Published on 11 September 2014. Downloaded by Indian Institute of Science Education and Research – Bhopal on 15/10/2014 11:42:36.
Analyst View Article Online
PAPER
Cite this: Analyst, 2014, 139, 5747
View Journal | View Issue
A kinetic study of ferrocenium cation decomposition utilizing an integrated electrochemical methodology composed of cyclic voltammetry and amperometry† Archana Singh, Debarati Roy Chowdhury and Amit Paul* A novel, easy, quick, and inexpensive integrated electrochemical methodology composed of cyclic voltammetry and amperometry has been developed for the determination of the kinetic stability of higher oxidation states for inorganic complexes. In this study, ferrocene and its derivatives have been used as model systems and the corresponding ferrocenium cations were generated in situ during the electrochemical experiments to determine their kinetic stabilities. The study found that the ferrocenium cations decompose following the first-order kinetics at 27 3 C in the presence of ambient oxygen and water. The half-lives of the ferrocenium, carboxylate ferrocenium, and decamethyl ferrocenium cations were found to be 1.27 103, 1.52 103, and [11.0 103 s, respectively, in acetonitrile solvent having a 0.5 M tetrabutylammonium hexafluorophosphate electrolyte. These results are in agreement
Received 21st July 2014 Accepted 11th September 2014
with the previous reports, i.e. the ferrocenium cation is unstable whereas the decamethyl ferrocenium cation has superior stability. The new methodology has been established by performing various
DOI: 10.1039/c4an01325e
experiments using different concentrations of ferrocene, variable scan rates in cyclic voltammetry,
www.rsc.org/analyst
different time periods for amperometry, and in situ spectroelectrochemical experiments.
Introduction Cyclic voltammetry (CV) has been proven to be a powerful electroanalytical technique for different branches of chemistry such as inorganic chemistry, organic chemistry, physical chemistry, and biochemistry. CV utilizes a linear cyclic potential scan between two potentials. During this process, the molecules are oxidized/reduced to a higher oxidation/reduction state and then reverted back to their initial oxidation state (redox chemistry). The versatility of CV results from its capability of observing the redox processes over a wide range of potentials and in as little as a fraction of a minute which otherwise requires a tedious and time consuming chemical oxidation/ reduction methodology. This simple redox chemistry based on CV has been utilized in different research elds such as electron transfer,1 electrocatalysis,2,3 supercapacitors,4 etc. In the eld of electrocatalysis, the higher oxidation states of redox moieties are generated in situ and used to oxidize/reduce different substrates such as water,2 alcohols,3 carbon dioxide,5 etc. In the case of a supercapacitor, redox chemistry is used to generate pseudocapacitance.4 For both the research areas, it is of Department of Chemistry, Indian Institute of Science Education and Research (IISER), Bhopal, MP, 462066, India. E-mail:
[email protected] † Electronic supplementary information (ESI) available: Integration methodology, additional UV-Vis spectra, and rate law for the ferrocenium cation decomposition mechanism. See DOI: 10.1039/c4an01325e
This journal is © The Royal Society of Chemistry 2014
importance to understand the kinetic stability of electrochemically generated higher oxidation/reduction states of the redox moieties, since the stability of the redox moieties is the key for success in both the research areas. On the other hand, amperometry is another important electroanalytical technique where the potential of the working electrode is maintained at a constant value with respect to the reference electrode and the current is measured as a function of time. The technique has been widely used for different research purposes such as bulk electrolysis, potentiometric titration, etc.6,7 Ferrocene is one of the most widely studied redox probes because of its reversible electrochemistry. Interestingly, it was observed that although ferrocene is a very stable molecule, the ferrocenium cation does not have a long term chemical stability.1 Decomposition of the ferrocenium cation is dependent on the solvent, pH, counter anion, etc.8–10 Detailed mechanistic studies were performed in the seventies. In order to address the issues related to the chemical stability of the ferrocenium cations, different research groups have worked on procedures to increase the stability of the ferrocenium cation by using antioxidants9 or by synthesizing the analogs of ferrocene.11 In this work, an integrated electrochemical methodology based on CV and amperometry has been developed for the determination of kinetics associated with decomposition of
Analyst, 2014, 139, 5747–5754 | 5747
View Article Online
Published on 11 September 2014. Downloaded by Indian Institute of Science Education and Research – Bhopal on 15/10/2014 11:42:36.
