How To Measure Conductivity Of Water
The more soluble salts in water, the more ions, and the greater the conductivity of water. Therefore, we can indirectly measure water’s conductivity to reflect the number of dissolved solids in water.
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Conductivity and water hardness
The conductivity of the solution is directly proportional to the concentration of dissolved solids, and the higher the concentration of dissolved solids, the greater the conductivity. The relationship between conductivity and dissolved solids concentration is approximately expressed as: 1.4 μ S / cm = 1ppm or 2 μ S / cm = 1ppm (CaCO3 per million units).
We can indirectly get the total hardness value of water by conductivity meter or total solid dissolution meter. As mentioned above, for the convenience of approximate conversion, 1 μ S / cm conductivity = 0.5ppm hardness.
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Determination of water conductivity by electrode method
When the two electrodes are inserted into the solution, we can measure the resistance R between the two electrodes. According to Ohm’s law, when the temperature is constant, there is r= ρ L / A, where ρ is the resistivity, l is the spacing between electrodes, and A is the cross-sectional area of electrodes.
Since A and L are fixed, L / A is a constant called the conductivity cell constant Q.
Conductance s is inversely related to resistance R: S = 1 / R;
Conductivity K and resistivity ρ Reciprocal relationship: k = 1/ ρ；
S = 1/R = 1/( ρ Q) → Q/R = 1/ ρ = K
Calibrate with the standard solution to get the conductivity cell constant Q, measure the water sample to get the resistance R, and calculate K.
Steps:
Clean the electrode with a standard solution three times
Add the standard solution into the conductivity cell and take a constant temperature water bath at 25 ℃ for 15 minutes
Calibration: if the standard solution R is measured and the standard solution K is known, we can measure the conductivity cell Q value.
Test: clean the electrode with pure water, moisten it with water, and test the R-value of the water sample (if the deviation between two times is less than 2%, take the average value). Know the Q value, calculate the K value of the water sample, and record the temperature.
Note:
1. Water sample collection and storage:
The water sample should be stored in polyethylene bottles, filled and sealed, refrigerated at 4 ° C, and it should be analyzed as soon as possible after collection within 24 hours.
2. Pretreatment:
When the water sample contains suspended substances, oil and grease, it can interfere with the test. We can measure the water sample first and then test the standard solution to understand the interference. If there is interference, we should filter or extract the water sample should be.
3. Influencing factors:
Influence of electrode polarization: if there is high concentration polarization, it will make the electrode surface and solution concentration unable to reach equilibrium, resulting in test error.
Elimination method: Brush platinum black on the platinum sheet to increase the surface area (reduce the current density); Use alternating current to offset concentration polarization.
Capacitance factor: when AC power work, the capacitance affects the resistance between the electrodes.
Elimination method: change the constant of electrolytic cell and increase the resistance value; Increase the power frequency and reduce the capacitance.
Temperature factor: the conductivity increases by 2% when the temperature increases by 1 ℃. we must keep solution temperature stable. K25 ℃ = kt / [1 + a (t-25)] temperature coefficient a = 0.022
Background factor: when ammonia or carbon dioxide in the air is absorbed by solution, it will affect the test results; Impurities in solution, pure water and utensils will also affect the test results.
How to measure the conductivity of purified water
There is no conductivity for pure solvents, so the conductivity is close to zero. Only solutions with ions or polar bonds have conductivity. In other words, completely purified water is a pure solution and theoretically has no conductivity.
However, water molecules in pure water will produce hydrogen ions and hydroxyl ions due to ionising to some extent. Although the conductivity of pure water is very weak, it also has measurable conductivity.
In addition, the water still contains impurity ions due to the preparation process of purified water. The more impurity ions, the stronger the conductivity and the higher the conductivity. Therefore, we can determine the content of impurity ions in the water by conductivity to evaluate whether the water quality meets the requirements.
The temperature has a significant influence on the measured value of the conductivity of the sample. If the conductivity of water is calculated by the temperature correction method, it will cause a large error. So, the conductivity of purified water is measured by non-temperature compensation mode. We can measure it and record the measured temperature by online conductivity meter, then calculate the limit value by linear interpolation method. If the measured conductivity is less than the limit value, it is considered that it is conforming to the regulations; if the measured conductivity is greater than the limit value, it is considered as not conforming to the regulations.
Factors affecting the conductivity test of purified water
a) When the water contains other impurity ions, the more impurity ions, the higher the conductivity.
b) When carbon dioxide and other gases in the air dissolve in water and interact with water, it will produce corresponding ions, increasing water’s conductivity.
c) The conductivity of water is closely related to temperature. The conductivity of purified water will increase by the increase in sample temperature.
d) The tiny bubbles attached to the electrode surface will increase the resistance of the sample in the electrode and reduce the measured conductivity value. Therefore, ensure no bubbles in the electrode before each measurement. We can remove the bubbles by flicking or alternately lifting the electrode.
How to measure the conductivity of ultrapure water
Assuming that the conductivity of pure water is measured by the off-line conductivity meter in the laboratory, it needs to follow the below thing, and the measurement results are only for reference:
1) Electrode constant of conductive electrode: electrode with C = 0.1 or 0.01 is necessary; otherwise, it will decrease the measurement accuracy.
2) The conductivity of pure water will rise rapidly by absorbing CO2 and other carbonates in the air in an open container. Therefore, it must be read quickly, preferably within 30 seconds.
3) it had better use a closed flow cell or conductivity cell for measurement. If not, wash it repeatedly with a 250ml polyethylene cup (without quartz glass), insert the pure water outlet pipe close to the bottom of the cup, adjust the flow of pure water, and slowly fill the cup until it overflows to prevent any bubbles, and insert the conductive electrode near the outlet pipe orifice for measurement when the water overflows continuously. Only in this way the result is close to the real conductivity value.
The conductivity can not be accurately measured in the general experimental environment, especially the off-line conductivity measurement. If you want to measure the resistivity of ultrapure water accurately, you must use a special electrode with temperature compensation to measure it in a closed state while the ultrapure water is flowing; otherwise, you cannot measure the accurate resistivity value.
Conductivity is a measure of the content of ions in water. In ultrapure water, there is no ion, and the ion concentration is very low. Therefore, the determination results are inaccurate and only referenced.
When using a desktop conductivity meter, it is not recommended to measure the conductivity of ultrapure water or pure water conductivity < 1US/ cm@25 ℃. We can use online conductivity measurement instead of that.
