Conductivity Measurement

1. Definition of conductivity
Conductivity is the ability of an object to conduct electric current. The measuring principle of a conductivity meter is to place two parallel plates in the solution to be measured, apply a certain potential (usually a 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 current, the conductivity of the solution is proportional to the ion concentration in the solution.
However, the conductivity of some solutions has no direct relationship with the ion concentration. The curve on the right shows the relationship between the conductivity and ion concentration of two common solutions. Note that the linear relationship in the graph is for sodium chloride solution rather than high concentration sulfuric acid. The interaction of some high concentration solution ions will change this linear relationship.
2. Measurement unit
The basic unit of conductivity is Siemens (S), formerly known as Ohm. Because the geometry of the conductivity cell affects the conductivity value, the standard measurement uses unit conductivity S/cm to compensate for the differences 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). Here L is the length of the liquid column between the two plates, and A is the area of ​​the plates. (See the figure on the right)
C=Gx(L/A)
If the conductivity cell constant is 1cm-1, the unit conductivity is the same as the conductivity of the measured solution. Although the shape of the electrode is different, each electrode can be represented by a corresponding theoretical conductivity cell.
Although we specify the conductivity as: μS or mS due to space limitations, this series of units should be understood as the corresponding unit conductivity units: μS/cm or mS/cm.
1μS/cm=0.001mS/cm=0.000001S/cm=1μmho/cmThe
following table lists the optimal conductivity ranges for three different conductivity cell constants:
Conductivity cell constant optimal conductivity (us/cm)
0.1 0.5to400
1.0 10to2000
10.0 1000to200,000
3. Calibration of conductivity meter and maintenance of conductivity cell
The conductivity meter and conductivity cell should be calibrated with standard solutions before use. Select a standard solution that is closest to the conductivity of the solution being measured. Electrodes that have been polarized and soiled should be re-plated or cleaned to restore the active surface of the conductivity cell. In most cases, hot water with some mild liquid detergent 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 use abrasives to clean the electrode. The
following table lists the conductivity of some commonly used solutions.
Solution conductivity us/cm (resistivity)
Pure water 0.055us/cm 18.2MΩ
Power plant boiler water 1.0us/cm 1MΩ
City water 50us/cm 0.02Ω
Seawater 53mS/cm 20Ω
31.0%HNO3 31.0% Nitric acid 865mS/cm 1Ω
4. Temperature compensation of
conductivity Conductivity measurement is temperature-dependent. The degree of influence of temperature on conductivity varies depending on the solution, and can be calculated using the following formula.
Gt=Gtcal{1+α(t-tcal)}
Where:
Gt=Conductivity at a certain temperature (°C)
Gtcal=Conductivity at standard temperature (°C)
α=Temperature coefficient of solution at standard temperature (°C)
The following table lists the α values ​​of commonly used solutions. To obtain the α value of other solutions, just measure the conductivity within a certain temperature range and draw the corresponding conductivity change curve with temperature as the vertical axis. The curve point corresponding to the standard temperature is the α value of the solution.
Solution (25°C) Concentration Alpha (α)
Hydrochloric acid 10wt% 1.56
Potassium chloride solution 10wt% 1.88
Sulfuric acid 50wt% 1.93
Sodium chloride solution 10wt% 2.14
Hydrofluoric acid 1.5wt% 7.20
Nitric acid 31wt% 31.0
All conductivity meters currently available on the market can be adjusted to a standard temperature (usually 25°C) or automatically temperature compensated. Most conductivity meters with fixed temperature compensation have an α adjustment of 2%/°C (approximately the α of sodium chloride solution at 25°C). Conductivity meters with adjustable temperature compensation can adjust α to be closer to the α of the solution being measured.