Analyst
Paper
electrochemically generated higher oxidation states of inorganic complexes. Ferrocene, ferrocene carboxylic acid, and decamethyl ferrocene have been used as the model molecules and the kinetics associated with decomposition of the respective electrochemically generated ferrocenium cations were studied. The study found that the ferrocenium and carboxylated ferrocenium cations decompose very fast following the rstorder kinetics, but the kinetic stability of the decamethyl ferrocenium cation was signicantly higher than the ferrocenium or carboxylated ferrocenium cations.
Experimental section Chemicals Ferrocene (Spectrochem, India, 98%), ferrocene carboxylic acid (Spectrochem, India, 97%), decamethyl ferrocene (Sigma Aldrich, 97%), sodium dihydrogen phosphate (NaH2PO4) (Merck, 99%), disodium hydrogen phosphate (Na2HPO4) (Merck, 98%), tetrabutylammonium hexauorophosphate (Bu4NPF6), and acetonitrile (ACN, 99.9%, HPLC grade) were purchased from Sigma-Aldrich. The materials were used in this study without any further purication. Transparent indium tin oxide (ITO) coated on a glass electrode (resistivity < 10 U sq1) was purchased from Shilpa Enterprises, India. Milli-Q water was used throughout the study. Preparation of ferrocene and ferrocene derivative solutions Ferrocene, ferrocene carboxylic acid, and decamethyl ferrocene were dissolved in ACN having 0.5 M Bu4NPF6 electrolyte. Ferrocene solution was also prepared in ACN : aqueous phosphate buffer (50 : 50) (pH ¼ 8, ionic strength (I) ¼ 0.2 M). All these solutions were used for the kinetics associated with decomposition of the electrochemically generated ferrocenium cation. The concentrations of ferrocene and ferrocene derivatives used in this study were in the range of 0.2 to 0.7 mM. The concentration range was intentionally kept low so that the electrode fouling due to the decomposition products from the respective ferrocenium cations remains insignicant during the electrochemical experiments. The strategy was successful and conrmed by electrochemical studies (vide infra). Electrochemical experiments Electrochemical experiments were carried out on a CH Instruments, Austin, TX (Model CHI 760D) bipotentiostat. A CH Instruments SEC-C thin layer quartz crystal spectroelectrochemical cell was used as the electrochemical cell. The three electrode electrochemical cell consisted of a high surface area Pt mesh electrode as the working electrode, a Pt wire as a counter electrode, and an Ag/AgNO3 (10 mM AgNO3) nonaqueous electrode as a reference electrode. The dimensions of the working electrode were 6.1 mm 6.5 mm 0.55 mm (length width thickness) and the surface area was 3 cm2. Fig. 1 shows digital images of the spectroelectrochemical cell having all the electrodes placed inside. The cell has two regions. The lower and upper portions of the cell have path lengths of 0.1
5748 | Analyst, 2014, 139, 5747–5754
Fig. 1 Digital images of the thin layer spectroelectrochemical cell used in this study are shown: (A) front view and (B) side view.
and 1 cm respectively. Fig. 1 shows that the high surface area Pt mesh working electrode was kept close to the Pt counter electrode and the two electrodes were placed in the narrow path length portion of the cell of 0.1 cm. The reference electrode was placed just above the working and counter electrodes in the 1 cm path length region. A small volume of electrolyte solution (300 mL) was used in the present study. This cell design ensured that the oxidation of the analyte can be achieved quickly so that the mass transfer within the cell can be neglected. The conclusion has been further veried by spectroelectrochemical studies (vide infra). The Teon cap of the cell has two small holes which allowed passage of air into the cell. The experiments were performed at 27 3 C in the presence of ambient O2 and water. In the present study, the potential of the Ag/ AgNO3 reference electrode was found to be 0.54 V versus normal hydrogen electrode (NHE). The potential was veried once in a week and reported here against NHE throughout the manuscript.
Spectroelectrochemical experiments The spectroelectrochemical studies were performed using a thin layer quartz crystal spectroelectrochemical cell as discussed in the Electrochemical section. The electrode arrangements for these experiments were the same as discussed in the Electrochemical section. For such studies, along with the electrochemical experiments, simultaneously ultraviolet-visible (UV-Vis) spectroscopy experiments were performed using a Shimadzu UV 1800 spectrophotometer. However, a higher concentration of ferrocene and ferrocene derivatives (2 mM) was taken in ACN/0.5 M Bu4NPF6 solution for these studies. The concentrations of the analytes for these studies were kept higher compared to the electrochemical studies since the molar extinction coefficients (3) of ferrocene or its derivatives are low (
