Water conductivity measurement experiment

Summary

Electrical conductivity (Electric conductivity) is the ability of a solution to conduct an electric current expressed as a number. Pure water has a small conductivity, which increases when the water contains inorganic acids, bases, salts, or organic charged colloids. Conductivity is often used to indirectly infer the total concentration of charged substances in water. The conductivity of an aqueous solution depends on the nature and concentration of the charged substance, the temperature and viscosity of the solution.

Operation method

Conductivity meter method

Principle

Since conductivity is the inverse of resistance, when two electrodes (usually platinum electrodes or platinum black electrodes) are inserted into a solution, the resistance between the two electrodes can be measured R. According to Ohm's law, when the temperature is certain, this resistance value is directly proportional to the electrode spacing L (cm), and inversely proportional to the electrode cross-sectional area A (cm2), i.e., R = ρ × L/A Since the electrode area A and the spacing L are both fixed, therefore L/A is a constant, called the conductivity cell constant (expressed as Q). The constant of proportionality, ρ, is called resistivity, and its reciprocal, 1/ρ, is called conductivity, which is expressed as K. S = 1/R = 1/(ρ × Q), and S represents conductivity, which reflects the strength of the electrical conductivity. Therefore, K = QS or K = Q/R, when the conductivity cell constant is known and the resistance is measured, the conductivity can be found.

Materials and Instruments

Pure water
Pure water Potassium chloride
Conductivity meter Thermometer

Move

I. Instrumentation
1. Conductivity meter: error not to exceed 1%.
2. Thermometers: capable of reading to 0.1°C.
II. Reagents
1. Pure water (conductivity less than 0.1 mS/m).
2. the calibration solution supplied with the instrument, or the following homemade solution.
3. 0.010 mg/L potassium chloride standard solution: weigh 0.7456 g of potassium chloride dried at 105°C for 2 h and cooled, dissolve in pure water and volume at 25°C to 1000 mL, the conductivity of this solution at 25°C is 141.3 mS/m. Appropriate dilution is made when necessary, and the conductivities of the various concentrations of potassium chloride solutions (25°C) are shown in Table 1.


III. Measurement steps
(1) Pour a sufficient amount of conductivity calibration solution into the beaker to immerse it into the small hole in the electrode.
(2) Place the electrode and thermometer into the solution at the same time,, with the electrode touching the bottom to ensure that air bubbles in the electrode sleeve are removed and that the temperature reaches equilibrium after a few minutes.
(3) Record the measured calibration fluid temperature.
(4) Pressing the ON/OFF key will turn the conductivity meter on.
(5) Press COND/TEMP to display the temperature and adjust the temperature knob until the display shows the recorded calibrant temperature plant.
(6) Press COND/TEMP again to display the conductivity measurement block and select the appropriate measurement range. If the instrument display is out of range, you need to select the next measurement block.
(7) Using a small screwdriver, adjust the calibration button on the side of the instrument to display the conductivity value at the temperature of the calibration solution at which all subsequent measurements were compensated.
(8) The instrument is calibrated and ready for measurement.
IV. Calculation of results
1. Determine the conductivity of the water sample at a constant temperature of 25°C. The reading of the instrument is the conductivity of the water sample (25°C) expressed in units of μS/cm.
2. Determination at any water temperature, the water sample temperature must be recorded, the sample measurement results are calculated according to the following formula:K25=Kt/[1+a×(t-25)]
Style:K25 - Conductivity of water samples at 25°C (μS/cm);Kt - conductivity of water sample at t℃ (μS/cm);a - the average temperature coefficient of various ionic conductivity, take 0.022/1 ℃;t - temperature of the water sample at the time of determination (℃).

Common Problems

The standard unit of conductivity is S/m (i.e., Siemens/meter), and the unit of general practical use is mS/m, commonly used unit μS/cm (micro Siemens/cm). Interchangeable units: 1 mS/m = 0.01 mS/cm = 10 μS/cm New distilled water conductivity of 0.05 ~ 0.2 mS/m, stored for a period of time, due to the dissolution of carbon dioxide or ammonia in the air, the conductivity can rise to 0.2 ~ 0.4 mS/m; drinking water conductivity in the range of 5 ~ 150 mS / m between the seawater conductivity of about 3,000 mS / m; clean river water conductivity of 3,000 mS / m; clean river water conductivity of 3,000 mS / m; clean river water conductivity of 5 ~ 150 mS / m between the water and the sea water. The conductivity of clean river water is 10 mS/m. Conductivity varies with temperature, and increases by about 2% for every 1°C rise in temperature, and 25°C is usually specified as the standard temperature for the determination of conductivity.

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