Hanna Instruments HI 4521 - Manual (Page 56)

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• The measured concentration value will be displayed on the LCD. If pressing , the “AutoHold” indicator
will start blinking on the LCD until the stability criterion is reached. The concentration value will be frozen on the
LCD, along with “AutoHold” indicator.
• To return to normal Measure mode press .
Note: If the reading is out of range, “-----” will be displayed on the LCD.
Note: Press at any time to stop the measurement and return to ISE Measure mode.
KNOWN ADDITION
To measure the concentration of a sample using the Known
Addition incremental method:
• Press and then to select ISE Measure
mode for the selected channel.
• Select the Known Addition method (see ISE Setup for details).
• Press to start the measurement. The first step of the
method will be displayed on the LCD in which the user is notified
to add the appropriate ISA Buffer Volume (if different from 0 mL)
to the sample and to immerse the electrode into the Sample
Volume. The method parameters are also displayed on the LCD.
• Press to set the method parameters. Press
/ to select next/previous parameter to be edit, then
press and use or to set the
desired parameter value. Press to save the modified value
and then press to exit method parameters edit menu.
• Make sure that the set parameters values are the used ones and
submerge the Ion Selective Electrode tip approximately 4 cm
(1½”) into the sample to be tested.
• Press to take the first mV reading.
• When the reading is stable, press to store the first
mV reading. The second step of the method will be displayed
on the LCD in which the user is notified to add the Standard
Volume of set concentration to the sample. The method
parameters are also displayed on the LCD.
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Actual samples that are more concentrated have much smaller activity coefficients ( <1). The addition of an
inert background salt to standards and samples stabilizes the activity coefficient so that concentration measurements
may be made directly. Hanna’s Ionic Strength Adjustment Buffer (ISAB) formulations also may optimize pH, and
complex interferences in addition to standardizing the ionic strength.
The Nernst equation can be rewritten:
ION SELECTIVE ANALYSIS METHODS
Direct Analysis Direct Analysis Direct Analysis Direct Analysis Direct Analysis
This method is a simple procedure for measuring multiple samples. It should only be used in the linear
working regions of the sensor. A direct reading instrument such as the
HI 4522 determines concentration of
the unknown by a direct reading after calibrating the instrument with the standards. The instrument is
calibrated as described in “ISE CALIBRATION” section, with two or more freshly made standards that are in the
measurement range of the unknowns. Ionic strength adjustment is made to samples and standards. Unknowns
are measured directly by the instrument.
At lower concentrations, in non-linear regions of the electrode response, multiple calibration points will extend
measurements to a practical detection limit. Calibrations must be performed more frequently in these cases.
Incremental Methods Incremental Methods Incremental Methods Incremental Methods Incremental Methods
Incremental methods are useful for the measurement of samples whose constituents are variable or
concentrated. Incremental techniques can reduce errors from such variables as temperature, viscosity, or pH
extremes and will provide indirect analysis of ions for which there is no ISE sensor for a direct measurement.
There are four different incremental methods for sample measurement. They are Known Addition, Known
Subtraction, Analyte Addition and Analyte Subtraction.
HI 4522 allows the analyst to use these techniques as
a simple routine procedure thus eliminating calculations or tables. The method once set up can be used for
repetitive measurements on multiple samples.
Known Addition and Known Subtraction
With Known addition,
standard is added to a sample being measured. The standard and sample contain the
same ion. mV are taken before and after the standard addition. From the mV the sample concentration is
determined.
With Known subtraction, a known standard is added to an ionic sample being measured. The standard reacts
with the measured ion in the sample in a known manner thus removing measured ions from the solution.
From the change in mV the concentration of the sample is determined.