of the electrodic process in organic systems, chemical bond~ are tbrmed and broken, in such a way that chemical reactions take place before and/or after charge transfer [3]. The coupled reacti~ms interfere in the polarographic signals causing asymmetry, overlapping and shift of peaks. As a result, this gives polarograms of low analytical quality if they are analysed in a classical way, where it is necessary to use specific and selective signals to quantify a compound.

The quality of the signal can be improved, in general, by modifying some experimental variables such as pH, concentration of the supporting electrolyte, etc. The influ- ence of the pH in most of the chemical processes involved in the electrochemical signals imposes severe limitations

* Corresponding author. Fax: +34 47 258831. E-mail: aherrero@ubu.es.

0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PI! S0022-0728(97)00215-5

upon the conditions in which the analysis should be carried out [4,5]. However, the great quantity of electrochemical data that present~day instrumentation provides allows one to analyse calibration problems with multivariate techo niques. The latter permits the use of all the information contained in the polarogram instead of only the peak current, as in classical univariate methods, avoiding the problems of non-selectivity of the electrochemical signals.

The aim of this work is the application of partial least squares regression [6,7] (PLS) to a calibration problem where, in addition to electrodic reactions that give the polarographic peaks, coupled chemical reactions coexist. This multivariate regression has been successfully applied to electrochemical problems in which complex relation~ between sensors exist [8,9]. in the present paper it has been used to carry out the determination of benzaldehyde at different pH with DPP.

Benzaldehyde is usually determined in food, mainly in products manufi.ctured with almond extract, by mean~ of gravimetric and ultraviolet s~c~rophotometfic method~ [ 10]. Likewise, its determination in waste gas [ 1 ! ] ha~ been carried out by high perfi~mance liquid chroma~ographyo HPLC, although this organic compound can also be tound as an impurity in benzyl alcohol solution~ [I 2], together with traces of a derivative compound, hydrobenzoin [13]. Polarographic analysis [14,15] is an alternative that allow~

224 A. Herrero, M. Cru: Orti: / Journal of Electroanalytical Chemistr), 432 (1997~ 223-227

more accurate determinations than gravimetric methods and cheaper instrumentation than spectroscopy or chro- matography.

Special importance has been conferred on the interpreta- tion of the latent variables in the PLS models built, since they should be related to the chemical and electrodic reactions taking place.

2. Experimental

2,1. Chemicals

Benzaldehy~ was obtained from Merck (> 99%). All reagents were of analytical grade and were used without further purification. All solutions were prepared with deioni~d water obtained with a Bamstead NANO Pure II sys~m, Nitrogen (99,997%) was used to remove dissolved oxygen,

Standard solutions were prepared 98.9 mM m media containing 20% of alcohol. The buffer solutions were Mcllvaine's standards made by mixing different quantities of 0,1 M citric acid and 0,2 M Na,HPO4 to give ten equally spaced pH values th3m 2.2 to 7.6,

2.2. Instrumentation

-8

-7

-6

.5 <

-3

-2

-I

0

~ i 2.2 - p t i 2.8

.... p l l 3.4 pH 4.0 pH 4.6

- - pH 5.2 - - r~S.s ...... pa64

i . . . . pH 70

A /

/ /,'

/ / /

[ /

/ /

/ /

/

.800 .900 -I0~0 -I I00 .1200 .1300 o1400 -|500

E~mV

°x B

-7 /'"~ /

/ ', /

: / , \ ~/ , ; J '~ / x,~ /

• ~X} - ~ 1 -Hkk} -1100 °!~0 -I.~00 -1400 .I.SO0 E/mV

Fig. Io Pohrogrems r~or,.led at dilTerem pH values for ¢oncemr~tions of I~n~ldehyd~: (A} OA9 mM and (B) 3.34 raM. The legend in (A) also applies Io (B),

Polarographic measurements were carried out using a Metrohm 646 VA processor with a 647 VA stand in conjunction with a Metrohm multimode electable (MME) used in the static mercury drop electrode (SMDE) n ~ e , The three-electrode system was completed by means of a platinum auxiliary electrode and an AgIAgCItKCi 3M ref- erence el~trode, Additions were done with a Metrohm

Dosimat burette, Analysis of data was with PARVUS [16] ~ STATGRAPHICS [171,

2,3, P~t-'e:dH~

The, experimental procedure was as follows: tl'e solu- tion was placed in the polarographic cell and purged with nitrogen for i0 rain, Once the solution had been deoxy- genated, polarograms were recorded from -800 m V to

