Hach-Lange 1200-S sc - Manual (Page 8)

6
General information
This voltage is expressed by the following Nernst equation:
E = Eo – (2.3 RT/F)xlog a[H+]
E = Eo – (gradient) xlog a[H+]
Where the following applies:
With every unit change in the pH (or decimal change in the ion concentration), the
EMF produced by the pair of electrodes changes by 59.16 mV at 25 °C. This
value is termed the gradient of the electrode.
The pH electrode pair is calibrated using solutions with a known and constant
hydrogen ion concentration. These solutions are termed buffer solutions. The
buffer solutions are used for the calibration of both the electrode isopotential and
the gradient.
2.3.2 REDOX measurement
In a Redox system the measurements are made using a balanced instrument that
comprises a Redox electrode and a reference electrode. The potential to be
measured is termed the Redox potential and depends on the relationship between
the activities of the two components of a Redox system and the number of
electrons transferred. In many cases, the pH value of the solution also affects the
potential.
The half-cell potential εB of the reference electrode has a large effect on the
potential E of the measuring chain. To rectify this effect, the potential of the
measuring electrode can be referred to the hydrogen electrode. If
εB is the half-
cell potential of the reference electrode used, the calculation is performed using
the following formula:
ε(H) = E + εB
Such standardised Redox potentials provide a certain amount of information on
the oxidisation or reduction potential of a Redox system. Increasing positive
values indicate increasing oxidation energy. The more negative the potential, the
higher the reduction energy. The area that is of interest in practice is between
+1500 and –1000 mV.
Standard potentials for a Redox system for aOx = aRed (a=activity) and for pH = 0
can be defined. This is turn corresponds to a standardised hydrogen ion activity of
aH+ = 1 mole per litre.
The stability and ability to reverse a Redox system have a significant effect on the
reproducibility of the Redox potential measured.
E
Eo
R
T
a[H+]
F
=
=
=
=
=
=
EMF value of the cell
Zero voltage (isopotential) of the system (depends on the internal
design of the glass electrode and reference electrode)
Gas constant
Temperature in Kelvin
Activity of the hydrogen ions
(corresponds to the concentration of the hydrogen ions)
Faraday constant
ε(H)
εB
=
= EMF value of the cell
Half-cell potential