Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 7)

Instruction Manual Fluoride Electrode
- 4 -
Sample Requirements
All samples must be aqueous and not contain organics which can dissolve the epoxy electrode body
and/or the cement bonding the sensing crystal to the electrode body. Inorganic solutions will not
affect the electrode. Infrequent measurements in solutions containing methanol, acetone, or dioxane
are permitted. Highly polar solvents, such as CHCl
3
or DMF, should not be contained in the
samples. Please check with Eutech Instruments Pte Ltd. before using the electrode in other organic
solvents.
The addition of TISAB to samples and standards will adjust the pH to 5.0-5.5. Samples must be
above pH 5 to avoid forming complexes with hydrogen ions and below pH 7 to avoid interference
by hydroxide ions.
The temperature of the standard solutions and of the sample solutions should be the same and below
80
o
C. The use of TISAB 1 also preferentially forms complexes with aluminum and with iron,
breaking the complexes that fluoride forms with these ions. With 1 ppm fluoride present, up to 3-5
ppm aluminum or iron is complexed. If higher levels of aluminum or iron are present, use TISAB 3.
Units of Measurement
Fluoride concentrations are measured in units of ppm as fluoride, moles per liter, or any other
convenient concentration unit. Table 1 indicates some concentration units and conversion factors.
TABLE 1: Concentration Unit Conversion Factors
ppm F
-1
moles/liter
190.0 1.0X10
-2
M
19.0 1.0X10
-3
M
1.9 1.0X10
-4
M
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with TISAB for all fluoride solutions. The temperature of both
sample solution and standard solution should be made the same.
Direct Measurement of Fluoride (using a pH/mV meter)
1. By serial dilution, prepare three standard solutions from the 0.1M, 1,000 ppm, or the 100
ppm stock standard. The resultant concentrations should be 10
-2
M, 10
-3
M, and 10
-4
M or
100, 10, and 1 ppm. Add 50 ml of TISAB 1 or TISAB 2 to each 50 ml of standard. When
calibrating, assume that the added TISAB has no effect on the standard concentration.
2.
Place the most dilute solution on the magnetic stirrer and begin stirring at a constant rate.
After assuring that the meter is in the mV mode, lower the electrode tips into the solution.
After the reading has stabilized, record the mV reading.