Instruction Manual
Cyanide Electrode
9
4.
On a semi-logarithmic paper, plot the concentration (log axis) against the millivolt reading
(linear axis) as in Figure 1. Reserve the final solution for checking the electrode each hour.
5.
To a 150 ml plastic beaker, add 100 ml of sample and 1 ml of ISA. Place the beaker on the
magnetic stirrer and begin stirring at a constant rate. After rinsing the electrodes, blot dry
and lower the electrode tips into the solution. After stabilization of the reading, read the
mV potential and determine the concentration from the calibration curve. A new low level
calibration curve should be prepared daily using fresh standards.
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors like temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is
independent of concentration within the electrode's operating range.
Interferences
A layer of silver metal may form on the electrode surface in the presence of strongly reducing
solutions, such as photographic developer. Ions forming very insoluble silver salts will cause the
electrodes to malfunction if present in solution at sufficiently high levels to form a layer of silver
salt on the membrane surface. Electrode performance can be restored by polishing if the surface of
the sensing element becomes contaminated. See section
Electrode Response
for proper polishing
procedure.
Solutions containing oxidizing agents such as Fe
+3
, Cu
+2
, and MnO
4
- will not affect electrode
performance. All samples must be free of mercury.
The maximum allowable ratio of interfering ions to cyanide ions is given in Table 3. The ratio is
expressed as the ratio of the interfering ion concentration in moles per liter to the sample cyanide
concentration in moles per liter. Readings will be in error if this ratio is exceeded. Neither the
accuracy of the measurement nor the 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 Cyanide Ion
Interferences Maximum Ratio
Cl
-1
1X10
6
Br
-1
5X10
3
I
-1
1X10
-1
S
-2
must be absent
When using the cyanide ion electrode, an example of the use of Table 3 follows:
What is the maximum level of bromide allowable in a sample whose cyanide concentration is 1X10
-
5
M?