NAVI F-52, Instruction Manual

Page: 243 / 270

Chapter 16 Reference
16.3 Ion measurement
230 HORIBA
Generally, the effect of coexistent ions on the electric potentials at the ion electrode is shown
by the allowable coexistence limit. The smaller is this value, the greater is the effect.
Response time
The response time of ion electrodes varies depending on their types and structures as well as
ion species, ion concentration, and ion strength. In addition, the response time of electrodes of
even the same type varies depending on whether their performance is good or bad and
whether they are new or old. Generally, the response time tends to become longer as the ion
concentration is higher; or it tends to become shorter as the concentration is lower. It also
tends to become shorter when the measurement of low concentration is shifted to that of high
concentration; or it also tends to become longer in the reverse case.
he change of 1 mV in electric potential difference gives a measurement error equivalent to a
concentration error of approximately 4% for univalent ions or 8% for bivalent ions. There-
fore, it is essential to read the displayed electric potential difference or concentration after it
has been stabilized.
Effect of light
The electric potential at a certain type of ion electrode vary with the effect of light (non-crys-
tal solid membrane electrode). When this type of electrode is used, it is not desirable to per-
form measurement in a place where light intensity changes. In this case, it is essential to avoid
light by using a brown-colored beaker or the like.
Generally, the electrodes which are influenced by light are those using silver halide as a main
component in its responsive membrane. (Cl, Br, I, CN, etc.)
Standard solution
To measure the ion concentration of a solution, previously calibrate the ion meter by using a
conditioned standard solution whose concentration is known. The calibration must be per-
formed the number of times required for attaining the desired accuracy. Normally, however, it
should be carried out once every day or before measurement. If the solution is stirred with a
stirrer or the like when calibration and measurement are carried out, the response stability is
improved.
●
To calibrate the pH meter, basically prepare two or more standard solution
whose concentrations are different. When the concentrations the solution to be
measured is roughly known, prepare standard solution so that the concentration
value falls between their concentrations. In this case, it is recommended that the
concentration of the low-end standard buffer be one tenth that of the high-end.
●
If the concentration of the solution to be measured is unknown, this factor should
be larger than in the above case. However, take care that any of the concentra-
tions does not exceed the linearity and detection limits of the ion electrode.
Temperature of standard solution and measured solution
Your pH meter has a temperature compensation capability, but make the temperature of stan-
dard solution during calibration as equal as possible to that of the target solution during mea-
surement.
This is necessary because both the ion electrode and the reference electrode has output fluctu-
ations resulting from temperature changes. The error of the measured value becomes greater
as the difference in temperature between the standard solution and the measured solution
becomes larger.
Handling standard solutions after use
Never put the used standard buffers back to their containers.