Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 16)

Instruction Manual Fluoride Electrode
- 13 -
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.
Electrode Life
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.