Carbon disks

J. Slobodnik, O. Ostezkizan, H. Lingeman and U. A. Th Brinkman, Solid-phase extraction of polar pesticides from environmental water samples on grapliitised carbon and Empore-activated carbon disks and on-line coupling to octadecyl-bonded silica analytical columns , J. Chromatogr. 750 227-238 (1996). 

Explain clearly why a carbon composite disk electrode offers improved signal-to-background characteristics compared to a carbon disk electrode of the same geometric area. 

The electrodeposition of tellurium and silver has been investigated in dilute aqueous solutions of tellurous acid and Ag " ions (concentrations in the order of 10 to 10 " M) in 0.1 M HCIO4 [164], In particular, cyclic voltammetry experiments were conducted with rotating glassy carbon disk electrodes in baths with various concentration ratios of Ag(I) and Te(IV) precursors, and their outcome was discussed in terms of the voltammetric features. For a Ag(I)/Te(IV) ratio close to 0.8, formation of quasi pure silver telluride, Ag2Te, was reported. The authors, based on their measurements and on account of thermodynamic predictions, assumed that silver is deposited first on the electrode (Ag" + e Ag), and then Te(IV) is reduced on the previous silver deposit with formation of Ag2Te according to the reaction... 

By electrodeposition of CuInSe2 thin films on glassy carbon disk substrates in acidic (pH 2) baths of cupric ions and sodium citrate, under potentiostatic conditions [176], it was established that the formation of tetragonal chalcopyrite CIS is entirely prevalent in the deposition potential interval -0.7 to -0.9 V vs. SCE. Through analysis of potentiostatic current transients, it was concluded that electrocrystallization of the compound proceeds according to a 3D progressive nucleation-growth model with diffusion control. 

Activated carbon disks have also been used for the extraction of polar pesticides from environmental water samples. Slobodnik et alP used an Empore activated carbon disk (EACD) and Envi-Carb graphitized carbon black (GCB) and CPP-50 graphitized carbon for the trace enrichment of polar pesticides from water. 

Fig. 2 Cyclic voltammograms recorded at a glassy carbon disk electrode (diameter = 1mm) at a sweep rate of lVs-1 in 0.2 M Bu4NPF6/THF for Zn-reduced solutions of...

Research related to the use of vanadium phosphates or V2O5 as oxidants of gases such as CO and SO2 in commercial processes shows that solid vanadyl sulfate can serve as a gas-permeable solid-phase electrolyte [102]. Two reversible redox features are observable at slow scan rates (20-150 mV s ) by CV in a gas-tight two-electrode cell packed with powdered VOSO4 3H2O between a 10-mm carbon disk and a 3-mm glassy carbon electrode. The V(IV/V) couple was observed at 0.55 V versus C, and the V(IV/III) couple was observed at —0.97 V. Unlike in aqueous solution where vanadyl sulfate is reduced to [V(H20)6] ", the V=0 bond in the solid remains intact. The oxidation of CO(g) can be observed when it is introduced into this cell. 

Tab.6 Cyclic voltammett7 data for the reduction of 10 M solution of a-K4SiWi2O40 or a-K6P2Wig062 in various solvents. T = 25 °C working electrode glassy carbon disk. Sweep rate 100 mV s (taken from Ref 34)...

Fig. 54. Cyclic voltammogram of H SiW O recorded at conventional size glassy carbon disk electrode, (a) Single-crystal sample (b) 4 mM H4SiW1204o in 0.5 M H2SO4. (From Ref. 269.)...

The results of two studies of silicide films are presented in Tables IV and V. For the results shown in Table IV, molybdenum silicides were deposited on vitreous carbon disks. One set was stripped with base (2 N NaOH), and the other set was stripped with acid (HF-HN03-H20, 1 1 10). The study of chromium silicides (Table V) gave similar results. 

