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. Use of a hood and protective clothing is strongly
advised when handling cyanide samples and standards.
A direct reading ISE meter (HI 4222 or equivalent) deter-
mines 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 measurement range of the unknowns.
One mL of ISA (HI 4001-00) is added to each 100 mL
volume of standard or sample. More calibration standards
are required in non-linear regions. Unknowns are read
directly.
Samples with higher concentration 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.