Electrical conductors can be classified into two types: (1) electronic conductors and (2) electrolytic conductors. Solid and molten metals, semiconductors, and some salts are examples of electronic conductors. Conduction takes place in electronic conductors by direct migration of electrons through the conductor under the influence of an applied potential. Here the atoms or ions that compose the conductor remain stationary (except for vibrations about their equilibirum positions) and these conductors carry a current as electrons pass through the orbitals of the atoms or ions. (See pages 449-501 in your text.)
Electrolytic conduction is observed in solutions of strong and weak electrolytes, in molten salts, and in some ionic solids. Conduction occurs in electrolytic conductors as both positive ions and negative ions migrate toward electrodes. In contrast to electronic conduction, electrolytic conduction involves a transport of ions from one part of the conductor to another. Further, the flow of current in an electrolytic conductor is accompanied by chemical changes at the electrodes. (Examples of such reactions can be found on pages 743 -747 of your text.) Electrolytic conduction plays an important role in the function of electrochemical cells, batteries, electrolysis, and electroplating.
We know that solutions can be nonelectrolytes (nonconductors), weak electrolytes (poor conductors), or strong electrolytes (good conductors). In this experiment we will determine the conductivity of such solutions in a more quantitative manner by measuring their conductance. We will examine how the conductivity of various solutions changes as the concentration and the identity of ions change. We will also see how conductivity can be used to follow chemical reactions.
The conductance of an electrolytic or electronic conductor is the reciprocal of its resistance in ohms. At one time the unit of conductance was called a mho (the reverse of ohm). This unit has been renamed siemens after W. Siemens, a noted German physical scientist who did extensive research into the behavior of electricity. Electrolyte solutions have conductances that are lower than those of metals. Their conductances are often reported in units of microsiemens (1 S = 106 S).
We will measure the conductance of several electrolytic conductors using a conductivity probe similar to that shown in the figure to the right. When the probe is in a solution with ions, an electrical circuit is completed across the electrodes which are on either side of the hole in the probe. A potential difference is applied to the two electrodes and a current results which is proportional to the conductance of the solution. This conductance is converted to a voltage that is read by an interface and displayed by the LabView program on a laboratory computer.
Our conductivity probe uses an alternating current in order to prevent the complete ion migration to the electrodes (a situation called polarization) and consumption of the solute by reactions at the electrodes. With each half-cycle of the alternating current the polarity (sign) of the electrodes is reversed. This reverses the direction of the ion flow and reverses any chemical reaction that may have occurred at the electrode in the previous half-cycle. Thus the solutions under study retain their identity and the electrodes are not contaminated by oxidation-reduction reactions occurring on their surface.
Calibrating the Conductivity Probe
The computer must be interfaced with a conductivity probe. Make sure that your multimeter and computer are on and that the power supply for the probe is plugged in. Click on the conductivity titration icon on your computer. The multimeter should now say "remote" indicating a correct probe-computer connection. You are ready to calibrate the probe.
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