Many people become more active in summer than in winter. The ions in an electrolyte solution likewise become more active when the temperature rises, and this makes it easier for electricity to flow. (Of course you know that heat is the movement of molecules and ions that make up a substance.)
In other words, as the temperature of a conductive solution rises, its conductivity increases. Also, just as each human has a different personality, ions have different characteristics. Even with the same temperature variation, ions become active differently, depending on the kinds of ions and the density of the solution. This indicates that, when making various judgments on the basis of the value for conductivity (when comparing density of salt water, for example), the temperature of the solution to be measured should be kept constant.
But, that can't be done so easily. Therefore, the temperature of a solution is measured, and the value for conductivity obtained at that temperature is converted to the value for conductivity if the temperature were 25 °C. Since, however, the variation in conductivity in relation to temperature differs according to the kind of solution and its density, the conversion must adjust for these factors as well. For simplicity, conversion is made with an assumption of a 2% variation in conductivity per °C. (This is the JIS standard.)
A little more detail may be in order regarding the conversion of temperature, as it is not easily understandable.
For example, the conductivity of a solution of 100 µS/cm at a temperature of 25 °C will be 98 µS/cm at 24 °C, and 102 µS/cm at 26 °C--a change of 2% against a temperature variation of per °C. The following formula represents this:
Accordingly, the conductivity of a solution of 80 µS/cm at 15 °C will be 100 µS/cm when converted to 25 °C.ÅB
Hopefully, you may now have a rough idea of what conductivity is all about. The next topic is how to measure conductivity, including some of the basic methods for taking measurements.
An electrolyte solution contains positive ions, each of which has a positive electrical charge, and negative ions, each of which have has a negative electrical charge. As illustrated in Fig. (A), we will now place a pair of metal plates at opposites sides in an electrolyte solution, and connect a battery. With such a setup, the positive ions move toward the plate connected to the negative terminal of the battery, and the negative ions move toward the plate connected to the positive terminal of the battery, and thus electric current flows through the solution.
When a voltage is applied, the ions move straight toward the respective oppositely charged metal plates, as illustrated in Fig. (B). At this point, the following formula, discussed in 1-2, is relevant.
As long as the metal plates remain in the same positions, the value L/S remains the same. Since conductivity is inversely proportional to resistance, the conductivity can be known if the resistance is measured. Remember Ohm's law, discussed in 1-1, which is:
The above formula can be converted to R=E/I, and hence, when it is inserted to the formula for conductivity,
The voltage (E) of the battery being constant, the conductivity (k) and the current (I) are proportional; therefore, the conductivity can be obtained if we measure the current. From the above formula also, you will see that conductivity is an index of how easily electric current flows. Now, about the relationship between length (L) and area (S): According to the formula regarding conductivity we have so far discussed, the value for conductivity to be measured naturally changes as length (L)/area (S) changes. Generally, the ratio of this length and area is called the cell constant, and the following formula is used:
The above cell constant can be obtained in principle by measuring the distance between the metal plates and the area of the metal plates with a ruler. If the area of one metal plate is 1
, and two metal plates are placed with a distance of 1 cm in between, the cell constant will be K = 1/1 = 1 (
).
When the above distance is widened to 10 cm, the cell constant will be = 10/1 = 10 (
). When the same liquid, at 1 (S/cm) for example, isused for these cells, the resistance value of K = 1 (
) cell is 1 (
),andthe resistance value of K = 10 (
) cell is 10 (
).
When we discussed Fig. (B), we suggested that ions moved in straight lines. Actually, however, all ions do not move in straight lines (remember, the name ion originally meant wanderer). Some ions detour, as illustrated in Fig. (C), and the cell constants can't be measured with a ruler.
Hence, the resistance of a standard solution with a known conductivity is measured to determine the cell constant, as follows:
Cell constant (K) = Resistance (R) X Conductivity (k)
Frequently used as a standard solution is potassium chloride solution (KCl dissolved in water). It was even used by Kohlrausch, who laid the foundation for measurement of conductivity.
Standard KCl solution (JIS KO102)
