11
applying gentle pressure. Continue polishing until
you are satisfied with the surface. If dark deposits
appear on polishing strip move the paper slightly
and continue polishing.
•
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. Use of protective clothing is advised when han-
dling lead samples and standards. A direct reading ISE
meter (HI 4222 or equivalent) 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 standards that are in the linear mea-
surement range of the unknowns. Two mL of ISA (HI 4001-
12) is added to each 50 mL volume of standard or sample.
Fifty milliliters of the methanol/formalin mixture is also
added to each sample or standard. More calibration stan-
dards are required in non-linear regions. Unknowns are
read directly.
Samples with concentration greater than 0.1 M should be
diluted to be within the working range of the electrodes.
The final result must be multiplied by the corresponding
dilution factor to determine the actual concentration.
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.
