Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 13)

Instruction Manual Nitrate Ion Electrode
11
E = Ep - S log X
where: E = measured electrode potential
Ep= reference potential (a constant)
S = electrode slope (~56 mV/decade)
X = level of nitrate ions in solution
The activity, X, represents the effective concentration of the ions in solution. The total nitrate ion
concentration, C
t
, is the sum of free nitrate ion, C
f
, and complexed or bound perchlorate ion, C
b
. The
electrode is able to respond to only the free ions, whose concentration is :
C
f
= C
t
- C
b
Since nitrate ions form very few stable complexes, the free ion concentration may be equated to the
total ion concentration.
The activity is related to the free ion concentration, C
f
, by the activity coefficient, g, by:
X = g C
f
Activity coefficients vary, depending on total ions strength, I, defined as:
I = ½ SC
x
Z
x
²
where: C
x
= concentration of ion X
Z
x
= charge of ion X
S = sum of all of 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, g, 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 (ISA) is
added to samples and standards. The recommended ISA for nitrate is (NH
4
)
2
SO
4
. 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 nitrate 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 inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally across the
solution boundary resulting in a potential difference between the two solutions, since ions diffuse at
different rates. When making measurements, it is important to remember that this potential be the same
when the reference is in the standardizing solution as well as in the sample solution or the change in
liquid junction potential will appear as an error in the measured electrode potential.
The composition of the liquid junction filling solution in the reference electrode is most important. The
speed with which the positive and negative ions in the filling solutions diffuse into the samples should
be as nearly equal as possible, that is, the filling solution should be equitransferent. No junction
potential can result if the rate at which positive and negative charge carried into the sample is equal.
Strongly acidic (pH = 0-2) and strongly basic (pH = 12-14) solutions are particularly troublesome to
measure. The high mobility of hydrogen and hydroxide ions in samples make it impossible to mask