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Analytical Procedures - Fluorine in Gas Exit
Drying Tower
December 19, 2002
Introduction
A
fluorine selective electrode allows fluorine in aqueous solution to be measured quickly,
and accurately. The electrode consists of a single-crystal lanthanum fluorine
membrane, and an internal reference, bonded into an epoxy body. The crystal is an
ionic conductor in which only fluorine ions are mobile. When the membrane is in
contact with a fluorine solution, an electrode potential develops across the
membrane. This potential, which depends on the level of free fluorine ions in
solution, is measured against an external constant reference potential with a digital
pH/mV meter or specific ion meter. The measured potential corresponding to the level
of fluorine ions in solution is described by the Nernst equation. Samples should be
at the same temperature as standard solution. A 1°C difference in temperature will give
about a 2% of measurement error.
This
technique can be used also in the determination of fluorine (as HF) in a gas stream. The
gas is passed through an absorption apparatus where fluorine is selectively absorbed over
an alkaline solution, then the solution is measured to determine fluorine content and the
result is related to the volume of the gas washed.
Equipment
- Isokinetic sampling apparatus
- PTFE
membrane (teflon)
- Stainless steel holder for membrane filter
- pH/mV Meter like ORION ionalyzer for
precision laboratory measurements, equipped with selective fluorine electrode
- Reference electrode (single -junction
reference electrode, filled with equitransferent filling solution)
- Magnetic stirrer - recommended
for laboratory measurements
- Graph paper - (for use with digital pH/mV
laboratory meters) 2 or 4 cycle semiloga-rithmic paper for preparing calibration curves
- Plastic labware - (samples and standards
should always be stored in plastic containers, as fluorine reacts with glass)
- Plastic impingers
Reagents
- Distilled water to prepare all solutions and standards
- Standard solution (100 ppm fluorine
standard solution)
- Total ionic strength adjuster (TISAB II) to
provide a constant background ion strength, decomplex fluorine and adjust solution pH
- Low level TISAB for use when measuring in
samples containing less than 0.4 ppm (2x10-5
M ) fluorine and no fluorine complexing agents
Procedure
Preparation of Low Level TISAB Solution
Place about 500 ml distilled water in a
1000 ml beaker. Add 57 ml of glacial acetic acid and 58 grams of sodium chloride.
Place the beaker in a water bath for cooling and immerse a calibrated pH electrode
into the solution. Slowly add 5 M NaOH until the pH is between 5.0 to 5.5.
Cool to room temperature. Pour into a 1 litre volumetric flask and dilute to mark
with distilled water. It is essential that all reagents used be as pure as
possible to keep the fluorine level in the buffer low.
Sampling
In the isokinetic sampling apparatus set up four impingers containing a total
of 1 litre of NaOH 1 N solution after the filter membrane. Draw about 100 litres of
gas through the apparatus at a flow rate of 5 litres per minute allowing the gas to bubble
through the solution. Gather all the solution together for analysis.
Calibraton Curve
Prepare 0.25, 0.5, 1, and 2 ppm fluorine standards by serial dilution of the
100 ppm standard. Add 50 ml TISAB, to each 50 ml standard. Insert the probe
and record the reading for each standard. Use the data to plot a calibration curve.
Analysis
Draw about 50 ml of the collected solution, adding the same quantity of TISAB
low level solution. Before adding TISAB solution to the sample, if necessary, adjust
pH to 5-6 with acetic acid 4 M. Insert the probe and record the reading. The
fluorine concentration can be determined from the calibration curve.
Calculation
None
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