8
9
X.
X.
X.
X.
X. Corrective action
Corrective action
Corrective action
Corrective action
Corrective action
•
Verify protective cap has been removed.
•
Verify electrode is connected properly to meter and
meter is powered.
•
Verify dilute standards are freshly made and stored.
Remake solutions if appropriate.
•
If the sensor slope just misses the suggested slope
window, soaking the sensor in the storage solution
may solve the problem.
•
A scratched sensing surface will have a slugish be-
havior. Use conditioning /etch procedure discussed in
section VIII.
•
If the membrane is damaged, the response becomes
extremely sluggish, or the slope of the electrode has
decreased significantly, and procedures above have
not helped, the sensor should be replaced.
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. The
meter is calibrated with two or more freshly made stan-
dards that are in the linear measurement range of the
unknowns. More calibration standards are required in non-
linear regions. Unknowns are read directly. At very low
levels of sodium, special precautions must be employed for
reproducible measurements. Water used for standards must
be sodium free and sensors and glassware must be rinsed
repeatedly with this water to prevent carry over. In the
region where the electrode response appears curved, more
calibration points are needed, and calibration will need to
be repeated more frequently.
A pH/mV meter in mV mode with 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.
Typical Linearity of FC300 Sodium Electrode
-300
-250
-200
-150
-100
-50
0
50
100
-1
0
1
2
3
4
5
-log[Molar Concentration]
m
V
Procedure
1) Follow sections VIII and IX to prepare sensors for
measurement.
2) Follow section VI to prepare standards/ solution. Stan-
dards should bracket and fall within the range of
interest.
Ten mL HI 4016-00 ISA, or 10 mL HI 7090L ISA is
added to 100 mL of both samples and standards.
Add stir bar and mix before taking measurements.
3) Follow section VII; General Guidelines to optimize test
set-up.
4) During calibration it is best to start with lower concen-
tration samples first. Wait for a stable measurement
before recording values. Slightly longer equilibra-
tions are required at lower concentrations .
5) To prevent carry over and contamination of samples,
rinse sensors with DIW and blot dry between samples.