Thermo Fisher Scientific Ion Selective Electrodes Bromide - Manual (Page 11)

Instruction Manual Bromide Electrode
11
Complexation
Total concentration (C
t
) consists of free ions (C
f
) and complexed or bound ions (C
c
) in solutions:
C
t
= C
f
+ C
c
Since the electrode only responds to free ions, any complexing agent in the solution reduces the
measured concentration of ions.
Bromide ions complex with some metal ions. Table 4 lists the levels of complexing metals causing
a 20% error at 1.0X10
-4
M bromide.
TABLE 4: Levels of Complexing Agents Causing a 20% Error at 1.0X10
-4
M Bromide
Ion Concentration
Bi
+3
4.0X10
-4
M (80 ppm)
Cd
+2
2.0X10
-3
M (200 ppm)
Pb
+2
8.0X10
-3
M (1,600 ppm)
Sn
+2
2.0X10
-2
M (2,400 ppm)
Tl
+3
2.0X10
-5
M (4 ppm)
Temperature Influences
Samples and standards should be at the same temperature since electrode potentials are influenced
by changes in temperature. A 1
o
C difference in temperature results in a 2% error at the 1.0X10
-3
M
concentration level. Because of solubility equilibrium on which the electrode depends, the absolute
potential of the reference electrode changes slowly with the temperature. The slope of the electrode,
as indicated by the factor "S" in the Nernst equation, also varies with temperature. Table 5 gives
values of the "S" factor in the Nernst equation for the bromide ion.
TABLE 5: Temperature vs Values for the Electrode Slope
Temp (
o
C) "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 recalibrated.
The temperature range of the Eutech Bromide Ion Electrodes is 0
o
- 80
o
C, provided that temperature
equilibrium has occurred.