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4.37 WATER CONDUCTIVITY

      Electrical conductivity in water is a measure of the ion-facilitated electron flow passing through it. Water molecules dissociate into ions as a function of pH and temperature and result in a very predictable conductivity. Some gases, most notably carbon dioxide, readily dissolve in water and interact to form ions, which predictably affect conductivity as well as pH. For the purpose of this discussion, these ions and their resulting conductivity can be considered intrinsic to the water.

      Water conductivity is also affected by the presence of extraneous ions. The extraneous ions used in modeling the conductivity specifications described below are the chloride and sodium ions. The conductivity of the ubiquitous chloride ion and the ammonium ion represent a major portion of the allowed water impurity level. A balancing quantity of cations, such as sodium ions, is included in this allowed impurity level to maintain electroneutrality. Extraneous ions such as these may have significant impact on the water’s chemical purity and suitability for use in pharmaceutical applications.

      Water conductivity must be measured accurately using calibrated instrumentation. On-line conductivity testing provides real-time measurements and opportunities for real-time process control, decision, and intervention. Precaution should be taken while collecting water samples for off-line conductivity measurements. The sample may be affected by the sampling method, the sampling container, and environmental factors such as ambient carbon dioxide concentration and organic vapours.

Apparatus

CONDUCTIVITY CELL

      – Electrodes of a suitable material.

      – The conductivity cell constant must be within ±2 per cent of the given value.

      – The cell constant can be verified directly by using a solution of known or traceable conductivity, or indirectly by comparing the instrument reading taken with the conductivity sensor in question to readings from a conductivity sensor of known or traceable cell constant.

CONDUCTIVITY METER 

      – Meter calibration is accomplished by replacing the conductivity sensor with a traceable precision resistors (accurate to ±0.1 per cent of the stated value, traceable to the official standard) or an equivalently accurate adjustable resistance device, such as a Wheatstone Bridge, to give a predicted instrument response. Each scale on the meter may require separate calibration prior to use.

      – The frequency of recalibration is a function of instrument design, degree of use, etc. However, because some multiple-scale instruments have a single calibration adjustment, recalibration may be required between each use of a different scale.

      – The instrument accuracy must be ±0.1 μS/cm, excluding the conductivity sensor cell constant accuracy.

SYSTEM VERIFICATION

      Periodic verification of the entire equipment shall be performed. This could be done by comparing the conductivity/resistivity values displayed by the measuring equipment with those of an external calibrated conductivity-measuring device. The two nontemperature-compensated conductivity or resistivity values must be equivalent to within ±20 per cent of each other, or at a difference that is acceptable on the basis of product water criticality and/or the water conductivity ranges in which the measurements are taken. The two conductivity sensors should be positioned close enough together to measure the same water sample in the same environmental conditions.

      A similar verification performed in temperaturecompensated mode could be performed to ensure an appropriate accuracy of the equipment when such a mode is used for trending or other purposes.

CONDUCTIVITY TEMPERATURE COMPENSATION

      Many instruments automatically correct the actual reading to display the value that theoretically would be observed at the nominal temperature of 25º. This is typically done using a temperature sensor embedded in the conductivity sensor and an algorithm in the instrument’s circuitry. This temperature compensation algorithm may not be accurate. Conductivity values used in this method are nontemperature-compensated measurements.

      – Temperature measurement is required for the performance of the Stage 1 test. It may be made using the temperature sensor embedded in the conductivity cell sensor. An external temperature sensor positioned near the conductivity sensor is also acceptable. Accuracy of the temperature measurement must be ±2º. 

Procedure

FOR BULK WATER

      Stage 1

      Stage 1 is intended for on-line measurement or may be performed off-line in a sufficient amount of water to a suitable container.

      1. Determine the temperature of the water and the conductivity of the water using a nontemperaturecompensated conductivity reading.

      2. Using the Stage 1 temperature and conductivity requirements table, find the temperature value that is not greater than the measured temperature, i.e. the next lower temperature. The corresponding conductivity value on this table is the limit. (Note Do not interpolate.)

      3. If the measured conductivity is not greater than the table value, the water meets the requirements of the test for conductivity. If the conductivity is greater than the table value, proceed with Stage 2.

Stage 1 Temperature and Conductivity Requirements (for Nontemperature-compensated Conductivity Measurements Only) 

      Stage 2​

      4. Transfer a sufficient amount of water (100 ml or more) to a suitable container, and stir the test sample. Adjust the temperature, if necessary, and, while maintaining it at 25º±1º, begin vigorously agitating the test sample while periodically observing the conductivity. When the change in conductivity (due to uptake of atmospheric carbon dioxide) is less than a net of 0.1 μS/cm per 5 minutes, note the conductivity.

      5. If the conductivity is not more than 2.1 μS/cm, the water meets the requirements of the test for conductivity. If the conductivity is more than 2.1 μS/cm, proceed with Stage 3.

      Stage 3

      6. Perform this test within approximately 5 minutes of the conductivity determination in Step 5, while maintaining the sample temperature at 25º±1º. Add a recently prepared saturated solution of potassium chloride to the same test sample (0.3 ml per 100 ml of the test sample), and determine the pH to the nearest 0.1 pH unit, as directed under “Determination of pH” (Appendix 4.11).

      7. Referring to the Stage 3 pH and Conductivity Requirements table, determine the conductivity limit at the measured pH value. If the measured conductivity in Step 4 is not more than the conductivity requirements for the pH determined in Step 6, the water meets the requirements of the test for conductivity. If either the measured conductivity is greater than this value or the pH is outside the range of 5.0 to 7.0, the water does not meet the requirements of the test for conductivity.

Stage 3 pH and Conductivity Requirements (for Atmosphere- and Temperatureequilibrated Samples Only)

FOR STERILE WATER

      Transfer a sufficient amount of water to a suitable container, and stir the test sample. Adjust the temperature, if necessary, and, while maintaining it at 25º±1º, begin vigorously agitating the test sample while periodically observing the conductivity. When the change in conductivity (due to uptake of ambient carbon dioxide) is less than a net of 0.1 μS/cm per 5 minutes, note the conductivity.

      For containers with a nominal volume of 10 ml or less, if the conductivity is not more than 25 μS/cm, the water meets the requirements. For containers with a nominal volume more than 10 ml, if the conductivity is not more than 5 μS/cm, the water meets the requirements.