Zinc oxide/platinum electrodes

The electrochemical oxidation of tyramine in NaOH/MeOH media gives films of polytyramine (25). The film, on a platinum electrode, can complex copper(II) ions from aqueous media and cobalt(II), iron(II), manganese(II) and zinc(II) from organic media. X-ray photoelectron spectroscopy established that coordination of the metal ions had occurred. For cobalt, evidence of coordination to both ether and amine functions is obtained, but for the other metal ions evidence of ether coordination is less definitive. 

The measured voltage of the Daniel cell is 1.10 V. This is the overall voltage of the cell consisting of two half-reactions, namely the oxidation of zinc and the reduction of cupric ion. The individual potentials of the half-reactions cannot be measured. The potentials of the half-reactions can be obtained relative to a standard. The standard is a hydrogen electrode. This consists of a platinum electrode immersed in 1M HC1 with hydrogen gas bubbling through at 1 atmosphere pressure. The reaction is ... 

Determining Ehaif-ceii The Standard Hydrogen Electrode What portion of ceii for the zinc-copper reaction is contributed by the anode half-cell (oxidation of Zn) and what portion by the cathode half-cell (reduction of Cu ) That is, how can we know half-cell potentials if we can only measure the potential of the complete cell Half-cell potentials, such as Ezine and °opper. are not absolute quantities, but rather are values relative to that of a standard. This standard reference halfcell has its standard electrode potential defined as zero (E fereiice — 0.00 V). The standard reference half-cell is a standard hydrogen electrode, which consists of a specially prepared platinum electrode immersed in a 1 M aqueous solution of a strong acid, H (fl ) [or H30 (a )], through which H2 gas at 1 atm is bubbled. Thus, the reference half-reaction is... 

Practice Exercise 2 (a) The first reaction occurs at the anode and the second reaction at the cathode, (b) Zinc is oxidized to form Zn as the reaction proceeds and therefore the zinc electrode loses mass, (c) The platinum electrode is not involved in the reaction and none of the products of the reaction at the cathode are solids, so the mass of platimun electrode does not change, (d) The platimun cathode is positive. 

An electrochemical cell is considered chemically irreversible if reversing the current leads to different electrode reactions and new side products. This is often the case if a solid falls out of solution or a gas is produced, as the sohd or gaseous product may not be available to participate in the reverse reaction. When a solid zinc electrode is oxidized in an acidic system with a platinum electrode the following two reactions take place ... 

This reduction step can be readily observed at a mercury electrode in an aprotic solvent or even in aqueous medium at an electrode covered with a suitable surfactant. However, in the absence of a surface-active substance, nitrobenzene is reduced in aqueous media in a four-electron wave, as the first step (Eq. 5.9.3) is followed by fast electrochemical and chemical reactions yielding phenylhydroxylamine. At even more negative potentials phenylhydroxylamine is further reduced to aniline. The same process occurs at lead and zinc electrodes, where phenylhydroxylamine can even be oxidized to yield nitrobenzene again. At electrodes such as platinum, nickel or iron, where chemisorption bonds can be formed with the products of the... 

The electrochemical reduction of azulene with carbon, platinum, lead or zinc cathode does not give any product, whereas that with magnesium electrode yields a dimeric compound as the only reduction product, though the dimeric compound is easily transformed to the corresponding monomeric compound by a mild oxidation as shown in equation 2825. 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

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