Polarographic and voltammetric investigation of new synthesized βdiketone (۲ methoxy Benzene Thiol Acetyl Acetone)

دسته شیمی و آنالیز دستگاهی
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری بیست و پنجمین گردهمائی علوم زمین
نویسنده Sima Evazmoghadam
تاريخ برگزاری ۳۰ بهمن ۱۳۸۵



۱. Introducton

βdiketones compounds constitute a very important class of organic compounds because of their widespread application in many areas of technology and Industry, They are well known for their use as analytical and extraction reagent in dye industry or in elemental separation technique. These diketone complexes are very useful for medical means such as decreasing blood sugar in diabetic or for preparing plate or capsule.(۱)

They play a role catalyzor to accelerate the chemical reaction: their catalyzor activity decrease as following: 


Mn۳+ > Fe۳+ > Mn۲+ > Cu۲+ > Cr۲+ > Cr۳+ > Co۳+

The Iron complex has antimicrobial role. (۲, ۳). In this study polarographic and voltammetric behavior of ۲ methoxy Benzene Thiol Acetyl Acetone (۲MBTAA) was investigated. The presence of conjugated bond system and chelating moiety gives rise to a considerable shift of the polarographic and voltammetric peak potentials and currents. Therefore such compounds can be used as metallochromic indicators. Electrochemical methods such as polarography and voltammetry are sufficiently informative tool for studying protolytic equilibria of compound having a conjugated bond system in different solution media.(۴, ۵).The aim of this study is to explain the electrochemical reaction mechanism for the diketone compound using square wave voltammetry (SWV), differential pulse polarography (DPP) and cyclic voltammetry (CV) in different media. At the same time, the ionization constants were determined from the pH dependences of the electrochemical measurements and protolytic equilibria of the compound having a conjugated bond system in aqueous-ethanol medium. This compound has not hitherto been investigated by polarographic and voltammetric methods, the molecular structure of the ۲MBTAA is given as below.



Scheme ۱.


۲. Experimental

۲MBTAA synthesized and was checked with HNMR and IR, Boric acid, Phosphoric acid, Acetic acid and Sodium hydroxide for Britton Robinson buffer were obtained from Merck product. All solution were prepared using ultra pure water obtained by passing deionized water   through an advantec Millipore F۲ HN ۶۴۹۶SC system.SWV, DPP and CV polarograms and voltamograms of the βdiketone were recorded with a Metrohm VA ۷۹۷ computrace Electrochemical Analyzer three electrode combination system was used, this consisted of a Multi Mod Electrode (DME, SMDE and HMDE), a Ag/AgCl reference electrode and a Pt wire auxiliary electrode.Pulse height of ۵۰mV and step height of ۴mV were used. Solutions were degassed with nitrogen for ۵۰۰s prior to measurements and for an additional ۵s before each scan. A nitrogen atmosphere was maintained throughout the experiments.Each scan was done on a separate mercury drop at room temperature.


۳. Results and Discussion:

The polarograms and voltammograms of the solution containing the ۲MBTAA were taken in BR buffer solutions containing ۵۰% ethanol at pH values between ۲.۰ and ۱۲.۰ ۲MBTAA has one electrochemistry reducible group and its structure is shown in scheme ۱. Differential pulse polarograms and square wave voltammograms of ۲MBTAA in BR buffer at different pH values are shown in Fig. ۱.As shown in Fig. ۱, peak currents and potentials are dependent on pH.Differential pulse polarography and square wave voltammetry of the ۲MBTAA give one well defined polarographic reduction peak Ic at the HMDE at a sweep rate of ۰.۰۵V/s for DPP and ۰.۹۰۰V/s for SWV in aqueous-ethanol buffers in the pH range ۲.۰-۱۰.۰.

The shape of peak Ic became broad at higher pH for DPP and SWV polarograms and voltammograms. The peak potential (EP) of reduction peak Ic for the ۲MBTAA is dependent of pH in the pH range ۲.۰-۱۲.۰ studied as shown in Figs.۱ and ۳.Plots of peak potential (EP) versus pH for the ۲MBTAA are depicted in Fig. ۲ for DPP and SWV. As the pH was gradually increased, the peak potential shifted towards more negative values. As shown in Fig.۲, the dependence of the peak potentials of the ۲MBTAA for SWV and DPP shows a break at a pH value of about ۹.۵-۱۰.۵. Below this value a proton transfer precedes the electrode process above pH ۱۰.۵ the peak potential are pH independent. Generally the polarographic and voltammetric reduction of an organic compound can be represented by (۶-۹):

O+aH++ ne =RHa

The approximate relationship between Ep and pH for a reversible reaction at ۲۵۰C is:

