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The Nernst equation can be rewritten:
ION SELECTIVE ANALYSIS METHODS
Direct AnalysisDirect Analysis
Direct AnalysisDirect 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 4222 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 & MEASUREMENTS” 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 MethodsIncremental Methods
Incremental MethodsIncremental 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 4222 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.