12
2. Unscrew upper cap and slide down cable toward con-
nector.
3. Move spring and outer body down cable also.
4. Dry off inner stem and module with a soft tissue.
5. Hold inner stem and unscrew module and replace
with a new one. (HI 4104-51).
6. Reassemble electrode (see section VII), and refill with
electrolyte. Soak new membrane in calcium solution
without ISA to condition before calibration.
XI.
XI.
XI.
XI.
XI. Direct Calibration and Measurement
Direct Calibration and Measurement
Direct Calibration and Measurement
Direct Calibration and Measurement
Direct Calibration and Measurement
This method is a simple procedure for measuring many
samples. A direct reading ISE meter (HI 4222 or equiva-
lent) determines concentration of the unknown by a direct
reading after calibrating the meter with the standards. Add
HI 4004-00 to adjust ionic strength at a dose of 2 mL of per
100 mL sample or standard. The meter is calibrated using
freshly made standards that are in the measurement range
of the unknowns. Unknowns are read directly. In the region
where the electrode calibration becomes less linear, more
calibration points are needed, and calibration will need to
be repeated more frequently.
A pH/mV meter in mV mode and semi log graph paper
may also be used. Two or more freshly prepared standards
that are in the measurement range of the unknowns are
measured in mV mode on the meter.
These values are plotted on the semi-log paper and the
points are connected to form a straight-line curve. When
samples are measured, their mV values are converted to
concentration by following the mV to the concentration axis
on the semi-log plot.
