Activity Versus Concentration - Radiometer Analytical ISE25Pb Mode D'emploi

Activity Versus Concentration

The lead ion activity, a
the following equation:
Where f is the activity coefficient. In very dilute solutions,
Pb2+
f is by definition 1. Hence a
Pb2+
For concentrations up to 0.01 M, f
accurately by means of the Debye-Hückel equation:
-log f
Where I is the total ionic strength of the solution (see the follow-
ing), and A and B are constants which are 0.5115 and 0.3291
respectively at 25°C. å is an ion size parameter which is approxi-
mately 4.5 for Pb
By inserting the constants and z = +2 (the valency of the lead
ion), the equation can be simplified to:
Thus, it can be seen that the ionic strength is an important
parameter in measurements with ion-selective electrodes. The
ionic strength expresses the concentration of all ions in the
solution in terms of both their molarity and their charges. The
ionic strength is calculated as follows:
As all ion species present are included in the summation evalu-
ating I, f will remain almost constant for solutions with a high
Pb2+
background concentration of ions. This means that in most
cases the divergence between activity and concentration can be
overcome by adding an electrolyte which gives a high and
constant ionic strength to both the sample and the calibration
solutions without interfering. In the "Guidelines for Sample
Preparation" section, information is provided concerning the
addition of electrolyte.
is related to concentration c
Pb 2+
a = f
2+
Pb
Pb
2 • √I/ (1 + B • å • √I)
= A • z
Pb2+
2+
.
= 2 • √I / (1 + 1.5 • √I)
-log f
Pb2+
I = 1/2 • Σ c
12
• c
2+ 2+
Pb Pb
= c .
2+ 2+
Pb
can be determined fairly
Pb2+
2
z
i i
by
2+
Pb