Ultrapure Water Applications – Conductivity Measurement

GB1576—2001

A5 Determination of Conductivity

A5.1 Method Summary

Acids, bases, and salts dissolved in water dissociate into positive and negative ions in the solution, giving the electrolyte solution the ability to conduct electricity. This conductivity can be expressed as conductivity.

The conductivity of an electrolyte solution is usually determined by inserting two metal plates (electrodes) into the solution and measuring the resistivity between the electrodes. Conductivity is the reciprocal of resistivity, defined as the conductivity of the solution when the electrode cross-sectional area is 1 cm² and the distance between the electrodes is 1 cm.

The unit of conductivity is SiS/cm (S/cm). In water analysis, it is often expressed as parts per million (µS/cm).

The conductivity of a solution is related to the properties and concentration of the electrolyte, as well as the solution temperature. Generally, the conductivity of a solution refers to the conductivity at 25°C.

A5.2 Instruments

A5.2.1 Conductivity Meter (or Conductivity Analyzer): The measurement range should be within the standard range; a DDS-.11 model is suitable.

A5.2.2 Conductivity Electrode (Electrode): Commonly used laboratory conductivity electrodes are platinum or platinum black electrodes. Each electrode has its own cell constant, which can be divided into three categories: below 0.1 cm⁻¹, 0.1–1.0 cm⁻¹, and 1.0–10 cm⁻¹.

A5.2.3 Thermometer: The accuracy should be higher than 0.5℃.

A5.3 Reagents

A5.3.1 1 mol/L Potassium Chloride Standard Solution: Weigh 74.5513 g of analytical grade potassium chloride (or primary standard) dried at 105℃ for 2 h. Dissolve it in freshly prepared Grade II reagent water (20℃±2℃), transfer to a 1 L volumetric flask, dilute to the mark, and mix well.

A5.3.2 0.1 mol/L Potassium Chloride Standard Solution: Weigh 7.4551 g of analytical grade potassium chloride (or primary standard) dried at 105℃ for 2 h. Dissolve in freshly prepared Grade II aqueous solution (20℃±2℃), transfer to a 1 L volumetric flask, and dilute to the mark. Mix well.

A5.3.3 0.01 mol/L Potassium Chloride Standard Solution: Weigh 0.7455 g of analytical grade potassium chloride (or primary standard) dried at 105℃ for 2 h. Dissolve in freshly prepared Grade II aqueous solution (20℃±2℃), transfer to a 1 L volumetric flask, and dilute to the mark. Mix well.

A5.3.4 0.001 mol/L Potassium Chloride Standard Solution: Before use, accurately pipette 100 mL of 0.01 mol/L potassium chloride standard solution into a 1 L volumetric flask, dilute to the mark with freshly prepared Grade I reagent water (20℃±2℃), and mix well.

The above potassium chloride standard solutions should be stored in polyethylene plastic bottles (or hard glass bottles) and sealed. The conductivity of these potassium chloride standard solutions at different temperatures is shown in Table A4.

Table A4 Conductivity of Potassium Chloride Standard Solutions

A5.4 Operating Procedures

A5.4.1 The conductivity meter should be operated according to the instructions in the user manual.

A5.4.2 Different water samples have different conductivity levels, so electrodes with different conductivity cell constants should be used. Refer to Table A5 for selecting electrodes with different conductivity cell constants for water samples with different conductivity levels.

Table A5 Selection of Electrodes with Different Conductivity Cell Constants

Rinse the selected electrode with Grade II reagent water, then rinse 2-3 times with Grade I reagent water and soak it in Grade I reagent water for later use.

A5.4.3 Take 50-100 mL of water sample (temperature 25℃±5℃) and place it in a plastic or hard glass cup. Rinse the electrode with the water sample 2-3 times, then immerse it in the water sample for conductivity measurement. Repeat the sampling and measurement 2-3 times. The relative error of the measurement reading should be within ±3%, which is the measured conductivity value (when using a conductivity meter, the reading is the conductivity value). Record the water sample temperature simultaneously.