I ~ mV, All the results presented were obtained using differential-pulse mode with a pulse amplitude of - 5 0

mV, The drop time was 0,6 s, the drop area was 0,40 mm ~ and the ~ rate was - 10 mV s'J,

• Reruns and ~

3,1, Pdarographic ~ v i o u r ¢~ ~nzatdehyde

When a coupled reaction occurs in a electrodie process, different schemes can be prescnL depending bodl on the

number of steps in which it is developed and on the order they take place [18]. Among coupled chemical reactions. dimerization processes are characterized because the prod- uct of an electrochemical reaction combines with itself to give a dimer:

O+I~-~*R:R+R ~R~ (I)

This is the scheme followed in the electrochemical reduction of benzaldehyd¢, an organic compound whose complex mechanism of reduction on mercury electrodes has been widely studied [19,20]. Several electrochemical techniques have been used for this purpose: differential- pulse polarography [21,2211 alternating current polarogra- phy [23,24]; derivative polarography [25]: cyclic staircase voltammetry [2611 sonoelectrochemistry [2711 etc.

it has been reported that the modification of the acidity of the medium allows one to obtain information about the chemical reactions coupled to the electrodic process. Fig. I shows the effect of the pH variation on the differential pulse polarograms recorded at two different benzaldehyde concentrations, This makes it evident that processes which take place on the electrode depend to a great extent on the concentration of analyte in solution as well as on pH value.

When the reduction of benzaldehyde is carried out in acid medium at a mercury electrode, two successive po- iarographic peaks are observed, Fig. 2A. The first corre-

A. Herrero. M. Cntz Ortiz / Journal of Electroanalytical Chemisto, 432 (1997) 223-227 2 2 5

3,

2,

- I

0

-I

6

! 4

Addilion

A

-I,4 -0,8 -I,O -i,2

It. I V

4~

3~

6

Addition

II

9 . .

-0,8 -I 0 -!,2 E I V

4,

0 ~

6

Addition

C

E / V

Fig. 2, Polarograms recorded (once the blank signal has been subtracted) at the following pH values: (A) 2.2; (B) 4.6; (C) 7.6. Benzaldehyde concentrations range from 0.49 mM to 3.34 raM.

sponds to the reversible addition of one electron and one proton, which is associated with the formation of a free radical:

C6HsCHO + e- + H + ~ C6H~CHOH (2)

Whether the proton transfer precedes or succeeds the electron transfer depends on the pH of the medium [19]~ In the more acidic media the existence of one preprotonation step has been proved, whereas at higher pH values the proton exchange follows the electron transfer.

The radicals formed in this fast electron transfer dimer-

ize, giving the corresponding pinacol, named hydroben- zoin, through an irreversible chemical reaction:

2C6HsCHOH ~ C6HsCH(OH ) - CH(OH)C6H 5 (3)

However, if the potential reaches sufficiently low val- ues, the radicals are reduced to benzyl alcohol and a second monoelectron peak arises, being obscured by the discharge of protons at ~e lowest pH. This peak appears at more negative potential i:han the first, and it is assumed that it corresponds to a slow.~:lectron transfer [28]:

C6HsCHt])H + e- + H + ~ C6HsCH2OH (4)

The influence of the dimerization reaction on the second peak concerns both the shape and the peak potential. However, the fact that the second peak is smaller than the first is due to the partial formation of the dimer, as can be deduced from the report of Andrieux and Say,ant [29].

At higher pH values, Fig. 2B,C, the two peaks are gr~dually replaced by only one peak, associated with an electrodic reaction of two electrons. The third peak correo sponds to the formation of benzyl alcohol and is the result of the following ECE process:

CoHsCHO + 2e-+ 2H+~ C6HsCH2OH (5)

The interference of the dimerization reaction tend." s to separate the peak potentials, passing from two monoelec- tron peaks to a bielectron peak at sufficiently high pH values. The height and peak potential depend on the concentration of benzaldehyde, which has been understood to be an interference due to the simultaneity of electron transfer and radical dimerization.

3.2. Univariate analysis

To check the possibility of using the univafiate least squares regression (LS) in the determination of benzal- dehyde at the ten pH values cited before, LS models were built taking as dependent variable the peak current corre- sponding to the best defined peak versus the concentration of benzaldehyde. However, because of the high degree of overlapping the signal at pH 6.4, analysis under this experimental condition was not possible.

