China Education Equipment Purchasing Network News: According to China Chemical Instrument Network, electrical conductivity is the ability of objects to conduct electrical current. The measurement principle of the conductivity meter is to put two parallel plates in the solution to be measured, add a certain potential (usually a sine wave voltage) to both ends of the plate, and then measure the flow between the plates Current. According to Ohm's law, the reciprocal of conductivity (G)-resistance (R) is determined by the conductor itself. 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 differences caused by various electrode sizes. The unit conductivity (C) is simply the product of the measured conductivity (G) and the 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 .
The conductivity of the aqueous solution is directly proportional to the dissolved solids concentration, and the higher the solids concentration, the greater the conductivity. The relationship between conductivity and dissolved solids concentration is approximately expressed as: 1.4 μS / cm = 1 ppm or 2 μS / cm = 1 ppm (CaCO3 per million units). The total hardness value of water can be obtained indirectly by using a conductivity meter or a total solid dissolution meter. As mentioned above, for convenience of approximate conversion, 1 μs / cm conductivity = 0.5 ppm hardness. Electrical conductivity is the ability of a substance to transmit current. It is relative to the resistance value. The unit is Siemens / cm (S / cm). 10-6 in this unit is expressed in μS / cm, and 10-3 is expressed in mS / cm. However, it should be noted that: (1) The hardness of water is measured indirectly by conductivity. The theoretical error is about 20-30ppm. (2) The conductivity of the solution determines the movement of molecules. Temperature affects the movement of molecules. Set to 20 ℃ or 25 ℃ (3) Use the reagent test to get more accurate water hardness value.
The conductivity of water has a certain relationship with the amount of inorganic acids, alkalis and salts it contains. When their concentration is low, the conductivity increases with increasing concentration. Therefore, this indicator is often used to speculate the total concentration or salt content of ions in water. Different types of water have different electrical conductivity. The conductivity of fresh distilled water is 0.2-2μS / cm, but after a period of time, due to the absorption of CO2, it increases to 2-4μS / cm; the conductivity of ultra pure water is less than 0.10 / μS / cm; the conductivity of natural water Between 50-500μS / cm, mineralized water can reach 500-1000μS / cm; the conductivity of industrial wastewater containing acid, alkali and salt often exceeds 10000μS / cm; the conductivity of seawater is about 30,000μS / cm. The cell constant is often measured with a standard potassium chloride solution of known conductivity. The conductivity (25 ° C) of different concentrations of potassium chloride solution is listed in the table below. The conductivity of the solution is related to its temperature, the polarization phenomenon on the electrode, the distributed capacitance of the electrode and other factors. Compensation or elimination measures are generally adopted on the instrument.
The water sample should be measured as soon as possible after collection. If it contains coarse suspended substances, oil and fat, which interferes with the measurement, it should be filtered or extracted to remove. Specific steps are as follows:
1) Immerse the platinum black electrode in deionized water for several minutes.
2) Adjust the head screw M so that the pointer is at zero.
3) Move the calibration and measurement switch K2 to the "calibration" position.
4) After turning on the power switch K and preheating for a few minutes, adjust the correction regulator Rw3 so that the pointer is on the full scale.
5) Move the high cycle and low cycle switches K3 to the appropriate gear.
6) Move the range selection switch R1 to the appropriate gear.
7) Adjust the electrode constant regulator Rw2 to correspond to the constant of the electrode used (this is equivalent to adjusting the electrode constant to 1, and the measured conductivity of the solution is equal to the conductivity of the solution in value)
8) After rinsing the electrode with a small amount of test solution, insert its plug into the electrode socket Kx and immerse it in the test solution.
9) After adjusting the calibration regulator Rw3 to full scale, move the calibration and measurement switch K2 to the measurement position. Read the indication number of the needle, and then multiply the multiple indicated by the range selection switch R1, which is the conductivity of the solution. Repeat the measurement once and take the average value.
10) Move the calibration and measurement switch K2 to the "calibration" position and take out the electrode.
11) After measuring, disconnect the power supply. After washing the electrode with deionized water, immerse it in deionized water and set aside.
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