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 4113-51).
6. Reassemble electrode (see section VII), and refill with
electrolyte. Soak new membrane in nitrate solution
without ISA to condition before calibration.
XI. XI.
XI. XI.
XI.
Direct Calibration and MeasurementDirect Calibration and Measurement
Direct Calibration and MeasurementDirect 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 4013-00 to adjust ionic strength at a dose of 2 mL of per
100 mL sample or standard. ISISA* may also be used at a
dose of 50 mL for 50 mL of 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 be-
comes 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.
*Note: ISISA is the recommended ISA used for procedure
4500-NO
3
-D. published in Standard Methods for the Ex-
amination of Water and Wastewater.
