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Solutions Specific Conductance
The electrical conductivity of solutions is reported as the specific conductance.
The fact that aqueous solutions of certain materials, called electrolytes, conduct an electric current provides the most direct evidence for the idea that ions capable of independent motion are present. More detailed studies of the electrical conductivity of such solutions provide information on the number and independence of these ions.
Measurements of the conductivity of aqueous solutions are made with a conductivity cell and an electric current like that shown in the fig. when an alternating current is used to prevent build up of charges of opposite sign near the two electrode surfaces, so that there is little electric resistance at the metal solution interface, the conductivity cell obeys Ohm’s law: the current flowing through the cell is proportional to the voltage across the cell. It is therefore possible to assign a resistance of so many ohms to such a cell, just as one assigns a resistance to a metallic conductor.
It is more convenient to focus on the conductance of an electrolyte solution rather than on its resistance. These quantities are reciprocally related and the conductance L is calculated from the measured resistance as
L = 1/R
Specific conductance of KCl solutions;
If R is the resistance in ohms, symbol Ω, and then L has the units of Ω-1. As for metallic conductors, the resistance and therefore the conductance depend on the cross section area A and the length l of the region between the electrodes. As for a metallic conductor,
R = p 1/A
Where p is the specific resistance and is the proportionality factor that corresponds to the resistance of a cell of unit cross section area and unit length. One can write:
L = k A/l
Where k, the specific conductance, can be thought of as the conductance of a cube of the solution of the electrolyte of unit dimensions.
The specific conductance can, in principle, be obtained from the measured value of R which gives L = 1/R, and of l and A of the cell. In practice, it is more convenient to deduce l and A, or rather the cell constant l/A, from a measurement of L when the cell is filled with a solution of non specific conductance. Once this geometric factor has been obtained for a cell, it can be used to deduce k for an unknown solution from a measured value of L.
The cell constant is always determined by using a solution KCL. Specific conductances of these reference solutions have been determined by measurements with rather elaborately designed electrodes which avoid the uncertainty in the effecting current carrying cross section that exists in ordinary cell. Since the strong temperature dependence is characteristic of all conductance results, it is necessary to make measurements of conductances in well thermostated cells.
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The fact that aqueous solutions of certain materials, called electrolytes, conduct an electric current provides the most direct evidence for the idea that ions capable of independent motion are present. More detailed studies of the electrical conductivity of such solutions provide information on the number and independence of these ions.
Measurements of the conductivity of aqueous solutions are made with a conductivity cell and an electric current like that shown in the fig. when an alternating current is used to prevent build up of charges of opposite sign near the two electrode surfaces, so that there is little electric resistance at the metal solution interface, the conductivity cell obeys Ohm’s law: the current flowing through the cell is proportional to the voltage across the cell. It is therefore possible to assign a resistance of so many ohms to such a cell, just as one assigns a resistance to a metallic conductor.
It is more convenient to focus on the conductance of an electrolyte solution rather than on its resistance. These quantities are reciprocally related and the conductance L is calculated from the measured resistance as
L = 1/R
Specific conductance of KCl solutions;
| Concentration mol L-1 | 0˚C (k, Ω-1 m-1) | 18˚C (k, Ω-1 m-1) | 25˚C (k, Ω-1 m-1) |
| 1 | 6.543 | 9.820 | 11.173 |
| 0.1 | 0.7154 | 1.1192 | 1.2886 |
| 0.01 | 0.07751 | 0.12227 | 0.14114 |
If R is the resistance in ohms, symbol Ω, and then L has the units of Ω-1. As for metallic conductors, the resistance and therefore the conductance depend on the cross section area A and the length l of the region between the electrodes. As for a metallic conductor,
R = p 1/A
Where p is the specific resistance and is the proportionality factor that corresponds to the resistance of a cell of unit cross section area and unit length. One can write:
L = k A/l
Where k, the specific conductance, can be thought of as the conductance of a cube of the solution of the electrolyte of unit dimensions.
The specific conductance can, in principle, be obtained from the measured value of R which gives L = 1/R, and of l and A of the cell. In practice, it is more convenient to deduce l and A, or rather the cell constant l/A, from a measurement of L when the cell is filled with a solution of non specific conductance. Once this geometric factor has been obtained for a cell, it can be used to deduce k for an unknown solution from a measured value of L.
The cell constant is always determined by using a solution KCL. Specific conductances of these reference solutions have been determined by measurements with rather elaborately designed electrodes which avoid the uncertainty in the effecting current carrying cross section that exists in ordinary cell. Since the strong temperature dependence is characteristic of all conductance results, it is necessary to make measurements of conductances in well thermostated cells.
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