Ep=E۰-۰.۰۵۹(a/n) pH

A plot of Ep against pH should be linear with a slop of ۰.۰۵۹a/n and an intercept corresponding to E۰(۱۰,۱۱).The linear pH dependence of the peak potential for SWV and DPP is given in Table۱.The equations (Table۱) show that protons participate directly in the reduction process at pH ۲.۰-۱۱.۰.Fig. ۲ indicated that the peak potential of the ۲MBTAA is constant at pH>۹.۵ for DPP an SWV.Protonation or deprotonation of the diketone groups is shown in scheme۲.




                 a                                                   b


Fig. ۱.DPP(a) and SWV(b)  ۰.۱۷ mM of ۲MBTAA voltammograms and polarograms (۳.۵o, ۵.۲۴, ۶.۱۶, ۷.۲۲, ۸.۱۵, ۹.۴۱, ۱۰.۵



Fig. ۲.Effect pH on the peak potentials of DPP and SWV obtained for ۴x۱۰ M solution of ۲MBTAA



Table۱Dependence of peak potential on pH



pH range                  Equation                                                      R۲                        Technique






Ep(V)= -۰.۱۰۰۱pH + ۰.۲۲۲۷


Ep(V)= -۰.۰۲۷۱pH - ۰.۳۱۴۶

۰.۹۹۷۳              DPP

۰.۹۹۶۳             SWV

۰.۹۹۸۷              DPP

۰.۹۹۱۹              SWV


In Scheme ۲ the protonations on the oxygen atom have been represented preferentially on the oxygen, therefore the variation with increasing pH of equilibrium potential consist of three linear segments with slope of -۶۸,

-۳۱mV and @ ۰ for DPP(Fig.۲) indicating the different electrode reaction mechanisms. Similar behavior of currents and peak potentials reaction with pH were observed in CV voltammograms compared with DPP and SWV (as given below).The introduction of electron-donor groups in o-o΄-positions complicated the reduction process of dyes and increases their irreversibility. The height of the reduction peak of the βdiketone increases because the rate of their protonization increases at pH ۲.۰-۷.۰(Fig ۳)                                                     



Fig. ۳: Effect of pH on the peak currents of DPP and SWV obtained for ۴x۱۰ M ۲MBTAA


۳.۱. The effect of ethanol

The potential peaks of reduction become negative and the height of the cathode peaks decrease with the increasing ethanol contents as shown in Fig.۴.The increase of ethanol concentration in aqua-ethanol mixes reduces proton-donor activity of water and reduces the rate of protonation.The rate of the electrode process strongly decrease in the case of the superficial reaction of protonization that precedes an electrochemical stage, because adsorption of the basic form of the depolarizer decreases(۱۲).It is confirmed by significant decease of the rate criterion at the increase of ethanol concentration in solutions.




Fig. ۴. Effect of the composition of the solvent on the peak currents and peak potentials in BR buffer at pH ۷.۰


۳.۲. CV and DCP studies

Fig. ۵ Shows typical DCP polarograms of the ketone compound in BR buffer containing ۵۰% ethanol; a well defined cathodic wave is produced. The reduction of the ۲BMTAA at HMDE was found to be pH –dependent E۱/۲ values were shifted to negative potentials upon the increase of pH A plot of  E۱/۲ versus pH shows a region of linearity with a break at about pH as DPP and SWV.Logarithmic analysis of the reduction waves obtained in BR buffer of different pH values (۲.۰-۱۲.۰) resulted in straight lines. The variable values of slopes do not prove that the reduction process is irreversible. The αn values were calculated according to the method of Heyrovsky Ilkovic equation (۷).At pH ۲.۰-۱۰.۵ αn values indicate that the electrode process consists of  two electrons (Fig. ۶).



Fig. ۵.DCP  polarograms ۲ mM of ۲MBTAA  (۳.۵o, ۵.۲۴, ۶.۱۶, ۷.۲۲, ۸.۱۵, ۹.۴۱, ۱۰.۵


Also the plots of Ep-pH of the reduction wave of the ۲MBTAA were straight line of slop values (S) reported in Table ۱.The number of hydrogen ions (Z+H) participated in the rate-determining step was calculated using the slop values S۱ and S۲ of both the Edc versus log (I/Id-I) and E۱/۲-pH plots, respectively.The estimated results indicated that two protons and two electrons (pH<۱۰.۵) participated in the rate determination step of the reactant centers(۷).The cyclic voltammograms obtained for ۲MBTAA in BR buffer at PH  ۲.۰-۱۱.۰ (scan rate ۱V/s) are shown in Fig. ۷  only one well defined reduction peak was seen in the range of potentials examinted at pH ۲.۰-۱۰.۰, this can be assigned to reduction of the βdikeone .At pH >۹.۵ CV voltammograms are broad . The elucidate further the electrode reaction of the βdikeone a cyclic voltammetry at HMDE was recorded. As shown in Fig ۷, the cyclic voltammograms of the ۲MBTAA in Britton Robinson buffer solution of pH ۲.۰-۱۰.۰ exhibit a single cathodic peak with no peak on the reverse scan indicating the irreversible nature of the electrode reaction.The  peak potentials (EP) shifted to more negative values upon rise of each of the pH ۲-۱۰ which confirmed the involvement of protons in electrode reactions as DPP, SWV and DCP (Fig.۲) (۹).

