Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 10)

Instruction Manual Fluoride Electrode
- 7 -
Prepare 2, 1, and 0.5 ppm fluoride standards by serial dilution of the 100 ppm fluoride standard. To
each 50 ml of standard, add 50 ml of TISAB 1 or TISAB 2. Calibrate the meter as previously
described by using the 2, 1, and 0.5 ppm standards. The calibration curve should be drawn on semi-
logarithmic 2-cycle graph paper or use an ion meter.
Direct Measurement of Fluoride in Acid Solutions
Hydrogen ion complexes a portion of the fluoride ion in solutions with a pH below 5, forming HF
or HF
2
-1
, which cannot be detected by the electrodes. Adjustment to weakly acidic/weakly basic
range before making the fluoride determination is necessary, but not with strongly basic solutions.
The use of sodium acetate buffers the pH above 5 and helps fix the total ionic strength of standards
and samples to the same level.
1.
Dissolve reagent grade sodium acetate (CH
3
COONa) in distilled water to prepare a 15%
solution. Prepare a sufficient quantity to dilute all standards and samples.
2.
Prepare a blank solution containing all components of the sample except fluoride. This
solution will be used to prepare standards.
3.
Add fluoride to the blank solution to prepare standards in the concentration range of the
unknown solutions. If using a standard pH/mV meter, prepare three standards and a
calibration curve as previously described. If an ion meter is used, only two standards are
necessary. Add 9 parts of sodium acetate to each 1 part of standard. Fresh standards should
be prepared every two weeks if the standard contains less than 10 ppm fluoride.
4.
Calibrate the electrodes as described in the section Electrode Slope Check .
5.
After diluting each unknown sample 10:1 with sodium acetate (as in 3 above), measure the
mV potential and determine the fluoride concentration.
Direct Measurement of Fluoride in Alkaline Solutions
Hydroxide ions interfere with fluoride measurements in basic solutions with a low fluoride content.
At a fluoride concentration less than 1.0X10
-4
M and at a pH of 9.5 or above, the electrode potential
reading, caused by the concentration of both hydroxide and fluoride ions, is higher than it would be
if fluoride alone were present. (See section entitled pH Effects.)
Using a 4M buffered potassium acetate solution to adjust the pH to between 5 and 6 eliminates
hydroxide ion error and raises the total ionic strength of both standards and samples to the same
value. The fluoride ion concentration can be determined in the usual manner after both standards
and samples are diluted 10:1 with the buffer solution.
1. To prepare a 4M buffered potassium acetate solution, add one part of distilled water,
slowly, to two parts of 6M acetic acid, CH
3
COOH, in a large beaker surrounded by a water
bath. Slowly add 50% KOH solution to the acetic acid mixture, with constant stirring,
until a pH of 5 is reached. Prepare enough buffer to dilute all standards and samples 10:1.
2.
Following the directions given in the preceding section
(Determination of Fluoride in Acid
Solutions ), prepare standards, calibrate the electrodes and measure the unknown samples.