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Primary and Secondary Cells
When two different metals are placed in contact with one another in air, one metal becomes positive and the other negative. This charge is very feeble.
The Voltaic Cell
Volta, a professor at the University of Pavia, experimented with this phenomenon. He took discs of zinc and copper, placed them in a pile alternately, and separated them with felt discs saturated with vinegar or other dilute acid. Such a stack is capable of giving a shock and will continue to do so as long as the felt discs are kept moist.
A small voltaic cell, which is named in honor of Volta, is shown in Figure 10-1. The zinc element or electrode, when immersed in the acid, starts to dissolve, leaving its electrons behind and the zinc goes into solution as Zn++ ions. This action stops very shortly, as the zinc plate becomes negative and ceases to throw off positive ions. The amount of zinc thus dissolved is very minute.
Figure 10-1 Elementary voltaic cell.
If the negative zinc plate is connected externally by a wire to the copper plate, an electrical current will flow from the negative zinc plate through the external wire to the copper plate (positive). As the
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negative charge of the zinc plate is thus removed, more Zn++ ions go into the solution; thus more electrons go on the zinc plate and the energy of the dissolution of the zinc into the acid is converted into electrical energy.
What about the copper plate? The same action occurs with the copper plate. Cu++ ions are given into the solution, leaving the copper plate negative also.
These two negative charges from the zinc and copper are unequal in quantity, as illustrated in Figure 10-2. The copper plate has a potential of 0.81 V and the zinc plate a potential of 1.86 V, both negative. Now 1.86 — 0.81 = 1.05 V in favor of the zinc plate. So when the circuit is closed, as in Figure 10-1, the current flows from the zinc to the copper, through the resistor.
Actually, one may say the difference of potential between the plates is a measure of the difference of their tendencies to oxidize.
Figure 10-2 Difference in potential of the plates.
You have noticed that this apparatus was called a voltaic cell. One such unit is a cell. If more than one is connected in series or parallel, the combination becomes a battery. See Figure 10-3.
(A) Cells in series,
■* “+T" 4
(B) Cells in parallel. Figure 10-3 A battery.
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In a cell there must be two metals that differ in that one oxidizes more readily than the other. Note that in Figure 10-1 the zinc is called the positive element and the copper is called the negative element. Now refer to Table 9-1, “Galvanic Series of Metals,” and you will find that zinc is anodic compared to copper, which is more cathodic than zinc.
Impure zinc will dissolve in sulfuric acid, while pure zinc won’t. In the voltaic cell in Figure 10-1, the zinc will dissolve when the external circuit is closed. The chemical formula is
The hydrogen is given up at the copper plate. The zinc is dissolved in proportion to the current flowing from the cell. This is the primary cell and is considered to be unrechargeable. It is replenished by replacing the zinc plate and the electrolyte.
Commercial zinc usually has impurities in it, such as iron, arsenic, or some other metal. These particles are small, but refer to Figure 10-4 for the local battery action that transpires, causing the zinc to waste away. The impurity, zinc, and acid form a small cell that is shorted, so current flows. This local action may be stopped by amalgamating the zinc by rubbing it with mercury. The amalgamation loosens the impurities and they float to the surface. Mercury is sometimes added to the zinc while it is in a molten mass.
H2SO4 2H = + SO4=; then Zn++ + SO4 s ZnSO4
Figure 10-4 Local battery action.
Some of the liberated hydrogen clings to the positive plate and thus effectively reduces the effective surface of this plate. This
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reduction reduces the effective emf from the cell. This action is known as polarization.
Depolarizers are added to the cell to stop polarization. Depolarizers may be liquids or solids. Some liquid depolarizers are
Nitric acid Chromic acid Bichromate of potash Bichromate of soda Nitrate of potash
Some solid depolarizers are
Black oxide of manganese Oxide of copper Peroxide of lead Oxide of lead
All depolarizers abound in oxygen. The current releases this oxygen, which combines with the hydrogen that caused the polarization.
A few primary cells will be discussed in brief, so that you may gain further knowledge of them. As stated before, the primary cell is replenished by replacing the negative plate, if it has been depleted, and replacing the electrolyte.
A secondary cell is one that is rechargeable, without replacing the plate or electrolyte. Of course, there is an end to this, as we have seen when we have had to replace an automobile battery, which is a rechargeable secondary-cell combination.