Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 18)

The activity, X, represents the effective concentration of free lead ion in the solution. Both bound,
Cb, and free, Cf, lead ions are included in the total lead ion concentration, Ct. The lead ion
electrode will only respond to free lead ions, the concentration of which is:
Cf = Ct - Cb
The activity is related to the free lead ion concentration, Cf, by the activity coefficient, γ, by:
X =
γ Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½
Σ CxZx 2
where:
Cx = concentration of ion X
Zx = charge of ion X
Σ = 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,
γ , is constant and the activity, X, is directly proportional to the concentration. The lead
ion activity coefficients depend, to some extent, on the anions present. Pure lead nitrate and lead
perchlorate solutions do not display the same activity coefficient, even though both solutions have
the same total ionic strength.
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 solution for the lead electrodes is sodium
perchlorate, NaClO
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 lead 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 solution diffuse into the sample
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 their effect on the junction potential with any concentration of an equitransferent salt. One
must either calibrate the electrodes in the same pH range as the sample or use a known increment
method for ion measurement.