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Instruction Manual 

 

Fluoride Electrode

 

 

6

 
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. 
 

 
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 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. 
 

 

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.