19
Stage 2
Determine the influence of CO
2
.
Note: Stability criteria <0.5 %/min corresponds in fact to a change in conductivity <0.02 µS/cm
per minute (equivalent to 0.1 mS/cm per 5 minutes), as required in the Stage 2 USP document.
• Transfer a sufficient amount of water (100 mL or more) to a thermostatic vessel, and stir the
test specimen. Adjust the temperature and maintain it at 25±1 °C. We recommend using a
thermostatic bath.
• Rinse the cell carefully with deionized water. Place the conductivity probe in the sample and dip
it in the thermostatic vessel containing the sample. Check that the 4 rings and sleeve holes are
immersed in the solution.
• Press
Stage 2.
• When the conductivity value is stable, if the conductivity is not greater than 2.1
µ
S/cm the water
meets the requirements of the test, for conductivity.
If the conductivity is greater than 2.1
µ
S/cm proceed with Stage 3.
Stage 3
Determine the combined effect of the CO
2
and pH.
Use a Hanna Instruments pH meter.
Take care that the instrument is calibrated in at least two points using pH 4.01 and pH 7.01 Hanna
buffers.
Perform the following test within approximately 5 minutes of the conductivity determination while
maintaining the sample temperature at 25±1 °C.
• Add the Saturated KCl Solution to the sample (0.3 mL per 100 mL of the test specimen), and
determine the pH to the nearest 0.1 pH unit.
• Press
Stage 3
key, then enter the corresponding pH reading.
• Referring to the Stage 3 pH and conductivity requirement table the instrument determines the
conductivity limit at the measured pH value. If the measured conductivity is no greater than the
conductivity requirements for the pH determined the water meets the requirements of the test for
conductivity. If the measured conductivity is greater than this value or the pH is outside of the
range of 5.0 to 7.0, the water does not meet the requirements of the test for conductivity.
USP MEASUREMENT