Fig. ۶. The change of n with pH for the ۲MBTAA in BR buffer.


In the pH range of ۲.۰-۱۲.۰ the electrochemical reaction mechanism appears to be an irreversible process at all scan rates (۵-۱۰۰۰mv/s), as no current is observed in reverse scan for the ۲MBTAA.In this situation it can be said that the reaction is irreversible. The peak current changes linearly with scan rate (υ) according to equation Ip=Aυx. The x values ۱.۰ and ۰.۵ are expected for adsorption and diffusion controlled reaction, respectively.For ۲MBTAA, the regression of log (Ip) versus log (υ) gave a slope value of ۰.۹۱, indicating that the reduction current had contributions from both diffusion and adsorption currents(۸,۹).

As scan rate was increased from ۵-۱۰۰۰mv/s the peak potential shifted towards more negative potential as expected for an irreversible reduction process (Fig.۸). The effect of scan rate on the peak potential and peak current was also analyzed by cyclic voltammetry on an HMDE.Upon analysis of the effect of scan rate, dependence of peak intensity on scan rate and the root square of scan rate were established within the range of scan rates studied (۵-۱۰۰۰ mv/s). The change of Ip versus υ۱/۲ is not linear. With increase of scan rate, Ip/ υ۱/۲ slope increases. This situation proved that the complex electrochemical electrode reaction of the ۲MBTAA occurs on the electrode surface, such behaviour is adopted as indicative of EC mechanism (۱۳).

Dependence of peak potential and peak current with pH arises from different mesomeric forms.However; protonation should increase the stability as given different mesomeric forms in Fig. ۹ and scheme ۲.

From polarographic and voltammetric measurements it is known that the reduction of this compound containg electron donating substitent,it is therefore reasonable to assume that the majority of the ۲MBTAA compound studied here is reduced to corresponding ketone at all the pH values. The reduction  of the ketone involve two electrons .These results are appreciably supported from SWV, DPP, DCP and CV polarograms and voltammograms .According to SWV, DPP, DCPandCV techniques the reduction mechanism can be suggested in Scheme ۳.


Fig. ۷. CV voltammograms of the ۰.۷ mM of ۲MBTAA in BR buffer (pH ۳.۵-۱۱, scan rate ۱v/s).






Fig. ۸. (a)CV voltammograms at different scan rates at pH ۷.۰,(b) The change of peak currents with scan rate ,(c) The change of peak currents with root square of scan rate,(d) The  change of peak potentials with scan rate

Fig. ۹.Effect of pH on the cathodic peak potentials of CV


۳.۳. Determination of ۲MBTAA

The redox property of ۲MBTAA on the electrode is influenced by the variations of pH values. Therefore pH is a critical factor for determination



Scheme ۳.Reaction mechanism diagram for ۲MBTAA


 of this compound. On the other hand the pH of the test solution also affects both linear range and determination power. The DPP polarograms and SWV voltammograms showed improved peak currents compared to the DCP.SWV has been preferred because of its convenience in analytical applications. The solution condition affects the enhancement of the peak currents. It was observed that SWV technique was suitable for analytical purposes among the techniques in question in this work (Fig.۱). Under the above experiment conditions the linear regression equation was determined show the dependence of the peak current of the diketone concentration, In the pH range the around ۷ the peak current was approximately constant, In our experiment pH was kept at ۷ using Britton-Robinson buffer for determination of ۲MBTAA .A plot of currents against concentrations is linear between ۵x۱۰ M and ۵x۱۰M (Fig.۱۰).The peak current reaches it maximum value at pH ۷. Selected as optimum value for quantitative determination of ۲MBTAA.



Fig. ۱۰. Calibration graph for different ۲MBTAA concentration at pH ۷.۰ in BR buffer solution.






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Sima Evazmoghadam Mohamad H.Arbab Zavar, Sayyed F.Tayarri, Mahmud Chamsaz


 Geology survey of  Iran & Ferdowsi Mashhsd university of Iran





Voltammertric and polarographic reduction on a hanging mercury electrode in aqueous-ethanol medium was performed for the 2 methoxy Benzene Thiol Acetyl Acetone by using SWV,DPP and CV techniques. Electrochemical behavior of the new βdiketone has been investigated depending on ethanol-water ratio. From the polarographic and voltammetric data, electrochemical reduction mechanism has been suggested. Adsorption effect of the βdiketone compound was investigated

At different pH values, optimum condition are given for its polarographic and voltammetric determination For SWV.


Keywords: Diketone, Polarography, Voltammetry, Reduction mechanism


کلید واژه ها: Diketone Polarography Voltammetry Reductionmechanism شیمی و آنالیز دستگاهی سایر موارد