Hanna Instruments HI 4101 - Manual (Page 14)

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XIII. XIII. XIII. XIII. XIII. Direct Calibration and Measurement Direct Calibration and Measurement Direct Calibration and Measurement Direct Calibration and Measurement Direct Calibration and Measurement
The direct method can be used in the linear working re-
gions of the sensor. (See figure for typical sensor response).
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 measurement range of the unknowns. HI 4001-
00 ISA is added just before measurement of the standard
or sample. Covering the vessel to prevent gas loss is ad-
vised.
A pH/mV meter in mV mode and semi-log graph paper
may also be used. Two or freshly prepared standards that
are in the measurement range of the unknowns (with ISA
added), are measured in mV mode on the meter. These
standards are plotted on semilog graph paper and their
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.
Method 4500-NH
3
D. from Standard Methods for the Ex-
amination of Water and Wastewater is a direct measure-
ment method for water samples.
For both direct reading and mV convertion, ISA is added
prior to measurement and the vessel should be covered to
prevent gas loss.
In the lower concentration ranges the electrode calibration
becomes less linear, many more calibration points are
needed, and calibration will need to be repeated more
frequently. Known addition method may also be used in
these regions provided the actual slope of the sensor has
been determined.
Direct Measurement Procedure
1) Follow section IX to prepare sensor.
2) Follow section VI to prepare standards and solutions.