Thermo Fisher Scientific Ion Selective Electrodes Bromide - Manual (Page 10)

Instruction Manual Bromide Electrode
10
Interferences
A layer of silver may form on the electrode surface in the presence of strong reducing agents.
Insoluble salts of silver may be deposited on the membrane, causing electrode malfunction if high
levels of ions forming these salts are present in the sample. Proper performance can be restored by
polishing. See the section
Electrode Response
for proper polishing procedure. Solutions
containing oxidizing agents such as MnO
4
-1
, Fe
+3
and Cu
+2
, may be measured without problem. All
samples must be free of mercury.
The maximum allowable ratio of interfering ion to bromide ion is given in Table 3. This ratio is
expressed as the ratio of the interfering ion molarity to the bromide molarity. Readings will be in
error if this ratio is exceeded. Neither accuracy of the measurement nor surface of the electrode
membrane will be affected if the ratio is less than that listed in the table.
TABLE 3: Maximum Allowable Ratio of Interfering Ion to Bromide Ion
Interference Maximum Ratio for Bromide
OH
-1
(2) 30000
Cl
-1
(1) 400
S
2
O
3
-2
(3) 20
NH
3
(3) 2
I
-1
(1) 2.0X10
-4
CN
-1
(4) 8.0X10
-5
S
-2
(4) 1.0X10
-6
(1) Gran's plot titration can be used to measure mixed halides in solution.
(2) Acidify with 1M HNO
3
to pH 4 to remove hydroxide interference.
(3) These substances represent complexing species whose maximum level can be
exceeded without electrode damage. Values shown represents a 1% error.
(4) Add Ni
+2
to remove sulfide or cyanide interferences.
As an example of Table 3's use when using the bromide ion electrode:
What is the maximum level of iodide tolerable in a sample whose bromide concentration is 10-2M?
Using Table 3, the maximum ratio is:
[I
-1
]
———— = 2X10-4
[Br
-1
]
or [I
-1
] = 2X10
-4
[Br
-1
]
[I
-1
] = 2X10
-4
(1X10
-2
)
[I
-1
] = 2X10
-6
M maximum iodide concentration for no interference