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Analytical Procedures - Fluorine in Gas Exit Drying Tower
December 19, 2002
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.
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.
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.
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.
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.