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Measurement of Electrical Conductivity and Related Parameters

1. Definition of conductivity
Conductivity is the ability of an object to conduct current. The measuring principle of the conductivity measuring instrument is to put two parallel plates into the measured solution, apply a certain potential (usually sine wave voltage) to both ends of the plates, and then measure the current flowing between the plates. According to Ohm's Law, conductivity (G), the reciprocal of resistance (R), is determined by voltage and current.
         1           I(amps)
G = ——  = ————
         R           E(volts)
Because the charge of the ions in the solution accelerates the conduction of the current, the conductivity of the solution is proportional to the concentration of the ions in the solution.
However, the conductivity of some solutions is not directly related to the ion concentration.
2. Unit of Measurement
The basic unit of conductivity is Siemens (S), which was originally called ohm. Because the geometry of the conductivity cell affects the conductivity value, the standard measurement is expressed in unit conductivity S/cm to compensate for the difference caused by various electrode sizes. The unit conductivity (C) is simply the product of the measured conductivity (G) and the conductivity cell constant (L/A), where L is the length of the liquid column between the two plates and A is the area of the plates.
    C = G x (L/A)
If the conductivity cell constant is 1 cm-1, the unit conductivity is the same as that of the measured solution. Although the shapes of the electrodes are different, each electrode can be represented by a corresponding theoretical conductance cell.
Although because of the limitation of space, we specifically refer to conductivity as μ S or mS, this series of units should be understood as corresponding unit conductivity units: μ S/cm or ms/cm.
    1 μ S/cm = 0.001 mS/cm = 0.000001 S/cm = 1 μ mho/cm
The following table lists the best conductivity ranges for three different conductivity cell constants:
Conductivity cell constant               Optimum conductivity(us/cm)
                    0.1                                     0.5 to 400
                    1.0                                     10 to 2000
                   10.0                                   1000 to 200,000
3. Calibration of conductance meter and maintenance of conductance cell
Conductivity meter and conductivity cell should be checked with standard solution before use. Choose a standard solution closest to the conductivity of the measured solution. Polarized and soiled electrodes should be plated with platinum or cleaned to restore the active surface of the conductance cell. In most cases, hot water with some mild liquid cleaner is a very effective cleaner. Most organic matter can be easily cleaned with acetone, and chlorous acid solution can remove algae, bacteria or mold. Do not clean the electrodes with abrasive.
 The following table lists the conductivity of some commonly used solutions:
Solution                          Conductivity(us/cm)             Resistivity
Pure water                                0.055 us/cm                 18.2 M Ω
Boiler water of power plant      1.0 us/cm                      1M Ω
Urban water consumption         50us/cm                      0.02 Ω
Seawater                                     53ms/cm                      20 Ω
31.0% HNO3                               865ms/cm                      1 Ω
4. Temperature compensation of conductivity
Conductivity measurements are temperature dependent. The effect of temperature on conductivity varies from solution to solution, which can be obtained by the following formula.
    Gt = Gtcal {1 + α (t-tcal)}
Among them:
    Gt = conductivity at a certain temperature (° C)
    Gtcal = conductivity at standard temperature (° C)
    Alpha = temperature coefficient of solution at standard temperature (° C)
Solution                     Concentration (25°C)    Alpha (α)
Hydrochloric acid                  10                    wt% 1.56
Potassium chloride solution  10                    wt% 1.88
Sulfuric acid                           50                    wt% 1.93
Sodium chloride solution      10                    wt% 2.14
Hydrofluoric acid                  1.5                    wt% 7.20
Nitric acid                              31                    wt% 31.0
All conductance meters currently on the market can be adjusted or automatically compensated with reference to the standard temperature (usually 25 ° C). The alpha of most fixed temperature compensated conductometer is adjusted to 2%/° C (alpha of sodium chloride solution at approximately 25 ° C). The conductometer with adjustable temperature compensation can adjust α closer to the α of the measured solution.


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