11
•
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 bromide, special precautions must be employed
for reproducible measurements. Water used for standards
must be bromide 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.