Accumet XL 20, User Manual, Page: 188 / 199

177
Appendix:
ISE Theory
XL25, 50 and 60 meters
This form of the Nernst equation states that the electrode potential varies directly with the log of
the concentration in straight line manner, and results are then expressed in the preferred
concentration units. The slope of the line is equal to the value of:
Slope = S = 2.3RT/nF
The table below gives theoretical slope values at 25°C.
Species Slope (mV/decade) Species Slope (mV/decade)
Monovalent cation +59.16 Divalent cation +29.58
Monovalent anion -59.16 Divalent anion -29.58
The Nernst equation thus reduces to:
E = E* + S [log c]
Ion-selective Analysis Methods
Direct Analysis
In this method, a sample concentration is determined by measuring the electrode potential and
comparing to those obtained in standard solutions. Normally, an ionic strength adjustor and/or pH
adjustor must be added to both the samples and the standards. This helps in that the closer the
standard solution matrix is to the sample matrix, the more valid is the analysis. A number of
standard solutions are used to standardize a pH/mV/ion meter, or to construct a calibration curve
(by plotting the electrode’s output in mV versus the log of the concentration). The sample
concentration is then read from the meter or the calibration curve.
The advantages of the direct method of analysis are:
1. Many samples (particularly those with relatively simple ionic backgrounds) can be quickly
measured after a single calibration procedure.
2. The technique is valid over a wide range of concentrations.
3. Sample volume values are not required, and may also vary over a wide range.