Debora Giovanelli

Electrochemical determination of sulphide at nickel electrodes in alkaline media: a new electrochemical sensor

The electrochemistry of a nickel hydroxide electrode has been studied both in the presence and absence of sulphide at microdisc and macroelectrodes. With sulphide present a new oxidative wave is observed. At a macroelectrode, this response produced a linear range from 20 to 200 μM with a corresponding limit of detection of 19 μM. Under the microelectrode regime, the response was found to be linear from 20 to 200 μM with a detection limit of 10 μM. The protocol has been developed into the design of a simple and cheap electrochemical sensing cell for the detection of sulphide in aqueous media.

Determination of ammonia based on the electro-oxidation of hydroquinone in dimethylformamide or in the room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

The results detail a novel methodology for the electrochemical determination of ammonia based on its interaction with hydroquinone in DMF. It has been shown that ammonia reversibly removes protons from the hydroquinone molecules, thus facilitating the oxidative process with the emergence of a new wave at less positive potentials. The analytical utility of the proposed methodology has been examined with a linear range from 10 to 95ppm and corresponding limit-of-detection of 4.2ppm achievable. Finally, the response of hydroquinone in the presence of ammonia has been examined in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide, [EMIM][N(Tf)(2)]. Analogous voltammetric waveshapes to that observed in DMF were obtained, thereby confirming the viability of the method in either DMF or [EMIM][N(Tf)(2)] as solvent.

Anodic stripping voltammetry of sulphide at a nickel film: towards the development of a reagentless sensor

The determination of sulphide at an electrochemically generated nickel oxide layer at glassy carbon and screen-printed electrodes in acidic media has been examined and appraised. The NiO layer was found to produce a stripping-like signal to sulphide and gave a linear peak current response from 20 to 90 muM. The response was further enhanced by repetitive cycling allowing accumulation of nickel sulphide at the electrode surface such that lower micromolar levels of sulphide (i.e. 5 muM) can be determined. The response at the NiO layer to sulphide is shown to be reproducible over a period of 24 h, thereby offering the development of a disposable amperometric sensor for sulphide.

Amperometric determination of sulfide at a pre-oxidised nickel electrode in acidic media

The electrochemical response of a pre-oxidised nickel electrode to increasing additions of sulfide has been examined and shown to produce a stripping-like voltammetric wave. A mechanism is described based on the formation of nickel sulfide at the electrode surface from a non-electroactive nickel oxide layer. The analytical utility of the approach has been examined and a linear range from 1 to 140 microM and a limit of detection of 0.8 microM is achievable, depending on the accumulation time.

Electrochemical characterization of sulfide tagging via its reaction with benzoquinone derivatives

Abstract The voltammetric response of a range of structurally diverse substituted benzoquinones species (p-benzoquinone, 2,3-dimethoxy- 5-methyl-p-benzoquinone, 2,6-dimethoxy-p-benzoquinone, and 3,5-ditert-butyl-o-benzoquinone) to increasing additions of sulfide has been examined using square wave voltammetry. In all cases the response shows that the quinone undergoes a reaction with sulfide to form a new redox active species. It is shown that by judicious choice of the pH of the solution analytical signals can be obtained for each species; the corresponding analytical parameters are detailed and the results critically appraised.

Voltammetry of immobilised microdroplets containing p-chloranil on basal plane pyrolytic graphite electrodesElectronic supplementary information (ESI) available: Comparison of the reduction of TCBQ in BN with other organic solvents (DMF, DMSO and PrCN). See http://www.rsc.org/suppdata/cp/b4/b405531d/

The electron transfer properties of p-chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, TCBQ) were investigated in both homogeneous and heterogeneous media. For the homogeneous study, the electrochemical reduction of TCBQ was carried out in different aprotic solvents (namely: benzonitrile (BN), N,N,dimethylformamide (DMF), propylcyanide (PrCN) and dimethylsulfoxide (DMSO)) and revealed two successive one-electron reductions according to a quasi-reversible EE mechanism. For the heterogeneous study, cyclic voltammetry with basal plane pyrolytic graphite electrodes modified with microdroplets of benzonitrile/TCBQ was employed. The droplets were found to be randomly dispersed with a degree of overlapping and average diameters of 5 μm giving the microdroplets individual volumes of ca. 33 fL. The redox processes within the electrically insulating microdroplets were shown to be very sensitive to the nature and concentration of ions in the surrounding aqueous phase as, in order to retain electroneutrality within the unsupported oil phase, electric field-induced migration of ions likely occurs during the Faradaic current flow. Depending on the lipophilicity of the aqueous electrolyte cation uptake into or electrochemical generated anion expulsion from the organic phase containing the electroactive specie TCBQ was induced electrochemically. Alkali metal cation uptake into the microdroplet environment was not observed. However less hydrophilic tetraalkylammonium cations NR4+ (R+ = Bu and Pe) inserted. Proton insertion into the oil phase was also shown to occur as the current|voltage shifted to more positive potentials, making the reductive process more facile, as the pH of the buffer solution was decreased. The higher efficiency of proton insertion as compared with Group I cations insertion was explained in terms of the formation of strong O–H covalent bonds which outweighs the ion phase transfer thermodynamics. Finally, the cross-phase electron transfer across the benzonitrile|water interface was examined when the TCBQ microdroplets were purposely made conductive by addition of a hydrophobic, nonpartitioning electrolyte in the oil phase. Again, the resulted voltammetry was found to change depending on the identity and concentration of the salt dissolved in the surrounding aqueous environment.