NAVI F-52, Instruction Manual

Page: 240 / 270

Chapter 16 Reference
16.3 Ion measurement
F-52, 53, 54, 55 227
interaction between ions and solvent differs from that in an infinitely diluted solution. As the
distance between ions becomes shorter. (infinitely larger in an infinitely diluted solution), the
electrostatic interaction is gradually intensified.As a result, the ionic behavior differs from
that in the ideal solution and consumes extra energy. Therefore, the ion concentration must be
shown as practical one in actual solutions. This practical concentration is called activity (a),
which is obtained by multiplying ion concentration (Cx) by activity coefficient (
γ
).
a=
γ ・
Cx
The activity coefficient (
γ
) decreases as the ion concentration increases.
(Example) This relation in univalent ions is shown in the table below:
Ion strength
Since the activity coefficient varies with the effect of ion strength, it causes a measurement
error. Therefore, the ion strength of the solution under measurement must be maintained at a
certain level. Generally, this is attained by adding indifferent base which does not react with
the target ions, nor influence the electric potential under measurement. the type and added
amount of this indifferent base vary depending on the species and concentration of the target
ions.
(Example)
For the calibration curve for the fluoride ion electrode in Fig.3, potassium nitrate in an
amount of 0.1 mol/L was added to the sample solution to maintain the ion strength.
Effect of pH
The applicable pH range is determined by the type and structure of the ion electrode used.
Generally, this range becomes narrower as the concentration of the target ions decreases.
The component of the reactive membrane on some ion electrodes may dissolve or electric
potential may fluctuate with the effect of pH, depending on pH values. In addition, the effect
of pH may lower the sensitivity of the ion electrodes or the calibration curve may horizontally
shift. in order to avoid these effects, previously obtain the proper pH value and them maintain
it.
Ion concentration
(Cx)
mol/L
Activity
coefficient
(
γ
)
10
-5 0.998
10
-4 0.988
10
-3
0.961
10
-2
0.901
10
-1 0.751