19
Pharmaceutical laboratories working in the US market are obliged to respect the regulations set down
by the US Pharmacopoeia (USP). The 5
th
supplement of
USP24-NF19
lays down the rules for checking
the quality of pure or fully deionized water used for the production of injection products.
The conductivity of water provides information on its chemical composition. It is therefore logical that
conductivity is the main parameter to measure.
The conductivity of water is a measure of the ion mobility through this water. The conductivity partly
depends on the pH, the temperature and the amount of atmospheric carbon dioxide, which has been
dissolved in water to form ions (intrinsic conductivity). The conductivity also depends on the chloride,
sodium and ammonium ions initially present in water (extraneous conductivity).
The conductivity (intrinsic and extraneous) of the water is measured at Stage 1 and compared to
values listed in a table to evaluate if the studied water is suitable or not for use in pharmaceutical
applications. If the sample fails Stage 1, additional tests have to be performed (Stage 2 and 3) in
order to determine if the excessive conductivity value is due to intrinsic factors or extraneous ions.
USP Requirements
Automatic temperature correction must not be used.
Instrument specifications
Minimum resolution of 0.1
µ
S/cm on the lowest range. Excluding the cell accuracy, the instrument
accuracy must be ±0.1
µ
S/cm.
Meter calibration
It is accomplished by replacing the conductivity cell with precision resolution traceable to primary
standards (accurate to ±0.1 % of the stated value) or an equivalently accurate resistance device.
Cell calibration
Meter conductivity must be measured accurately using calibrated instrumentation. The conductivity
cell constant must be known in ±2%.
Before starting water analysis calibrate in the lowest EC range or set the probe cell constant (enter
the value written in the calibration certificate delivered with the probe).
Stage 1
Determine the temperature and conductivity of the water.
• Rinse the probe carefully with deionized water. Check that the four-rings, sleeve holes and the
temperature sensor are immersed in the sample and that no air bubbles are trapped. Connect
the probe to the meter, enter USP mode and press the
Stage 1
key. The instrument will perform
a temperature and conductivity measurement (using a non-temperature corrected conductivity
reading).
USP MEASUREMENT
