Platinized electrodes

The glass tubes contain mercury and are firmly fixed in the ebonite cover of the cell so that the distance between the electrodes may not change during the experiment. Contact with the platinum electrodes is made by dipping the copper wires of the circuit in the mercury contained in the tubes. The coating of the electrodes with platinum black is carried out in order to inhibit polarization. When examining certain electrolytes, platinized electrodes cannot be used since platinum black may catalyze the decomposition or the oxidation of the electrolyte. The cell is maintained at constant temperature thermostatically, as conductivity increases rather significantly with temperature. 

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. 

Electrochemical biosensors based on detection of hydrogen peroxide at platinized electrodes were found to be more versatile allowing a decrease in detection limit down to 1 i,mol L 1 [109]. However, all biological liquids contain a variety of electrochemically easily oxidizable reductants, e.g. ascorbate, urate, bilirubin, catecholamines, etc., which are oxidized at similar potentials and dramatically affect biosensor selectivity producing parasitic anodic current [110]. 

Fig. 3-32 Cyclic voltammogram for smooth Pt and chemically platinized electrodes for methanol oxidation in 3 M H2SO4 dth 0.1 M CH3OH at lOOmV/s.

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]. 

The specific adsorption of H2P04 ions that accompany the UPD of Zn + ions on Pt (platinized electrodes obtained by Pt electrodeposition on the gold-plated plastic foil) electrodes was studied by using a radiotracer method with P phosphate species in the presence of a great excess of C104 ions [196]. It was found that the specific adsorption of species induced... 

The electric field in the cell is best established by means of reversible electrodes such as Ag-AgCl or Cu-CuS04. Care must be taken to prevent the electrolyte of the electrode from contaminating the dispersion. Platinized electrodes behave reversibly with low currents, but gas evolution causes troubles at higher currents. 

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... 

The behavior of the platinized platinum electrodes is entirely different from that of the Ag/Ag halide electrodes, both under flow conditions and at rest. Whereas the nonflow potential for the nonpolarizable electrodes remained constant with time, the rest potential for two platinized platinum electrodes increased with time. Also, the flow of liquid produced a more pronounced effect on the platinum electrodes than on the Ag/Ag halide electrodes. The erratic behavior of the platinum electrodes appears to be due to polarization effects which are difficult to eliminate. In Figure 3 the effect of flow on both Ag/AgCl and platinized electrodes can be assessed. The effect of flow on the platinum electrodes was known to Helmholtz (19) and still appears to remain unexplained. 

The electrical conductivity of the water samples was measured with the help of a slide wire bridge on D.C. in an electrolytic cell with platinized electrodes. The value of the electrical conductivity of degassed water was approximately equal to that of equilibrium water (the difference is not certain), 2.49 x 10 7 S/cm. 

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). 

A cell similar to that used by MatiaSovsky et al. (1970) was used for conductivity measurements of chlorides and fluorides also by Winterhager and Werner (1956). These authors obtained constant Rs values between 20 and 50 kHz. The distinctly lower limiting frequencies are probably caused by the use of larger platinized electrodes, i.e. electrodes with a much larger effective surface area. This results in greater double-layer capacitance and higher a values, so that Rg = Re at lower frequencies, according to Eq. (8.64). 

The first results on the electrocatalytic reduction of adsorbed CIOJ ions at rhodized and platinized electrodes were reported by Vasina and Petrii. ... 

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. 

Figure 3. Mass spectrometric determination of the gaseous products of the photoelectrolytic process, (a) Gaseous sample collected from illuminated platinized Chi a cell (b) gaseous content over a similarly irradiated, platinized electrode in the absence of Chi a. Molecular hydrogen and oxygen, the two main products of the water splitting reaction, are readily manifested by the lines observed at masses 2 and 32 in (a). Helium gas was used to purge the cell prior to the light reaction. See text for a more detailed...

Alternating currents are preferred to direct current because little or no polarization of platinized electrodes takes place. During electrolysis the platinum black... 

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