Metrohm 781 pH/Ion Meter - Manual (Page 33)

3 Short operating tutorial
780/781 pH/Ion Meter, Manual
23
3.2 Determination of an ion concentration (only
781: Conc mode)
3.2.1 Requirements
Carrying out a calibration followed by the direct potentiometric determi-
nation of fluoride is described as an example. As this tutorial is only in-
tended to provide a first introduction to ISE measurement with the 781
pH/Ion Meter, the automatic addition of the standard solutions is omit-
ted in order to limit the number of accessories required. Further possi-
ble ways of carrying out a potentiometric fluoride determination, such
as the automatic addition of the standard for calibration or standard
addition, are described in Application Bulletin 82.
The following instruments, accessories and solutions are required:
• 781 pH/Ion Meter (2.781.0010)
• 801 Magnetic Stirrer (2.801.0010)
Other stirrers can also be used (see Section 5.6).
• Fluoride ISE (6.0502.150) with cable (6.2104.020)
• Ag/AgCl reference electrode (6.0726.100) with cable (6.2106.020)
Electrolyte filling: inner and outer c(KCl) = 3 mol/L
• Temperature sensor (e.g. Pt 1000, 6.1110.100) with cable
(6.2104.140)
• Fluoride standard solution (6.2301.030)
c(NaF) = 0.1000 mol/L (1900 ppm F - )
• TISAB solution
NaCl, Na-acetate buffer pH 5.5; for preparation see below
3.2.2 Preparations
To prepare the TISAB solution dissolve 58 g NaCl in approx. 500 mL
dist. water and add 57 mL glacial acetic acid. Adjust the pH of the buff-
er with 5 M NaOH to 5.5. Then make up the solution to 1 L with dist. wa-
ter.
Prepare the following five fluoride standard solutions from the stock so-
lution: 0.5, 2.5, 5.0, 25.0 and 50.0 ppm. All standards should consist of
a 1:1 mixture of dist. water and TISAB buffer.
Before you start this tutorial you must ensure that the instrument and
accessories have been correctly installed as described in Section 2.
The most important points concerning the installation are briefly given
again below (please refer to the Section mentioned for details). If you
are not certain whether all the settings ( CONFIG and PARAMETER Conc )
correspond to the original conditions you can first reset any alterations
by initializing the memory ( RAM Initialization ) (see Section 8.5).