Thermo Fisher Scientific Ion Selective Electrodes Iodide - Manual (Page 9)

Instruction Manual Iodide Electrode
9
TABLE 2: Maximum Allowable Ratio of Interfering Ion to Iodide Ion
Interference Maximum Ratio
Cl
-1
(1) 1.0X10
6
S
2
O
3
-2
(2) 1.0X10
5
Br
-1
(1) 5.0X10
3
CN
-1
(3) 4.0X10
-1
S
-2
(3) 1.0X10
-6
(1) Gran's plot titration can be used to measure mixed halides in solution.
(2) These substances represent complexing species whose maximum level can be exceeded
without electrode damage. Values shown represents a 1% error.
(3) Add Ni
+2
to remove sulfide or cyanide interferences.
As an example of Table 2's use when using the iodide ion electrode:
What is the maximum level of iodide tolerable in a sample whose iodide concentration is 10-4M?
Using Table 2, the maximum ratio is:
[Br
-1
]
⎯⎯⎯⎯ = 5X10 3
[I
-1
]
or [Br
-1
] = 5X10
3
[I
-1
]
= 5X10
3
(1X10
-4
)
[Br
-1
] = 5X10
-1
M maximum bromide concentration for no interference
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.
Iodide ions complex with some metal ions. Table 3 lists the levels of complexing metals causing a
20% error.
TABLE 3: Levels of Complexing Agents Causing a 20% Error at 1.0X10
-4
M Iodide
Ion Concentration
Bi
+3
2.0X10
-5
M ( 4 ppm)
Cd
+2
5.0X10
-4
M ( 50 ppm)
Pb
+2
5.0X10
-3
M (1,000 ppm)