The preparation of carbon disk-shape UMEs is similar to that of the carbon fibre UME. The only difference is that the active part is sealed on epoxy resin. With this purpose, a micropipette tip (1 mL) is glued to the head-tip of a carbon fibre UME. The carbon fibre is maintained in vertical position with a metal hook and the micropipette tip (1 mL) is filled with epoxy resin. Once the resin is cured, the tip is cross-sectioned with a microtome or a blade. Then, the disk carbon UME is ready. 

The pretreatment of the carbon disk UMEs (like a greater part of plane electrodes) is commonly based on mechanical procedures, i.e. polishing [118]. Initially, carbon disk UMEs are grounded down on emery paper (600 grit). Then, the electrode surface is polished with successively finer grades of alumina slurries (0.1- and 0.05-gm diameter particles) on respective cloths. Usually, that is enough to obtain a clean and adequate electroactive electrode surface. 

The well-known equation i = 4nFDCr (i limiting current, n number of electrons implied in the electrochemical process, F Faraday constant, D diffusion coefficient, C electroactive specie concentration and r radius of the disk) describes the theoretical steady-state limiting currents of the disk UMEs. This equation is useful to determine the effective radius of a disk UME and to estimate diffusion coefficients. In this sense, the above-mentioned polished carbon disk UMEs have been characterised through the limiting currents obtained in solution with known parameters, i.e. ferrocyanide aqueous solutions (0.05 M and 2M KC1) [118]. The experimental limiting currents were fairly accurately described by this equation ( + 10%). When the effective radius is determined, this equation can be employed to obtain unknown diffusion coefficients. In this way, we have estimated the diffusion coefficients for /i-carotene in several aprotic solvents with different electrolytic concentrations [123]. 

Figure 23.9 Trace analysis of a metal (M) analyte in the presence of surfactants (S) using the vertically active and horizontally passive aligned nanowires. Such adaptive operation leads to opening and closing of the surface to allow measurement and protection of the transducer between measurements. Also shown are the optical images (top view) of the glassy-carbon disk electrode covered with the vertically (left) and horizontally (right) aligned nanowires.47 (Reprinted with permission from R. Laocharoensuk et al.,...

In a rotated ring-disk electrode experiment where the disk is controlled at +2.6 V versus SCE for the oxidation of HOOH and the ring electrode is controlled at -1.4 V versus SCE for the reduction of the oxidation products from HOOH, the observed collection efficiency (N = /r//d) is 0.384. This is slightly less than the theoretical value of 0.418 for the electrode. The products from HOOH oxidation at the glassy-carbon disk electrode (ED, + 2.6 V vs. SCE) can be characterized at the ring by scanning its potential from +1.0 to... 

Example 2.1 The reversible oxidation of dopamine (DA) is a 2e process. A cyclic voltammetric anodic peak current of 2.2 pA is observed for a 0.4-mM solution of dopamine in phosphate buffer at a glassy carbon disk electrode of 2.6 mm2 with a scan rate of 25mV/s. What will ip be for v = lOOmV/s and 1.2mM DA ... 

Differential pulse voltammetry at rotated glassy carbon disk cathodes proved successful for the accurate determination of inor-... 

Figure 3. Differential pulse voltammogram of a mixture of dibenzothiophene and benzothiophene in acetonitrile. Supporting electrolyte 0.1 M tetraethylammonium perchlorate. Indicator electrode glassy carbon disk, rotated at 1800 rpm. Linear potential ramp, 0.002 volt/s. Pulse amplitude, AE = 0.025 V. Pulse duration, 57 ms. Current sampling time, 17 ms.

Fig. 7. Cyclic voltammograms of 1.0 mM [(MeCp)Mn(CO)2NO]+ in CH2C12 at the indicated temperatures with P(OEt)3 present at a concentration of (A) none and (B)-(D) 10 mM. The scan rate was 0.5 V/s, and the working electrode was a 1-mm-diameter glassy carbon disk.

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