Lead ions, platinum electrode

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

The electrochemical oxidation is often more sensitive to the reaction conditions than to the substituents. Platinum electrodes are recommended for methoxylation and the equivalent acetoxylation procedures.290 In acetonitrile buffered by hydrogen carbonate ion, 3,4-diethylfuran affords the 2,5-dihydroxy-2,5-dihydro derivative (84%) and Jones oxidation readily leads to diethylmaleic anhydride in what is claimed to be the best general method for such conversions.291 In unbuffered methanol and under current density control, the oxidation of 2-methylfuran appears to eliminate the methyl group since the product is the acetal-ester 111 also obtained from methyl 2-furoate.292 If sodium acetate buffer is used, however, the methyl group is retained but oxidized in part to the aldehyde diacetate 112 in a... 

In the presence of Pb(II) ions in sulfuric acid, potential oscillations have been observed for galvanostatic oxidation of hydrogen on platinum electrode [129]. This behavior has been attributed to ad-sorption/oxidation/desorption processes of lead on the platinum surface. Lead at high values of coverage is oxidized to insoluble PbS04, which blocks the Pt surface. 

As the last example we discuss the recent study of Skiilason et al. who examined the hydrogen evolution at a platinum electrode. They studied the model system of Fig. 22 that contains a slab of Pt atoms, representing the Pt(lll) surface. On top of that, two layers of water molecules plus an excess of H atoms are placed. Thereby, protons become solvated in the water, leading to the formation of ions,... 

Lead perchlorate and bismuth nitrate were added to the electrolytic solution at 5 x 10-5 and 10 5 M, respectively, to modify a platinum electrode, which has a real surface area of 40 cm2. The electrolysis cell has two identical compartments separated by an ion-exchange membrane (Nation 423). A Ptgolrio sheet and the Hg/Hg2S04, SOf s i, electrode served as counter and reference electrodes, respectively. 

The electrode reaction is rarely as simple as described above. In many cases the product is either insoluble, or partly adsorbed at the electrode surface. Besides, the reactants of many reactions are also surface active. Furthermore, the electrode reaction can be either preceded or followed by chemical reactions. Hence, the choice of the working electrode also depends on the reaction mechanism. For instance, the reduction of lead ions on a platinum electrode is complicated by nucleation and growth of lead micro-crystals, while on a mercury electrode lead atoms are dissolved in mercury and the reduction is fast and reversible. Similarly, the well-known pigment alizarin red S and the product of its reduction are both strongly adsorbed on the surface of mercury and carbon electrodes [17]. In this case, the liquid mercury electrode is analytically much more useful because the adsorptive accumulation on the fresh electrode surface can be easily repeated by creating a new mercury drop. However, on the solid electrode, the film of irreversibly adsorbed substance is so stable that it can be formed in one solution and then transferred into another electrolyte for the measurement of the kinetics of the electrode reaction. After each experiment... 

Other alkali-metal chlorates are produced by analogous technology while sodium and potassium bromate are produced electrolytically starting both from bromide ion and bromine solutions. The production of bromate is, however, a very small-scale process and the cells have not been optimized to any extent for example while cells with lead dioxide and platinized titanium have been described, some plants still use solid platinum electrodes The mechanism of bromate formation is identical to that described for chlorate by reactions (5.10)—(5.13) the kinetics are, however, different. The hydrolysis of bromine is slower than chlorine but the disproportionation step is much faster (by a factor of 100) and it is therefore advisable to use a more alkaline electrolyte, about pH 11. 

In addition, electrooxidation of cystine and cysteine at platinum and gold electrodes has been described [158-160]. All a-amino acids have been found oxidizable at solid metal electrodes at approximately the same potentials [161, 162]. This oxidation leads to the formation of an imine intermediate, which is further oxidized to nornitril. At a silver electrode slow hydrolysis of this intermediate to noraldehyde also takes place. The electrochemical oxidation reactions of a- and jS-alanine at a platinum electrode in aqueous solutions produce free radicals accompanied by a second reaction involving loss of CO2 [163]. In the electrooxidation of a-alanine, the adsorbed intermediate species is either hydrolyzed anodically to acetaldehyde and ammonia, or is oxidized to a carbonium ion which is subsequently hydrolyzed to acetaldehyde and ammonia in solution, analoguous to the behaviour of glycine [164]. The mechanism for jS-alanine is similar except carbonium ion formation is accompanied by a hybrid transfer to form acetaldehyde. 

Between the electrodes, current is carried partly by the silver ions and partly by the negatively charged nitrate ions. This leads to a situation which is analogous to space charge formation in a thermionic vacuum tube. Since the Ag ions are only partly responsible for the current in the electrolyte, they are not transported away from the anode immediately upon formation. There is, then, an accumulation of silver ions about the anode and a depletion of silver ions at the cathode. These phenomena can be observed in a Hittorf transference apparatus. Similar phenomena occur in mixed systems, for example, platinum electrodes in contact with sodium chloride solution. 

However, a platinum electrode dipping into such a solution exhibits a finite potential value. The dissolution of a pure ferrous salt may, in a first stage, lead to such a weak potential value of the solution that water itself is reduced in a second stage. Therefore, water plays the role of an oxidant, of course. As a result, traces of ferric ions are formed. Both forms of the couple are present and the solution exhibits a finite potential value predicted by Nernst s law, which is that found experimentally. Experience indicates that the solution potential, in this case, is not lower than 0.50 V (E° = 0.77 V). Inversely, when the solution contains only ferric ions, the potential does not tend toward +oo as expected. In this case, water plays the role of a reductor. Traces of Fe + are formed. The potential value the solution takes does not exceed about 1.05 V. 

More recently, EC-AFM [11,66,67] has been applied to study the morphology and proton conductivity of Nafion membranes. Using EC-AFM, the spatially resolved proton current driven by the electrochemical reactions occurring on the two sides of the Nafion membrane can be measured. The experimental setup is shown in Fig. 5.15. A commercial AFM is equipped with an electrochemical cell and a biopotentiostat. The AFM tip is modified with a platinum electrode to act as the cathode catalyst for the fuel cell reaction, whereas the Nafion membrane coated with Pt catalyst on the side opposite the AFM tip serves as the anode of the electrochemical cell. The applied voltage leads to water oxidation and generates protons on the electrode with the Pt catalyst. The current can then be detected when the AFM tip is in contact with an ion channel that is connected with the ionic network in the membrane. Because water is required for the electrochemical reactions, the experiment must be performed... 

In spite of its disadvantages, platinum has been used for nonenzymatic detection of blood glucose. One of the drawbacks of the platinum electrode is its catalytic activity for the electrochemical oxidation of glucose drops which can be seriously affected by the chloride ion present in physiological fluids. On the other hand, amino acids, biochemicals like ascorbic acid, creatinine, epinephrine, and urea in blood can destroy the platinum electrode. In this way, if blood proteins occupy the catalytic sites on the platinum surface, the detection of glucose on platinum will be deteriorated. Due to the fact that glucose oxidation can be inhibited by many biochemicals and amino acids in blood, this can lead to a loss of sensitivity when glucose is detected with platinum. ... 

---