Electrochemical Studies of Potassium Ferricyanide in Acetonitrile-Water Media (1:1) using Cyclic Voltammetry Method

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1 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May 217 Electrochemical Studies of Potassium Ferricyanide in Acetonitrile-Water Media (1:1) using Cyclic Voltammetry Method Nkunu 1 Z. N., Kamau 1 G. N., Kithure 1 J. G., Muya 2 C.N. 1 Department of Chemistry, University of Nairobi 2 Department of Environmental Sciences, Technical University of Kenya nkunuzach@gmail.com (Z.N.Nkunu), gnkamau@uonbi.ac.ke (G.N. Kamau), jkithure@uonbi.ac.ke (J.G. Kithure), muyacatenjambi@yahoo.com (M. C. Njambi) Abstract Cyclic Voltammetry is a versatile electroanalytical technique for the study of electroactive species. This method monitors redox behavior of chemical species within a wide range potential. The current at the working electrode is monitored as a triangular excitation potential is applied to the electrode. The resulting voltammograms was analyzed for fundamental information regarding the redox reactions. Cyclic voltammetry are the electrochemical equivalent to the spectra in optical spectroscopy. The number of electrons involved in the redox reaction for a reversible couple is related to the difference in peak potential by 59mV/n. The formal reduction potential is the mean of Peak anodic potential and peak cathodic potential (Epc) and peak anodic current (Epa) and peak cathodic current are close in magnitude. The absolute ratio of peak anodic (ipa) and peak cathodic current (ipc) for both scan rate and concentration studies proved unity for a reversible redox reaction. The mean peak voltage separation value for scan rate and concentration study was found to be.617 and.7v respectively. Their standard deviation was found to be ±.493 and ±.382 respectively. The calculated value for peak voltage for scan rate study and concentration study were found to be slightly higher than the theoretical value of.59v/n. The peak anodic current and peak cathodic current versus scan rate graphs and peak anodic current and peak cathodic current versus concentration curves found to exhibit a high R 2 values close to unity at a temperature of 25±1 C. Key Words: Ferricyanide, Cyclic Voltammetry, acetonitrile-water media and Potentiostat. 1. Introduction Potassium ferricyanide is a bright red salt with a chemical formula K 3 Fe(CN) 6. The salt contains theoctahedrally coordinated [Fe (CN) 6] 3- ion (Sharpe, 1976). It is soluble in acetonitrile-water media (1:1) and its solution show some green yellow fluorescence.like other metal cyanides, solid potassium ferricyanide has a complicated polymeric structure. The polymer consists of octahedral [Fe(CN) 6 ] 3 centers crosslinked with K + ions that are bound to the CN ligands (Figgis,; Gerloch,; Mason,1969). The K + ---NCFe linkages break when the solid is dissolved in water-acetonitrile media.the Fe(CN)6 3- / Fe (CN)6 4- redox couple is used 52

2 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May 217 as an example of an electrochemically reversible redox system used to study some basic concepts of Cyclic Voltammetry. [Fe(CN) 6 ] 3 consists of a Fe 3+ center bound in octahedral geometry to six cyanide ligands. The complex has O h symmetry (Zanello, 23). The iron is low spin and easily reduced to the relatedferrocyanideion [Fe(CN) 6 ] 4, which is a ferrous (Fe 2+ ) derivative. This redox couple is reversible and entails no making or breaking of Fe-C bonds: [Fe(CN) 6 ] 3 + e [Fe(CN) 6 ] 4 For the reasons above, potassium ferricyanide redox is a standard in electrochemistry. This redox couple is also used in: preparation of Prussian Blue (the deep blue pigment in blue printing), as a chemical reducer in photographic processing to remove silver from negatives and positives (dot etching), in wine production potassium ferricyanide has been used to precipitate out copper if copper has been used as a fungicide on the grapes (Stroebel, 1993).In histology, it is used to detect ferrous iron in biological tissue (Carson, 1997). The main objective of this experiment was to illustrate the basics of cyclic voltammetry by studying the one-electron reversible redox system. The properties of the reaction: Fe III (CN) 6 3- Fe II (CN) 6 4- were investigated. The parameters of greatest interest for a reversible CV are the peak cathodic Potential (Epc), the peak anodic potential (Epa), the peak cathodic current (ipc) and the peak anodic current (ipa). The peak currents of a reversible process is given by the RandlessSevick equation: ip =(2.69 x 1 5 )n 3/2 AD 1/2 V 1/2 Where A is the electrode surface area (cm 2 ) obtained using geometrical measurements, D is the diffusion coefficient (cm 2 /s), C is the concentration of the electroactive species in the bulk solution (mol/cm 3 ) and V is the scan rate (v) (Muya C.T, 215). Therefore, ip is proportional to C and proportional to V 1/2. If A is known, then D can be calculated from the slopes of the linear (Wanjau T W, 215). The results obtained from this experiment below is used to demonstrate the determination of the following: the formal reduction potential(eº); the number of electrons transferred in the redox process (n); the diffusion coefficient (D); electrochemical reversibility; and the effects of varying concentrations (C) and scan rate and the results compared with the literature existing ones. 2. Methodology 2.1 Reagents A 1ml stock solution of 1Mm K 3 Fe (CN) 6 in.1 M KNO 3 was prepared. Serial dilutions of this solution was done to give 5ml solutions of 1,2,3,4 and 5mM K 3 Fe (CN) 6 in.1m KNO 3. Acetonitrile solvent was an HPLC grade from Chemoquipcompany Ltd. An alumina slurry was made for cleaning the working electrode. 2.2 Apparatus In the experiment below, a computer-controlled Potentiostat; Autolab (PGSTAT A) electrochemical analyzer; Princeton applied Research, PAR; software was used. Layout of a three-electrode glass cell was used 53

