Hanna Instruments HI 4011 User Manual

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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. 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 Iodide, special precautions must be
employed for reproducible measurements. Water used for
standards must be Iodide free and sensors and glassware
must be rinsed repeatedly with this water to prevent carry
over. It is advised that deareated water be used on the
lower concentration standards to prevent oxidation of io-
dide to iodine. In this region, 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.

This manual is related to the following products:

HI 4111