The Conductivity test measures the electrical conductivity, or ion-facilitated electron flow, in water. It is used as an indicator of inorganic dissolved solids. The measurement of product conductivity is a typical way to monitor and trend the performance of water purification systems.
Conductivity analysis is required to meet USP standards of Purified Water, Sterile Purified Water, Pure Steam, Water for Injection, Sterile Water for Injection, Sterile Water for Inhalation, Sterile Water for Irrigation and Water for Hemodialysis, as well as EP standards of Purified Water in Bulk.
- USP 645
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CTA101 - Conductivity Test
The Conductivity test is performed on water which has been purified using unit operations that include deionization, distillation, ion exchange, reverse osmosis, filtration or other suitable purification procedures.
The combined conductivities of the intrinsic and extraneous ions vary as a function of pH and are the foundation for the conductivity requirements found in the USP.
The USP Water Conductivity procedure describes three stages. If the test conditions and conductivity limits are met at either of the preliminary stages, the water meets the requirements of the test and the third stage, in which the pH is measured, is unnecessary.
Stage I: The temperature of the sample is determined and the conductivity of the sample is measured using a non-temperature compensated conductivity reading. The conductivity limit value is found using the USP Stage I: Temperature and Conductivity Requirements table. If the measured conductivity is equal to or less than the table limit value, the water meets the requirements of the test for conductivity. If the conductivity is higher than the table limit value, Stage II is required.
Stage II: The sample temperature is adjusted to 25 ± 1°C and the conductivity is recorded at this temperature. If the conductivity is not greater than 2.1 µs/cm, the water meets the requirements of the test for conductivity. If the conductivity is greater than 2.1 µs/cm, Stage III is required.
Stage III: Saturated potassium chloride solution is added to the same water sample and the pH is determined. The conductivity limit at the measured pH value is determined by referring to the Stage III: pH and Conductivity Requirements table. If the measured conductivity in Stage III is no greater than the conductivity requirements, 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 of the range 5.0 to 7.0, the water does not meet the requirements of the test for conductivity.