3 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May 217 together with a carbon working electrode (3.1mm;.71cm 2 ) standard calomel reference electrode (SCE) and Platinum wire auxiliary electrode. The connections between the Potentiostat and the cell are as follows: black = working electrode, blue=reference electrode and red = auxiliary electrode. All experiments were conducted at room temperature (25±1ºC). 2.3 Procedure The carbon working electrode was polished using alumina slurry (.5 micron alumina powder) on soft lapping pads and rinsed well with distilled water before experimentation to have fresh working surface. Each of the solutions prepared above was added to the cell and thoroughly purged with Oxygen-free Nitrogen 15 minutes before running a CV. In addition, the solution was stirred between experiments in order to restore initial conditions, but it was not stirred during the experiment..1m KNO 3 supporting electrolyte was to control electrode potentials, eliminate the transport of electroactive species from migrating in the electric field gradients, maintain constant ionic strength and maintain constant ph. This ensured a diffusion controlled electrode process. Direct reduction potential of iron was then determined. 3. Results and Discussion The redox couple (K 3 Fe (CN) 6 3- /K4Fe (CN) 6 4-) was studied using a computer-controlled Potentiostat and gave the results below. 3.1 Scan Rate study Table1: Results for scan rate and peak height, for 1mM K 3 Fe (CN) 6 in.1m KNO 3. Solution Scan rate (mv/s) V 1/2 (mv/s) Epa Epc ipa (x1-5 A) ipc (x1-5 A) /ipa/ipc/ E Ep For scan rate studiesit is evident that peak separation values ranged between.56 to.69v with a mean of.62v and a standard deviation of ±.493, /ipa/ipa/ ranged between.993 to 1.62V with a mean of 1.55E-5A and a standard deviation of ±

4 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May x1 -.5x1 -.25x1 i / A.25x1.5x1.75x E / V Figure 1: Figure 6. Influence of concentration on peak currents for ferricyanide solution.the peak current increases with increase in concentration (1mM -5mM The scan rate is proportional to the peak current. This is evident from the plot done above. CURRENT X1E-5(A) y = 5.244x R² = SQUARE ROOT OF SCAN RATE (V/S) y = x 6.56 R² =.95 Figure 2: A Plot of Peak current against square root of scan rate for a concentration of 1Mm K 3 Fe(CN)6 The linear regression for the above graph is good (value beyond.95). It signifies that there is a correlation between the values, however it does not show the cause. 55

5 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May Concentration Study Table2: Peak heights at varying concentrations of (K 3 Fe (CN) 6 ) at a scan rate of 3Mv/s. Solution Concentration (mm) Epa Epc ipa (x1-5 ) ipc (x1-5 A) Ratio /ipa/ipc/ E Ep For concentration studiesit is evident that peak separation values ranged between.66 to.72 V with a mean of.7v and a standard deviation of ±.382, /ipa/ipa/ ranged between 1.5 to 1.1V with a mean of 1.74E-5A and a standard deviation of ± x1 -.3x1 -.2x1 -.1x1 i / A.1x1.2x1.3x1.4x E / V Figure 3 A plot of peak current against concentration at a scan rate of 3Mv/s It was well observed from the voltammogram that the peak height increased with increase in concentration (from 1-5mM). 56

6 International Journal of Scientific Research and Innovative Technology ISSN: Vol. 4 No. 5; May 217 Peak anodic and cathodic current (A) y = 7E-5x -5E-6 R² = Concentration (mm) y = -6E-5x + 7E-6 R² =.996 Series1 Series2 Linear (Series1) Linear (Series2) Figure 4 A plot of ipa and ipc peak currents against concentration There was a good correlation coefficient between the peak current (ipa and ipc) and concentration. However correlation does not reveal causation. 4. Conclusion Taking into consideration of the peak current ratio rather than the individual peaks current values it was found that for the reversible process, the ratio approaches one; value significantly different from 1 are typically associated with chemical reactions coupled with electron transfer (Quasi-reversible reactions). For example, the ratio is less than 1 if the electron transfer is followed by a chemical reaction. The formal redox potential (E O of a reversible process is given by the mean of the peak potentials. The other characteristic potential parameter is the separation of the peak potentials Ep. The theoretical values of Ep for a reversible process is.59/nv, and it is independent of scan rate. However the measured value for reversible process is generally higher due to uncompensated solution resistance and non-linear diffusion. Larger values of Ep, which increase with increasing scan rate, are characteristic of a slow electron transfer kinetics. A low standard deviation indicates that the data points tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range of values. 5. Acknowledgement We Authors of this paper, acknowledge Almighty God for knowledge and competence given to us to undertake this study. We are also grateful to University of Nairobi Chemistry and Physics Department for material support. 57

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