NAVI F-52, Instruction Manual

Page: 241 / 270

Chapter 16 Reference
16.3 Ion measurement
228 HORIBA
To make appropriate the pH value of the solution under measurement, previously prepare a
pH buffer which does not interfere the ion electrode (does not contain interfering ions) and
add a small amount of this buffer to the solution to be measured. For the appropriate pH range
for each ion electrode, refer to the instruction manual which comes with that electrode.
(Example)
Fig.4 Effect of pH on fluoride ion electrode
Effect of temperature
The electric potential slope measured by the ion measurement method is influenced by the
temperature of the solution under measurement in accordance with the Nerst formula. As the
solution temperature increases, the electric potential slope becomes larger; as the solution
temperature decreases, the slope becomes smaller. According to the Nerst formula, every time
the solution temperature increases by 10
℃
, a change of approximately 2 mV occurs for uni-
valent ions or a change of approximately 1 mV for bivalent ions. This requires making the
temperature of the standard solution as equal as possible to that of the solution under mea-
surement when preparing a calibration curve.
When you actually prepare a calibration curve at various solution temperature, you will find
that electric potential stays constant at a certain point. This point is called an isothermal cros-
spoint. When you carry out measurement under temperature compensation with an ion meter,
you must know the isothermal cross point value for the ion electrode used. In the present
stage, the commercially available ion electrodes have no written information on their isother-
mal cross point values. In addition, the isothermal cross point values of some ion electrodes
may not be found even if they are checked or you may be unable to find which cross point
should be used if multiple cross points are obtained. In such a case, more accurate measure-
ment can be carried out when the temperature compensation capability is not used. However,
Influence by pH
EMF(ele
c
tromotive fo
rce)
Temp.( ℃
) 0
10 20 25 30 40 50
Univalent
ions 54.20 56.18 58.16 59.16 60.15 62.13 64.11
Bivalent
ions 27.10 28.09 29.08 29.58 30.08 31.07 32.06