The results obtained allow one to conclude that, when coupled reactions take place together with the eleetrodic reaction, the control of the experimental conditions is critical in determining the analyte. Usually these systems provide non-specific signals to be analysed from a univario ate point of view, since they lead to results of poor analytical quality, in this case, the multivariate alternative is a possible solution to the problem.

3.3. Multivariate analysis

A multivariate procedure is necessary that simultaneo ousb considers the presence of all electrodic and chemical reactions implied, in this work PLS regression has been

~6 A. Herrero, M. Cruz Ortiz / Journal of Electro~ncdytical Chemisto, 432 (1997) 223-227

c h o ~ because it extracts ~ information contained in the whole polarogram, instead of taking specific sensors, with- out being affected by the high correlation that exists between c ~ n t s recorded at near potentials.

in the same way, in the space of the predictor variables (currents) this multivariate regression searches for direc- tions with maximum variance avoiding those not related to

response (concentration). This characteristic inherent in PLS method leads to regression models with high

prediction ability when the more relevant components are c ~ by means of the cross-validation criteria [30].

Using ~ data corresponding to each pH value a full cross-validated PLS m ~ l has been built. Each data ma- ~,x ha~ 118 ~ i c t o r variables (currents r e c o r d at 118 i~.~¢ntials, once the blank signal has been subtracted) and 7 obj~ts (additions of benzaldehyde). Fig. 2 shows the raw data u ~ in the ~nalysis for pH 2.2, 4.6 and 7.6.

To determi~ the number of latent variables needod in • e PLS models, both explainod aM crossovalidated (ieaw ing o ~ out) variance were considered, in this way, the variability sources not related to the response are not i~luded in the models. Fig. 3 summarizes the evolution of the cxplaiaod and cross~validatod v~ance corresponding m ~ ~ l s built for the different pH values. Explained v~ance values arc always higher than 99.2%, which indicates that the models are well fitted to the data used in their construction. Likewise, the proximity of both vari. ages points out the stability and pg, diction ability of the ~ l s developed.

PLS models were built with those latent variables which maximi~.e the cross-validated variance. The number of

2,1/ "=if/---' ['::I,, °

y e 99,S H

~ ~ 4 I 2 3 4

Number of tatent wdables

F~g. 3. EsChWg,,d b~lid line) and ¢~s :~d ida~ (do~ed line) ~rian¢~ ¢em:slpeadi~ ~ the PLS models built f~r the f~lowing pit values: (A)

{B) 2.$; tO) 3.4; (D) 4,~, (E) 4.6; (F) 5.2; (G) 5,8; (X) 6.4; (O 7.0; (J) T,$,

° " V ..............

>; °:1 -1

-0.8 - 1.0 - 1.2 - ! .4 E / V

0 4 [ D

-o.: I W .0s ,t,o .~., .~ 4

E / V

o.4f c~ 0 , 2 |

°F-Ir- e12

~0 4 t

~Og 4 0 4 , ~ .1,4

o4r . . . . u"fi-] 04] c

:o:tt 2t - o . : L . J ~ o

-0,8 - ! .0 - 1.2 - i .4 -0,8 - I .O - 1.2 - 1.4 E / V E I V

" : ~ J "°':l ' " 1 -0,8 - 1.0 - 1.2 - 1.4 -0.8 - 1,0 - 1,2 - 1,4

E t V E t V

0.4J H ] O,4~: l -

~, .o 4L=..::= ............ L ~ _ = J

.0:t~ • | ,0 :~,2 - | 4 .0,~ o I 0 • I .~ , 1.4 t / I V E I V

Fig. 4. Loadings ¢on'¢npondiog to the PLS m{glels built lhr: (A-C) ptl L2'. (D-F) ptl 4.(~: qrf;-l) pt4 7.6 wilh I, 2 and 3 latcm ~ariabic,~ rest~cttvely

these varies from ! to 3 depending on the pH considered. To give a chemical interpretation of these variables, which is very interesting because they have to be related to the phenomena related to the coupled reactions that happen on the electrode, Ioadings and scores corresponding to the different PLS models were analysed. Fig, 4 shows the I~dings for the first 3 latent variables, co~sponding to Ih¢ PLS models built for pH 2,2, 4.6 and 7.6.

