Making an Electrochemical Cell

It will be helpful at this point to explain in detail how to make an electrochemical cell and what processes are occurring there. We will start with a classical copper-zinc cell. First, we find a copper block Cu(s) and immerse it into a solution of copper sulfate, Cu (aq) S04 (aq), Fig. 9.1. Next, we connect the copper block with a source of electrons. Fig. 9.2. When two electrons, 2e , are supplied to the copper block... 

Half cell Usually a pure metal in a solution of (fixed) concentration. The half reaction of the metal ions dissolving and reprecipitating creates a steady potential when linked to another half cell. Two half cells make an electrochemical cell that can be a model for corrosion or... 

Goller G, Salonia J (1981) Dry floe method for making an electrochemical cell electrode. US Patent 4,287,232, 1 Sept 1981... 

Recall that oxidation cannot occur separately from reduction. Both must occur in an electrochemical reaction. The two half-cells taken together make an electrochemical cell. In the Zn/Cu electrochemical cell, the electrons move from the Zn electrode through the wire and down the Cu electrode to the Cu2+ ions at the electrode-solution interface. The Cu + ions are reduced to solid Cu, and the resulting Cu atoms attach themselves to the surface of the Cu electrode. For this reaction, a charge is carried through the barrier by a combination of Zn +(aq ) ions moving from the anode to the cathode and the SOl ( aq) ions moving from the cathode to the anode. 

FIGURE 4.1 Standard hydrogen electrode (SHE) (1) acidic solution with the hypothetical unit activity of H+(aq), (2) platinized platinum electrode, (3) hydrogen blow, (4) hydroseal for the prevention of air interference, (5) a salt bridge through which another electrode can be attached to make an electrochemical cell. 

We can make an electrochemical cell from the two half-cells ... 

Anodic Oxidation. The abiUty of tantalum to support a stable, insulating anodic oxide film accounts for the majority of tantalum powder usage (see Thin films). The film is produced or formed by making the metal, usually as a sintered porous pellet, the anode in an electrochemical cell. The electrolyte is most often a dilute aqueous solution of phosphoric acid, although high voltage appHcations often require substitution of some of the water with more aprotic solvents like ethylene glycol or Carbowax (49). The electrolyte temperature is between 60 and 90°C. 

The overall rate of an electrochemical reaction is measured by the current flow through the cell. In order to make valid comparisons between different electrode systems, this current is expressed as cunent density,/, the current per unit area of electrode surface. Tire current density that can be achieved in an electrochemical cell is dependent on many factors. The rate constant of the initial electron transfer step depends on the working electrode potential, Tlie concentration of the substrate maintained at the electrode surface depends on the diffusion coefficient, which is temperature dependent, and the thickness of the diffusion layer, which depends on the stirring rate. Under experimental conditions, current density is dependent on substrate concentration, stirring rate, temperature and electrode potential. 

The two half reactions of any redox reaction together make up an electrochemical cell. This cell has a standard potential difference, E , which is the voltage of the reaction at 25 °C when all substances involved are at unit activity. E refers to the potential difference when the substances are not in the standard state. E for a particular reaction can be found by subtracting one half cell reaction from the other and also subtracting the corresponding voltages. For example for reduction of Fe to Fe by H2, E° = 0.77 - 0 = 0.77 V. A further example is the oxidation of Fe " by solid Mn02 in acid solution. The half cell reactions are. 

Much of the following text—and the results of the research to which it corresponds—deals with a single electrode ( a uni-electrode system )- One can only imagine uni-electrode systems. It is true that anyone who has a beaker of solution can place in it a single electrode, aplatinum wire, say, and connect it to a source of electrical power outside the beaker (Fig. 7.3). But one cannot operate with it, pass electrons in and out of it, say, unless there is a second electrode (Fig. 7.1). Thus, to make up an electrochemical cell, one has to have two electrodes, and these can then act in three ways as devices (see next section). 

Immersing an electrochemical cell in a water bath creates problems. The procedure makes electrical isolation of the connections in the cell more difficult. Electrical leaks ate then encouraged and may cause errors of measurement. In any case, there are few measurements in which the added sensitivity to temperature control that a water thermostat implies is necessary. 

Complex equilibria can be studied by making them part of an electrochemical cell. If we measure the voltage and know the concentrations (activities) of all but one of the reactants and products, the Nemst equation allows us to compute the concentration of the unknown species. The electrochemical cell serves as a probe for that species. 

An electrochemical cell reaction, like any oxidation-reduction reaction, can be written as the sum of an oxidation half-reaction and a reduction half-reaction. In the case of a cell, these half-reactions correspond to the reactions at the two electrodes. Since the cell reaction is the sum of the half-cell reactions, it is convenient to think of dividing the cell potential into half-cell potentials. Unfortunately, there is no way of measuring a half-cell potential—we always need two half-cells to make a cell, the potential of which is measurable. By convention, the half-cell reaction,... 

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