Thermo Fisher Scientific CyberScan pH 5000 - Manual (Page 139)

Instruction Manual CyberScan pH5500/ 5000
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
Monovalent anion
+59.16
-59.16
Divalent cation
Divalent anion
+29.58
-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 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.
Incremental Methods
These procedures can reduce errors caused by temperature
variations, complex matrices, and complexation. They are useful for
applications where only occasional samples are analyzed.
Incremental methods include: Known Addition, Known Subtraction,
Analate Addition, and Analate Subtraction.
134