Thermo Fisher Scientific Ion Selective Electrodes Chloride - Manual (Page 13)

Instruction Manual Chloride Electrode
Chloride ions complex with some metal ions. Table 4 lists the levels of complexing metals causing
a 10% error at 1.0X10
-4
M chloride.
TABLE
4: Levels of Complexing Agents Causing a 10% Error at 1.0X10
-4
M Chloride
Ion Concentration
Bi
+3
4.0X10
-4
M (80 ppm)
Cd
+2
2.0X10
-3
M (200 ppm)
Mn
+2
2.0X10
-2
M (1100 ppm)
Pb
+2
2.0X10
-3
M (400 ppm)
Sn
+2
6.0X10
-3
M (700 ppm)
Tl
+3
4.0X10
-5
M (8 ppm)
Temperature Influences
Samples and standards should be within ±1C of each other, since electrode potentials are influenced
by changes in temperature. Because of the solubility equilibrium on which the electrode depends,
the absolute potential of the reference electrode changes slowly with temperature. The slope of the
electrode, as indicated by the factor "S" in the Nernst equation, also varies with temperature. Table
5 gives values for the "S" factor in the Nernst equation for the chloride ion.
TABLE
5: Temperature vs. Values for the Electrode Slope
Temp. (oC) "S"
0 54.2
10 56.2
20 58.2
25 59.2
30 60.1
40 62.1
50 64.1
If changes in temperature occur, the electrodes should be re-calibrated. The temperature range for
the chloride ion electrode is 0
o
-80
o
C, provided that temperature equilibrium has occurred. If the
temperature varies substantially from room temperature, equilibrium times up to one hour are
recommended.
Electrode Response
Plotting the electrode mV potential against the chloride concentration on semi-logarithmic paper
results in a straight line with a slope of about 56 mV per decade. (Refer to Figure 1.)
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