In all the cases, the first latent variable reproduces the polarogram shape, which indicates that it is a size lhctor. 1he second latent variable, for pH 2.2 (Fig. 4B), is mainly related to the shift of fl~c polarographic signals with the concenuation of benzaldehyde, whereas the third is related to the zone of potentials where the reduction of the protons of the medium is observed, which cause interferences in d~ near ~ k [9]. These three !aleut variables complete the PLS model built at this pH value, as the cross-validated variance shows in Fig. 3A.

However, when pH 4.6 is considered, the structure of the Ioadings observed at pH 2.2 for the second latent variable (Fig. 4B) takes the place of the third latent variable for this new model (Fig. 4F). This indicates the existence of a new source of variability more important than the peak shift, which in this case corresponds to the presence of a new peak at more negative potentials, mod- elled by the second latent variable (Fig. 4E).

On the other hand, the Ioadings obtained for the PLS model built at pH 7.6 show a second latent variable related to the signal shift (Fig. 4H), whereas the third latent v{~able, which does not form part of the model, shows a andom structure (Fig. 41).

With regard to the predictions carried out with these models, Table I shows the relative errors corresponding to the samples of each calibration. These errors arc in the expected range in polarographic determinations at this concentration level. The results obtained corroborate the

A. Herrero, M. Cruz Ortiz / Journal of Electroanalytical Chemistr), 432 ~ 1997) 223-227 227

Table I Relative errors (%) corresponding to the concentrations of benzaldehyde calculated with the PLS models at different pH values

pH 103 [C~HsCHO] (mol 1- ')

0.49 0,98 1.46 1.94 2.41 2.88 3.34

2.2 8,64 - 3.00 - 2,69 1.84 0.23 !.13 - 1.07 2.8 14.63 - 9.27 - 1.62 1.55 2.96 -0.21 - 1.32 3.4 1.84 2.28 - 2.08 - 0.43 - 1.35 3.32 - 1.64 4.0 -6.54 -5 .13 -10 .39 3.80 3.00 0.85 - I . 9 9 4,6 0,38 0.11 -0.07 -0.63 0.36 0,41 -0.29 5.2 - 1,83 2.68 - i.18 -0.46 0.39 -0.08 0.03 3.8 4,00 1,82 - 7.28 4.53 - 2,29 0,87 - 0.04 6,4 - I ,45 -0.13 0,10 0.24 0.20 -0.36 0.11 7.0 - 2,83 0.74 0.03 - 2.88 !,54 1.50 - 1.02 7.6 2.19 0.65 !.48 - 0.10 - !.70 0.37 0.23

possibility of using differential pulse polarography in the determination of benzaldehyde, whatever the pH (i.e. inde- pendently of the specificity of the experimental signal).

To validate this affirmation and to compare the true and calculated concentration values, the Friedman test by ranks [31] and an LS regression analysis were used. The non- parametric test concludes, for a level of significance a- - 5%, that it is possible to consider the concentration calcu- lated at the different pH equal for each addition of benzai- dehyde. Therefore, it is possible to compare the concentra- tion calculated by the PLS regressions (for each concentra- tion of benzaldehyde there are 10 replicates) with its true concentration value by means of a least squares regression. If the regression line has slope ! and intercept O, it will mean that true and calculated concentration values could be considered equal.

The results of the analysis were the following: y - -0.0258 + 1.0006x, 0~0 .9990 (standard deviation of the regression: 0.4405). The parameters of the regression were evaluated using the joint confidence interval test [ 16], which accepts the hypothesis of zero intercept and unity slope for a level of significance a ,~ 3.54%. This indicates that, for all benzaldehyde concentrations used and indepen- dently of the pH of the medium (i.e. of the quality of the analytical signal) the concentrations found with the PLS models are statistically equal to the true values.

4. Conclusions

PLS regression has been successfully applied to model the interference of coupled chemical reactions on polaro- graphic signals. In particular, it has been applied in the polarographic determination of benzaldehyde, where dimerization and protonation processes take place, which have originated the shift and overlapping of the polaro- graphic peaks, giving complex polarographic signals. This multivariate technique allows one to successfully deter- mine this compound independently of the pH in the con-

centration range studied, so this is no longer a limiting condition in the analysis.

It has also been poss ible to give chemical sense to the

latent variables obta ined with the PLS models; this has

served to analyse the chemica l reality of the problem and

to unders tand the p r o c e s s e s that are taking place.

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[91

lie]

[1,]

[12] [131 [141

[151 [16]

[17] [18]

[19]

12o1 [21]

[221

[23]

[24]

[23] [26]

[27]

[28] [29] [3o1 [311