Platinum Electrode rotating

Part—III exclusively treats Electrochemical Methods invariably and extensively used in the analysis of pharmaceutical substances in the Official Compendia. Two important methods, namely potentiometric methods (Chapter 16) deal with various types of reference electrodes and indicator electrodes, automatic titrator besides typical examples of nitrazepam, allopurinol and clonidine hydrochloride. Amperometric methods (Chapter 17) comprise of titrations involving dropping-mercury electrode, rotating—platinum electrode and twin-polarized microelectrodes (i.e., dead-stop-end-point method). 

Dropping mercury electrode Standard calomel electrode Rotating platinum electrode Mercury pool electrode Rotating pyrolytic graphite electrode Hanging mercury drop Millimoles... 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Procedure. Place 25.0 mL of the thiosulphate solution in the titration cell. Set the applied voltage to zero with respect to the S.C.E. after connecting the rotating platinum micro-electrode to the polarising unit. Adjust the range of the micro-ammeter. Titrate with the standard 0.005 M iodine solution in the usual manner. 

Dilute solutions of antimony(III) and arsenic(III) (ca 0.0005M) may be titrated with standard 0.002 M potassium bromate in a supporting electrolyte of 1M hydrochloric acid containing 0.05 M potassium bromide. The two electrodes are a rotating platinum micro-electrode and an S.C.E. the former is polarised to +0.2 volt. A reversed L-type of titration graph is obtained. 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

KGS51), hydrogen peroxide (85KGS1042), and a rotating platinum disk electrode (84KGS318). 

Samec Z, Weber J (1972) Reduction of ferric ion on a rotating platinum electrode of the turbulent type in the presence and absence of adsorbed sulfur. J Electroanal Chem Interfacial Electrochem 38 115-126... 

FIGURE 16.3 Polarization curve for the ionization of molecular hydrogen at a rotating platinum electrode (/= 16 s ) in 0.5 MH2SO4 solution. 

The Pd lT) and Pt ll) complexes yilB dtc) HI, 46) are diamagnetic and have a square planar geometry 112,113) like the nickel analogues. In CH2CI2 they can be oxidised irreversibly on a rotating platinum electrode, the Pd compound at a higher potential than the Pt compound (1.21 and 0.92 V vs s.c.e., respectively) 150). 

Voltametric studies (150,162,163) have revealed that the electron transfers Cu(R2chc)2 Cu(R2chc)2 Cu(R2dfc)2 are reversible at a rotating platinum electrode. 

Fig. 10. Half wave potentials (at a rotating platinum electrode) vs. d-electron configuration for Et2dtc complexes. The E1/2 values depend upon solvent and reference electrode used (see text), but this is a minor effect as compared with the influence of the d-electron configuration.

Potentials were measured at room temperature relative to an aqueous calomel electrode saturated with NaCl, using a rotating platinum electrode data for R = H complexes from reference 13, all other data from references 5 and 9. 

An alternative to the DME is the rotating platinum micro-electrode. Steady currents can be obtained by rotating the electrode at a constant... 

Two identical stationary micro-electrodes (usually platinum) across which a potential of 0.01-0.1 V is applied can be used in place of either the DME or the rotating platinum micro-electrode. The equivalence point is marked by a sudden rise in current from zero, a decrease to zero, or a minimum at or near zero (Figures 6.16(a), (b) and (c)). The shape of the curve depends on the reversibility of the redox reactions involved. The two platinum electrodes assume the roles of anode and cathode, and in all cases a current flows in the cell only if there is a significant concentration of both the oxidized and reduced forms of one of the reactants. In general, two types of system can be envisaged ... 

The polarization of the measuring (working) electrode, which is typically a rotating platinum disk embedded in a Teflon sheath, is held constant at some value at which the analyte reduces or oxidizes. The solution is stirred due to the rotation of the electrode. The resulting current is then measured as the titrant is added. The titrant reacts with the analyte, removing it from the solution, thus decreasing its concentration. The measured current therefore also decreases. When all of the analyte has reacted with the titrant, the decrease will stop, signaling the end point. 

Amperometric titrations with a rotating platinum electrode... 

Figure 87 Voltammogram recorded at a rotating platinum electrode in a MeCN solution of [Conl(Me2[14]py-dieneN4)(MeCN)2][Cl04]3. Rotation frequency = 600 rpm (rpm = revolutions per minute)...

Oxidation of amides at a rotating platinum electrode in acetonitrile with tetraethylammonium 4-toluenesulphonate as electrolyte. 

It has recently been shown by Spikes150 that uracil and a number of substituted uracils are subject to dye-sensitized photooxidation under certain conditions. Methylene blue and Eosin Y were active photosensitizers in the pH range 8-11.5, but were inactive below pH 8. FMN was very active over the pH range of 2.4-11.5. Photooxidation was measured with a rotating platinum oxygen electrode. 

Rotating platinum screens have been used for electrogravimetric analyses. Commercial instruments employ two concentric cylindrical platinum screen electrodes with one or both electrodes rotating to increase convection. The cell itself is usually a beaker with a sample volume of about 150 mL. Typically, no cell top is used, and when running multiple analyses, there should be adequate ventilation to prevent accumulation of hydrogen. A simple operational amplifier circuit can be constructed for the instrument. The commercial instruments available are expensive given the simplicity of this type of experiment. 

Even reactions which are thermodynamically unfavoured, e.g. with Kaq as low as 5 x 10-6, may be electrocatalytically mediated. For example, rotating platinum electrodes covered with polymerized [Ru(4-vinyl-4 -methyl-2,2 -bipyridyl)3]2+wiil electrocatalytically mediate the oxidation96 of such species as [Ru(bipy)3]2+, [Ru(bipy)2(4,4 -bipy)2]2+, [Ru(bipy)2(py)(MeCN)]2+, [Ru(bipy)2-(MeCN)J2+ and [Ru(bipy)2(pyrazine)2]2+. 

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