Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 12)

Instruction Manual Fluoride Electrode
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Follow the previous procedure for fluoride measurements with an ion meter, except use low level
TISAB 2 instead of TISAB 1. Always use the blank correction.
Titration
Titration is a very accurate determination of fluoride. Eutech Fluoride Ion Electrodes can be used as
highly sensitive endpoint detectors for titration of a fluoride-containing sample. Though titration are
more time consuming than direct ion measurements, the results are more accurate and reproducible.
Titration accurate to ±0.2% of the total fluoride concentration of the sample can be performed using
lanthanum nitrate as the titrant. Total fluoride concentration should be at least 1.0X10
-3
M for
endpoint detection. Low results are given if aluminum, iron, or trivalent chromium are present at a
level of 1% or higher.
Special titration procedures for aluminum, lithium, lanthanum, and thorium also makes use of the
fluoride electrode as an endpoint indicator.
Titration Procedure for Fluoride Determination
1. Dissolve 43.3 grams of reagent grade lanthanum nitrate, La(NO
3
)
3
.6H
2
O, in about 500 ml
distilled water in a 1 liter volumetric plastic flask. Fill to the mark with distilled water.
This 0.1M lanthanum solution will be used for all titration.
2.
Using the 0.1M fluoride standard, standardize the lanthanum nitrate by titration. To a 150
ml plastic beaker, add approximately 9.0 ml of fluoride standard (accurately measured) and
about 50 ml of distilled water. Place the beaker on the magnetic stirrer and begin stirring.
Lower the electrode tips into the solution.
3.
Using a 10 ml plastic burette, add the La(NO
3
)
3
titrant in 0.5-1.0 ml increments. Record the
mV reading against the volume of titrant added. As the mV potential change increases, add
smaller increments, down to 0.1-0.2 ml increments. Continue to add titrant and record the
mV potential against the volume until little change is noted in the mV reading even when
adding 0.5-1.0 ml increments.
4.
Using linear graph paper, plot the mV readings (y-axis) against the volume (x-axis). The
end-point is determined as the steepest slope on the titration curve. Record the endpoint.
o
Vt
5.
To a 150 ml plastic beaker, add approximately 9.0 ml of sample solution (accurately
measured) and about 50 ml of distilled water. Place the beaker on a magnetic stirrer and
begin stirring. Lower the rinsed, dried electrode tips in the solution.
6.
Titrate the sample as in step 3 above. The endpoint is denoted as x
x
Vt
7. Calculate the sample concentration, Cs
x o
x Vt Vf o
Cs = ⎯⎯⎯⎯⎯ . 2Cs
o x
Vt Vf