Metrohm 781 pH/Ion Meter - Manual (Page 38)

3.2 Determination of an ion concentration (only 781: Conc mode)
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780/781 pH/Ion Meter, Manual
3.2.4 Direct measurement of fluoride
10 mL of the sample solution (e.g. 10 g table salt in 100 mL dist. water)
is also treated with 10 mL TISAB solution. This should first be diluted
1:1 with dist. water. This produces the calculation parameters which are
to be defined (see Section 6.5.4). As the measurement of the ions is
carried out unstirred, the stirrer is only used before the measurement to
ensure that the sample solution is properly mixed.
1 Set method parameters
• In the Conc Mode define the stirrer control as a measuring pa-
rameter (). Additionally the stirrer can also be
switched on and off manually before the measurement with
.
The stirring rate can only be set when a Metrohm 8XX Stirrer is
connected to the MSB interface.
• The calculation parameters () are then:
2 Determine measured value directly
• Immerse electrodes in the sample.
• Switch on stirrer with and – after sufficient time
has elapsed – off again.
• Wait until the electrodes have stabilized; i.e. until the drift sig-
nal in the display no longer blinks.
• Read off the value. This gives the fluoride concentration of the
sample solution without the addition of TISAB.
STIRRER
2
PARAMETER Conc
:
> calculation parameters
: smpl.size 10.0 mL
V total 20.0 mL
factor 1.0
smpl.size unit: mL
PARAM
PARAMETER Conc
:
> measuring parameters
: :
stirrer: control
stirring rate 5
prestir pause 0 s
stir time 15 s
poststir pause 5 s