Thermo Fisher Scientific Ion Selective Electrodes Copper - Manual (Page 20)

20
Nernst equation:
E = E
o
+ S logX
where:
E = measured electrode potential
E
o
= reference potential (a constant)
S = electrode slope (
−27mV)
X = level of cupric ions in solution
The activity, X, represents the effective concentration of free
cupric ions in solution. Both bound, C
b
, and free, C
f
, cupric ions
are included in the total cupric ion concentration, C
t
. The
cupric ion electrode will only respond to free cupric ions, the
concentration of which is:
C
f
= C
t
- C
b
The activity is related to the free cupric ion by the activity
coefficient,
γ , by:
X =
γ Cf
Activity coefficients vary, depending on total ionic strength, I,
defined as:
I =
23C
x
Z
x
2
where:
C
x
= concentration of ion X
Z
x
= charge of ion X
3
= sum of all the types of ions in the
solution
In the case of high and constant ionic strength relative to the
sensed ion concentration, the activity coefficient,
γ , is
constant and the activity, X, is directly proportional to the
concentration.
To adjust the background ionic strength to a high and constant
value, ionic strength adjuster is added to samples and standards.
The recommended ISA solution for the cupric electrodes is sodium
nitrate, NaNO
3
. Solutions other than this may be used as ionic
strength adjusters as long as ions that they contain do not
interfere with the electrode's response to cupric ions.
The reference electrode must also be considered. When two
solutions of different composition are brought into contact with
one another, liquid junction potentials arise. Millivolt
potentials occur from the interdiffusion of ions in the two
solutions. Electrode charge will be carried unequally across the