A5.4.4 If the water sample humidity is not 25℃, the measured value should be converted to the conductivity value at 25℃ according to formula (A3).

A5.4.5 For electrodes with unknown cell constants or when cell constant calibration is required, the electrode can be used to measure the conductivity of a potassium chloride standard solution with known conductivity (temperature 25℃±5℃) (see Table A4). The cell constant of the electrode can then be calculated based on the measured results. To reduce error, a potassium chloride standard solution with conductivity close to that of the water sample should be used for calibration. The cell constant of the electrode is calculated according to formula (A4).

K=S1/S2.................................(A4)

Where: K—Cell constant of the electrode, cm－1;

S1—Conductivity of the potassium chloride standard solution, μS/cm;

S2—Conductivity of the potassium chloride standard solution measured using an electrode with an unknown cell constant, μS.

A5.4.6 If the humidity of the potassium chloride standard solution is not 25℃, the measured value should be converted to the conductivity value at 25℃ according to formula (A3) and substituted into formula (A4) to calculate the cell constant. A5.5 Relationship between Conductivity and Salinity

For the same type of natural freshwater, taking 25°C and humidity as a benchmark, conductivity and salinity are roughly proportional, with a ratio of approximately 1 μS/cm equivalent to 0.55–0.90 mg/L. Corrections are needed at other temperatures; that is, salinity changes by approximately 2% for every 1°C change. Use negative values for temperatures above 25°C and positive values for temperatures below 25°C.

Note: At 20°C, the conductivity of a certain natural water is measured to be 244 S/cm. Calculate the approximate salinity of this water. Solution: Assume that 1 S/cm of conductivity corresponds to 0.75 mg/L of salinity. Then the salinity is 244 × 0.75 + 244 × 0.75 × 2% × 5 = 2.0 × 10² mg/L.

Based on practical experience, generally within the pH range of 5–9, the ratio of conductivity to dissolved substances in the aqueous solution of natural water is approximately 1:(0.6–0.8). For general boiler water, if the OH- ions, which have the highest conductivity, are neutralized to form a neutral salt, the ratio of conductivity to dissolved solids is approximately 1:(0.5–0.6) (i.e., 1 μS/cm is equivalent to 0.5–0.6 mg/L).

Table A6 Conductivity of Different Water Qualities

S2—The conductivity of a potassium chloride standard solution is measured using an electrode with an unknown cell constant, in μS.

A5.4.6 If the humidity of the potassium chloride standard solution is not 25℃, the measured value should be converted to the conductivity value at 25℃ using formula (A3), and substituted into formula (A4) to calculate the cell constant.

A5.5 Relationship between Conductivity and Salt Content

For the same type of natural freshwater, taking 25℃ humidity as the standard, the conductivity and salt content are approximately proportional, with a ratio of approximately 1 μS/cm equivalent to 0.55–0.90 mg/L. Corrections are needed at other temperatures; that is, the salt content changes by approximately 2% for every 1℃ change. Use negative values for temperatures above 25℃ and positive values for temperatures below 25℃.

Note: At 20℃, the conductivity of a certain natural water is measured to be 244 S/cm. Calculate the approximate salt content of this water. Solution: Let the salt content be equivalent to 0.75 mg/L when the conductivity is 1 S/cm. Then the salt content is 244 × 0.75 + 244 × 0.75 × 2% × 5 = 2.0 × 10² mg/L.

Based on practical experience, the ratio of conductivity to dissolved matter in natural water is generally about 1:(0.6–0.8) within the pH range of 5–9. For general boiler water, if the OH⁻ ions with the highest conductivity are neutralized to form a neutral salt, the ratio of conductivity to dissolved solids is approximately 1:(0.5–0.6) (i.e., 1 μS/cm is equivalent to 0.5–0.6 mg/L).

Table A6: Conductivity of Different Water Quality

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