Platinum electrodes aqueous solution

Fig. 5.61. Time-resolved SEIRAS. Pump pulse 532 nm, 35 ps duration, 3 mJ cm , temporal profile indicated by dotted line position and infrared absorption intensity of CO stretch mode plotted as a function of time, platinum electrode, aqueous solution of 0.1 M HCIO4 (based on data in [310])...

On platinum or gold electrodes, aqueous solutions of disulphides are not reduced and only the oxidation of thiols could be studied. 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

It is stated that in time the acidity (up to 2,5 units) of 0,1-1,0 M HMTA aqueous solutions changes maximally at 1°C, in comparatively to other temperatures (11, 16, 21°C). When the temperature arises the change of HMTA aqueous solutions pH values decreases in time. Formaldehyde and ammonium ions (end products of HMTA hydrolysis) have been fixed only in more diluted solutions (0,10 and 0,25M). The concentration of NH in them in some times is higher than H2C=0 concentration that is caused by oxidation of the last one to a formic acid, being accompanied by the change of the system platinum electrode potential. It is stated that concentration NH in solutions does not exceed 5% from HMTA general content. The conclusion the mechanism of HMTA destruction in H,0 to depend essentially on its concentration and temperature has been made. 

Electrolysis of acidified water using platinum electrodes is a convenient source of hydrogen (and oxygen) and, on a larger scale, very pure hydrogen (>99.95%) can be obtained from the electrolysis of warm aqueous solutions of barium hydroxide between nickel electrodes. The method is expensive but becomes economical... 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

General discussion. The limiting reactions in aqueous solution at platinum electrodes are ... 

The polarographic determination of metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

When the electrode is placed in an aqueous solution of glucose which has been suitably diluted with a phosphate buffer solution (pH 7.3), solution passes through the outer membrane into the enzyme where hydroxen peroxide is produced. Hydrogen peroxide can diffuse through the inner membrane which, however, is impermeable to other components of the solution. The electrode vessel contains phosphate buffer, a platinum wire and a silver wire which act as electrodes. A potential of 0.7 volts is applied to the electrodes (the apparatus shown in Fig. 16.17 is suitable) with the platinum wire as anode. At this electrode the reaction H202->02 + 2H+ +2e takes place, and the oxygen produced is reduced at the silver cathode ... 

Efficiencies Related to Composite Films Electrogenerated from 1 M Pyrrole plus 10 2 M Sodium Polyacrylate Aqueous Solution by Polarization of a 1-cm2 Platinum Electrode at 800 mV for Different Polymerization Times... 

Finally, the electrode potential may affect the overall process by determining the state of oxidation of the electrode surface. It is well known that m aqueous solution a platinum electrode has a bare surface only over the narrow potential range from approximately -t-0-4 V to -tO-8 V versus N.H.E. at more cathodic potentials it is covered by adsorbed hydrogen atoms while at more anodic potentials it is covered by... 

In contrast with these active electrodes, a passive electrode conducts electrons to and from the external circuit but does not participate chemically in the half-reactions. Figure 19-8 shows a redox setup that contains passive electrodes. One compartment contains an aqueous solution of iron(III) chloride in contact with a platinum electrode. Electron transfer at this electrode reduces Fe " (a q) to Fe " ((2 q) ... 

When an electrolytic cell is designed, care must be taken in the selection of the cell components. For example, consider what happens when an aqueous solution of sodium chloride is electrolyzed using platinum electrodes. Platinum is used for passive electrodes, because this metal is resistant to oxidation and does not participate in the redox chemistry of the cell. There are three major species in the solution H2 O, Na, and Cl. Chloride ions... 

The thermodynamic standard potential of the methanol electrode has a value of + 0.02 V (RHE) that is, it is quite close to the hydrogen electrode potential. The steady-state potential of a platinum electrode in aqueous methanol solutions is about + 0.3 V (RHE). 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

Takasu Y, Eujii Y, Yasuda K, Iwanaga Y, Matsuda Y. 1989. Electrocatalytic properties of ultra-fine platinum particles for hydrogen electrode reaction in an aqueous solution of sulfuric acid. Electrochim Acta 34 453-458. 

In this cell, the following independent phases must be considered platinum, silver, gaseous hydrogen, solid silver chloride electrolyte, and an aqueous solution of hydrogen chloride. In order to be able to determine the EMF of the cell, the leads must be made of the same material and thus, to simplify matters, a platinum lead must be connected to the silver electrode. It will be seen in the conclusion to this section that the electromotive force of a cell does not depend on the material from which the leads are made, so that the whole derivation could be carried out with different, e.g. copper, leads. In addition to Cl- and H30+ ions (further written as H+), the solution also contains Ag+ ions in a small concentration corresponding to a saturated solution of silver chloride in hydrochloric acid. Thus, the following scheme of the phases can be written (the parentheses enclose the species present in the given phase) ... 

Mamian M, Torres W, Larmat EE (2009) Electrochemical degradation of atrazine in aqueous solution at a platinum electrode. Portugaliae Electrochim Act 27(3) 371-379... 

Figure 30. In situ measurements of the time evolution of the Cu K-edge when a platinum electrode coated with a polymeric film of poly (methylthiophene) is cathodically polarized in an aqueous solution containing 50 mM CuCl2. (From Ref. 105, with permission.)... 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

Fortier [6] found that AQ polymer from Eastman was not deleterious for the activity of a variety of enzymes such as L-amino acid oxidase, choline oxidase, galactose oxidase, and GOD. Following mixing of the enzyme with the AQ polymer, the mixture was cast and dried onto the surface of a platinum electrode. The film was then coated with a thin layer of Nafion to avoid dissolution of the AQ polymer film in the aqueous solution when the electrode was used as a biosensor. These easy-to-make amperometric biosensors, which were based on the amperometric detection of H202, showed high catalytic activity. 

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

---