Cell, electrochemical copper/silver

Lead in water may he analyzed very precisely at low concentrations hy anodic stripping voltametry using an electrochemical analyzer static or controlled growth mercury drop electrodes, reference calomel or silver-silver chloride electrodes and silica or TEE cells. Copper, silver, gold, and certain organic compounds may interfere in the test. (APHA, AWWA and WEE. 1998. Standard Methods for the Examination of Water and Wastewater, 20 ed. Washington, D.C. American Public Health Association.)... 

Figure 8.11. Electrochemical cell for electroplating silver onto copper.

Because silver, gold and copper electrodes are easily activated for SERS by roughening by use of reduction-oxidation cycles, SERS has been widely applied in electrochemistry to monitor the adsorption, orientation, and reactions of molecules at those electrodes in-situ. Special cells for SERS spectroelectrochemistry have been manufactured from chemically resistant materials and with a working electrode accessible to the laser radiation. The versatility of such a cell has been demonstrated in electrochemical reactions of corrosive, moisture-sensitive materials such as oxyhalide electrolytes [4.299]. 

Let s begin our investigation of an electrochemical cell by assembling one. Fill a beaker with a dilute solution of silver nitrate (about 0.1 M will do) and another beaker with dilute copper sulfate. Put a silver rod in the AgN03 solution and a copper rod in the CuSO< solution. With a wire, connect the silver rod to one terminal of an... 

The moles of silver deposited per mole of copper dissolved are the same whether reaction (J) is carried out in an electrochemical cell or in a single beaker, as in Experiment 7. If, in the cell, electrons are transferred from copper metal (forming Cu+2) to silver ion (forming metallic silver), then electrons must have been transferred from copper metal to silver ion in Experiment 7. 

Now let s take a more detailed look into the electrochemical cell. Figure 12-5 shows a cross-section of a cell that uses the same chemical reaction as that depicted in Figure 12-1. The only difference is that the two solutions are connected differently. In Figure 12-1 a tube containing a solution of an electrolyte (such as KNOa) provides a conducting path. In Figure 12-5 the silver nitrate is placed in a porous porcelain cup. Since the silver nitrate and copper sulfate solutions can seep through the porous cup, they provide their own connection to each other. 

Electroplating is achieved by passing an electric current through a solution containing dissolved metal ions as well as the metal object to be plated. The metal object acts as a cathode in an electrochemical cell, attracting metal ions from the solution. Ferrous and nonferrous metal objects are typically electroplated with aluminum, brass, bronze, cadmium, chromium, copper, iron, lead, nickel, tin, and zinc, as well as precious metals such as gold, platinum, and silver. Common electroplating bath solutions are listed in Table 7-1. 

In most electrochemical measurements solutions are made up to an arbitrary volume that usually is at least 1 cm3. However a few microcells have been described for work with solution volumes that are well below 1 cm3. The coulometric determination of silver ion in cell volumes as small as 20 /iL (formed by a thin copper sheet and a cavity of beeswax) has been discussed.62... 

Many oxidation/reduction reactions can be carried out in either of two ways that are physically quite different. In one, the reaction is performed by bringing the oxidant and the reductant into direct contact in a suitable container. In the second, the reaction is carried out in an electrochemical cell in which the reactants do not come in direct contact with one another. A marvelous example of direct contact is the famous silver tree experiment, in which a piece of copper is immersed in a silver nitrate. solution (Figure 18-1). Silver ions migrate to the metal and are reduced ... 

An electrochemical cell consists of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells that will be of interest to us, the solutions surrounding the two electrodes are different and must be separated to avoid direct reaction between the reactants. The most common way of avoiding mixing is to insert a salt bridge, such as that shown in Figure 18-2, between tire solutions. Conduction of electricity from one electrolyte solution to the other then occurs by migration of potassium ions m the bridge in one direction and chloride ions in the other. However, direct contact between copper metal and silver ions is prevented. 

By now, you may be thinking that spontaneous electrochemical processes are always beneficial, but consider the problem of corrosion, the natural redox process that oxidizes metals to their oxides and sulfides. In chemical teims, coiTOsion is the reverse of isolating a metal from its oxide or sulfide ore in electrochemical terms, the process shares many similarities with the operation of a voltaic cell. Damage from corrosion to cars, ships, buildings, and bridges runs into tens of billions of dollars annually, so it is a major problem in much of the world. We focus here on the corrosion of iron, but many other metals, such as copper and silver, also conode. 

An electrochemical cell consists of a silver metal electrode immersed in a solution with [Ag+] = 1.0 Mseparated by a porous disk from a copper metal electrode. If the copper electrode is placed in a solution of 5.0 M NH3 that is also 0.010 M in Cu(NH3)4, what is the cell potential at 25°C ... 

The unstable behavior of some solder-replacement adhesives has been attributed to galvanic corrosion. Similar to most corrosion mechanisms, condensed or absorbed moisture on the surface and dissimilar metals are required to form a galvanic cell. The silver filler acts as a cathode while the substrate metallization acts as an anode and is oxidized. In the case of tin-lead solder surfaces, the solder, which has a lower electrochemical potential (0.13 V) than silver (0.79 V), becomes the anode at which corrosion and oxidation occur. A smaller potential difference between a copper surface and silver accounts for some improvement in contact resistance over the solder-silver couple. 

These few examples of the application of solid state galvanic cells in the field of solid state reactions can only present a very limited view of this important area of solid state science. The examples were chosen primarily in order to demonstrate the principles according to which solid state research in thermodynamics and kinetics should be conducted with the use of electrochemical tools and methods. Such measurements are only possible because of the existence of suitable solid electrolytes. The most important of these are Zr02(-f CaO) and Th02(+Y2 03) for oxygen, silver halides and Ag4Rbl5 for silver, copper halides for copper, some glasses in which certain ions are dissolved, and p — Al2 03(-hNaO) for sodium. 

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