Stage 1: This stage is intended for on-line measurement and involves measuring the on-line temperature and non-temperature-compensated conductivity of the water at the test measurement reporting point to ensure the conductivity is below specified limits in the monograph.
Stages 2 and 3 are for off-line testing.
Stage 2: This stage requires taking a sample and stirring it to equilibrate to atmospheric CO2. If the conductivity is less than 2.1 μS/cm at 25°C ± 1°C, it passes. If not, move to Stage 3.
Stage 3: Saturate the sample from Stage 2 with potassium chloride (KCl). Measure pH to the nearest 0.1 pH unit. Look up the conductivity limit for that pH in Table 2 in USP <645> or in the table below. If the measured conductivity (from Stage 2) is less than the conductivity limit, it passes. If the measured conductivity is greater than this value or the pH is outside the range of 5.0 – 7.0, the water does not meet the requirements of USP <645> for conductivity.
By following the three-stage test method, pharmaceutical companies can ensure the quality of their water and, ultimately, the safety and effectiveness of their drugs.