Platinizing electrodes solution

In the choice of an analytical technique for following the course of a reaction, it is important to recognize that no aspect of the measurement should affect the kinetic processes occurring in the system. For example, a solution conductivity method that uses platinized electrodes should not be used in the study of a reaction that is catalyzed by platinum black. 

In Fig. 4, voltammograms of platinized electrodes prepared under various conditions are shown. For the sake of comparison, the corresponding voltammograms in HCIO4 and H2SO4 solutions are presented [226]. 

Before platinization, electrodes should be cleaned by brief immersion in a solution prepared by combining 3 volumes of 12 M hydrochloric acid with 1 volume of 16 M nitric acid and 4 volumes of water (50% aqua regia). This mixture also can be employed to strip the platinum black from used electrodes. The electrodes are then washed in 16 M nitric acid, rinsed in water, cathodized... 

Every oxidation must be accompanied by a reduction (i.e., the electrons must have somewhere to go). So it is impossible to determine experimentally the potential of any single electrode. We therefore establish an arbitrary standard. The conventional reference electrode is the standard hydrogen electrode (SHE). This electrode contains a piece of metal electrolytically coated with a grainy black surface of inert platinum metal, immersed in a 1 MH+ solution. Hydrogen, H2, is bubbled at 1 atm pressure through a glass envelope over the platinized electrode (Figure 21-8). 

In most prescriptions for preparation of platinized electrodes, it is an almost general rule that, after the platinization step and washing, the electrode should be submitted to alternate anodic and cathodic polarization, for instance in H2SO4 solution in... 

A shiny Pt foil was platinized by passing a 30 mA current for 10 min through a 7 X 10 M chloroplatinic acid solution containing 6 X 10 M lead acetate. A layer of polycrystalline chlorophyll, containing 1.5 X 10 Chi a molecules, was deposited on the platinized electrode surface, using the procedure described by Tang and Albrecht (6). The Chi a-plated electrode then was platinized again in the same chloroplatinic acid solution, except that the 30 mA current was passed for only 15 seconds. 

The surface area of the electrodes and their arrangement in the conductometric cell influence the electric resistance via the quotient l/A, the so-called cell constant. However, these geometric quantities are often difficult to investigate, especially in the case of platinated electrodes. Therefore, the cell constant is determined by using a calibrating solution of known a value (usually a solution of potassium chloride). These days, commercial equipment for measuring conductivity (conductometer) is... 

Oxidation at the anode results in the formation of blue to black oxidized materials (thickness 1 mm — 0.61 pm) of high conductivity. The films show a yellow colour in the uncharged low conductivity state upon undoping. The mechanical properties of 1 mm thick polypyrrole films (obtained on platinized electrodes in propylene carbonate partly in the presence of water) were reported Polypyrrole (PP) was obtained in two different morphologies Electrochemical polymerization in an aqueous solution of results in a compact structure of aggregated spheres... 

The Convenient Hydrogen Electrode (21) (CHE) or the Self-Contained Hydrogen Electrode (22) (SCHE) is an RE with a hydrogen bubble contained within the electrode body that replaces the hydrogen gas supply required for typical hydrogen REs. The thin layer of solution drawn by capillary action onto the platinized electrode surface quickly... 

Wieckowski A. Kinetic isotope effects between light and heavy water in HCOOH and CH30F1 adsorption and oxidation on Pt. J Electroanal Cbem 1977 78 229-41. Wieckowski A, Sobkowski J. Comparative study of adsorption and oxidation of formic acid and methanol on platinized electrodes in acidic solution. J Electroanal Cbem 1975 63 365-77. 

Chemical-deposition Electrode. Chemical techniques can also be employed in the fabrication of catalyst layers by platinizing the surface of a Nafion membrane. One approach [65, 66] is to expose one side of the Nafion membrane to a reductant solution (e.g., hydrazine solution) and the other side to a platinic acid solution. The reductant diffuses across the membrane to react with platinic acid and form a Pt catalyst layer. In another impregnation-reduction method [3, 67, 68] a cationic salt, such as Pt(NH3)4Cl2, is first impregnated into the Nafion membrane, followed by exposing this membrane to a reductant, such as NaBUj. As shown in Figure 19.13, a dense and porous platinum film can be formed using this chemical deposition technique. 

The preparation of platinized electrodes consists of depositing colloidal platinum (platinum black) upon the working surfaces. This is accomplished by the use of conventional electroplating techniques. The electrode to be platinized is connected as the cathode, and a piece of pure platinum sheet forms the anode of an electrolytic cell. Kohlrausch and Holborn formula platinizing solution is employed as the electrolyte. Its chemical composition is platinum chloride (H2PtCl2) 3 % dissolved in 0.025 % lead acetate solution. This platinizing solution may be obtained in 2-oz bottles from the Hartman-Leddon Co. in Philadelphia or from the Arthur H. Thomas Co., also of Philadelphia. 

Various configurations have been developed for hydrogen electrodes and these are described in the literature (Ives and Janz, 1961). Platinum, gold, and palladium as well as other metals have been proposed for the metallic element of the electrode. Frequently platinized platinum is the preferred substance. The metal substrate may be wire, mesh, or foil. Preparation of the platinized electrode proceeds somewhat differently from the method outlined in Chapter 2. The platinum surface is usually cleaned in a solution of the following composition (Bates, 1954) ... 

Daniel -Bek and Glazatova [174] investigated the oxidation of formic acid on porous carbon platinized electrodes at low positive potentials (up to 200 mV) in 1 M sulfuric acid solutions, by... 

Sometimes the term normal hydrogen electrode (and respectively normal potential instead of standard potential) has been used referring to a hydrogen electrode with a platinized platinum electrode immersed in 1 M sulfuric acid irrespectively of the actual proton activity in this solution. With the latter electrode poorly defined diffusion (liquid junction) potentials will be caused, thus data obtained with this electrode are not included. The term normal hydrogen electrode should not be used either, because it implies a reference to the concentration unit normal which is not to be used anymore, see also below. 

FIGURE 10.9 Galvanostatic charging curve for a platinized platinum electrode in 0.1 M H2SO4 solution (1) anodic scan, (2) cathodic scan. 

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

Podlovchenko Bl, Petrii OA, Frumkin AN, Lai H. 1966. The behaviour of a platinized-platinum electrode in solutions of alcohols containing more than one carbon atom, aldehydes and formic acid. J Electroanal Chem 11 12-25. 

Christensen PA, Hamnett A, Weeks S A. 1988. In-situ FTIR study of adsorption and oxidation of methanol on platinum and platinized glassy carbon electrodes in sulphuric acid solution. J Electroanal Chem 250 127-142. 

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