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Instruction Manual 

 

Fluoride Electrode

 

 

16

 

 
When the level of hydroxide is greater than one-tenth the level of fluoride ion present, hydroxide 
ion interferes with electrode response to fluoride. As an example, no hydroxide interference with 
fluoride measurements take place at pH 7 when the hydroxide concentration is 1.0X10

-7

 or less. As 

the pH increases, the hydroxide interference becomes appreciable. At pH 10, hydroxide ion 
concentration is 1.0X10

-4

M and no error is found in measurements of 1.0X10

-2

M fluoride. At the 

same hydroxide ion concentration and a fluoride concentration of 1.0X10

-4

M, about a 10% 

measurement error appears. At a fluoride concentration of 1.0X10

-5

M, considerable error exists in a 

pH 10 solution.  Figure 5 illustrates these errors. 
 

 
The addition of TISAB 1 or TISAB 2 to all fluoride samples and standards buffers the pH between 
5.0-5.5 to help avoid hydroxide interference or the formation of hydrogen complexes of fluoride.  
 

 
A fluoride electrode will last six months in normal laboratory use. On-line measurements might 
shorten operational lifetime to several months. In time, the response time will increase and the 
calibration slope decreases to the point calibration is difficult and electrode replacement is required.