Wax impregnated graphite electrode

Jarbawi and Heineman have used differential pulse voltammetry and chronocoulometry to study the effect of immersing a wax-impregnated graphite electrode in an aqueous solution of chlorpromazine [157]. Extraction of substance into the electrode and absorption at the interface were both found to occur. 

The electron activity (or intensity) at redox equilibrium may be measured by a potentiometer. A pH meter or a millivolt meter may be used for measuring the potential difference between a reference electrode (such as a calomel electrode) and an oxidation-reduction indicator electrode (such as platinum, gold, or a wax-impregnated graphite electrode). 

As part of a study on antioxidants the halfwave oxidation potential of phenothiazine at a wax-impregnated graphite electrode was determined and the value -f- 239 (mv SCE) was found. 

Z. Xi, et. al.. Determination of vitamin E by single-sweep oscillo-polarography with wax-impregnated graphite electrode, Fenxi Huaxue, 19... 

Figure 11.8.5 Anodic stripping analysis of a solution containing 2 X 10 M Zn, Cd, Pb, and Cu at an MFE (mercury-plated, wax-impregnated graphite electrode). Stripping carried out by differential pulse voltammetry.

Antioxidants in gasoline, derived from aniline, have been determined using the anodic oxidation waves at a wax-impregnated graphite electrode directly in an alcoholic solution of the sample. 

Table 1 Oxidation Peak Potentials for Different Tyrosine, Tryptophan, Cysteine, and Cystine at a paraffin wax-impregnated graphite electrode...

FIGURE 18.10 Differential-pulse anodic-stripping voltammogram of tap water at a mercury-plated, wax-impregnated graphite electrode showing detection of metals at subparts-per-billion levels. 

Of recent years the use of mercury film electrodes based on substrates other than platinum has been explored, and increased sensitivity is claimed for electrodes based on wax-impregnated graphite, on carbon paste and on vitreous carbon a technique of simultaneous deposition of mercury and of the metals to be determined has also been developed. 

A variety of electrodes have been used in this technique, including rotating mercury coated vitreous carbon [756], wax impregnated graphite cylinders [758], and hanging mercury drops [753-755]. 

The electrode substrates typically considered for potentiometric stripping analysis are -> glassy carbon and wax-impregnated graphite. Application of metal substrates is usually limited due to the possibility of contamination of mercury by amalgamation. Also other intermetallic compounds can be formed. Very careful polishing is essential to obtain good results. 

Carbon electrodes are also employed in electroanalytical applications due to the very low residual current over a wide range of potentials that makes it possible to study electrochemical reactions even at the level of trace concentration. Among the different types of such electrodes, wax-impregnated graphite rods, carbon powder bound with an inert viscous liquid (carbon paste), glassy carbon, pyrolytic graphite and carbon fibers, and, more recently, nanotubes and fullerenes can be mentioned. Carbon fibers have radial, random, or anion distributions that lead to a different distribution of step and step—step interactions. 

The experimental protocol for this research involves several essential features. First, no redox transformations are allowed. Ferrous and ferric iron chlorides are introduced into strictly anaerobic aqueous systems, with careful exclusion of oxygen (Q. Second, pH and the potentials at platinum and wax-impregnated graphite (WIG) electrodes are recorded as a function of time, beginning shortly after the addition of ferric chloride. 

Extend the range of ferric species activities over which Pt electrode response is nernstian, and, provide evidence for the comparable behavior of another, physically and chemically distinct electrode material, wax-impregnated graphite. Experiments in laboratory and field situations indicate nernstian response of both Pt and WIG electrodes at pH s as high as 6.4, and thus total Fe(III) activities as low as 10 M. 

The mercury electrode used in stripping analysis is either a conventional HMDE or a mercury film electrode (MFE). In current practice, the MFE is typically deposited onto a rotating glassy carbon or wax-impregnated graphite disk. One usually adds mercuric ion... 

Physical Measurements. For the electrolyses, a Wenking potentiostat model 70TS1 and a Koslow Scientific coulometer model 541 were used. Voltammetry with wax-impregnated graphite and rotating platinum electrodes was performed as described elsewhere (7, 8). IR and electronic spectra were measured on Perkin-Elmer 225 and Cary 14 instruments. X-band ESR spectra were recorded at room temperature on a JEOL MES-3X spectrometer. Phosphorus-31 NMR spectra were recorded in the pulse mode on a Varian XL-100 instrument at 40.5 MHz using a deuterium lock, or on a Bruker HFX-90 instrument at 36.43 MHz using a fluorine lock. 

Ward [119] has discussed in some detail the determination of phenolic and amine types of antioxidant and antiozonant in polymers by the chronopotentiometric techniqne, using a paraffin wax impregnated graphite indicating electrode and solntions of lithinm chloride and lithium perchlorate and acetonitrile in 95% ethanol as snpporting electrolytes. The precision obtainable for repeated chronopotentiometric runs in acetonitrile was fonnd to be better than 1.0% in cases in which electrode fouling did not occnr, and 1.7% when the electrode was fouled by electrolysis products. 

Fichter and Kern O first reported that uric acid could be electrochemically oxidized. The reaction was studied at a lead oxide electrode but without control of the anode potential. Under such uncontrolled conditions these workers found that in lithium carbonate solution at 40-60 °C a yield of approximately 70% of allantoin was obtained. In sulfuric acid solution a 63% yield of urea was obtained. A complete material balance was not obtained nor were any mechanistic details developed. In 1962 Smith and Elving 2) reported that uric acid gave a voltammetric oxidation peak at a wax-impregnated spectroscopic graphite electrode. Subsequently, Struck and Elving 3> examined the products of this oxidation and reported that in 1 M HOAc complete electrochemical oxidation required about 2.2 electrons per molecule of uric acid. The products formed were 0.25 mole C02,0.25 mole of allantoin or an allantoin precursor, 0.75 mole of urea, 0.3 mole of parabanic acid and 0.30 mole of alloxan per mole of uric acid oxidized. On the basis of these products a scheme was developed whereby uric acid (I, Fig. 1) is oxidized in a primary 2e process to a shortlived dicarbonium ion (Ha, lib, Fig. 1) which, being unstable, under-... 

Fig. 1. Proposed mechanism of electrochemical oxidation of uric acid at a wax-impregnated spectroscopic graphite electrode in 1 M HOAc according to Struck and Elving 3)...

The response of pyrolytic graphite electrodes depends on whether the orientation is edge plane or basal plane [102]. Spectral grade graphite impregnated with Ceresin wax or paraffin wax under vacuum can also be used. 

Electrochemical oxidation of natural and synthetic nucleic acids at carbon electrodes pyrolytic graphite [34], paraffin-wax-impregnated spectroscopic graphite [35] and glassy carbon [36, 37] has been studied. It was shown that at pH 4.5 the electrochemical activity of nucleic acids is conditioned by the presence of purine residues in polynucleotide chains. 

Currently, work is being conducted on an in situ electrodeposition sampling device 20, 21, 22), It consists of a submersible, self-contained potentiostat, power supply, reference electrode, and working electrode. Metals are deposited on the 1-in. diameter, wax-impregnated, pyrolytic graphite working electrode 21, 22, 23) which can then be removed from the sampler at the surface and stored. The metal film can be either... 

---