Thermo Fisher Scientific Ion Selective Electrodes Iodide User Manual
Page 6
Instruction Manual
Iodide Electrode
6
Iodide ions can be measured in units of moles/liter, parts/million, or any other convenient
concentration unit. Table 1 indicates some of the concentration units.
TABLE 1: Concentration Unit Conversion Factors
moles/liter ppm I
-1
1.0X10
-4
12.7
1.0X10
-3
127.0
1.0X10
-2
1,270.0
1.0X10
-1
12,770.0
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with ISA for all iodide solutions. The temperature of both sample
solutions and of standard solutions should be the same.
Direct Measurement of iodide (using a pH/mV meter)
1.
By serial dilution prepare three standard solutions from the 0.1M or 1,000 ppm standard.
The resultant concentrations should be 10
-2
, 10
-3
, and 10
-4
M or 1,000, 100 and 10 ppm
iodide standards. Add 2 ml of ISA to each 100 ml of standard. Prepare standards with a
composition similar to the samples if the samples have an ionic strength above 0.1M.
2.
Place the most dilute solution (1.0X10
-4
M or 10 ppm) in a 150 ml beaker on the magnetic
stirrer and begin stirring at a constant rate. After assuring that the meter is in the mV mode,
lower the electrode tips into the solution. When the reading has stabilized, record the mV
reading.
3.
Place the mid-range solution (1.0X10
-3
M or 100 ppm) in a 150 ml beaker on the magnetic
stirrer and begin stirring at a constant rate. After rinsing the electrodes in distilled water,
blot dry and immerse the electrode tips in the solution. When the reading has stabilized,
record the mV reading.
4.
Place the most concentrated solution (1.0X10
-2
M or 1,000 ppm) in a 150 ml beaker on the
magnetic stirrer and begin stirring at a constant rate. After rinsing the electrodes in
distilled water, blot dry and immerse the electrode tips in the solution. When the reading
has stabilized, record the mV reading.
5.
Using semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the curve down to about 1.0X 10
-5
M or l ppm. A
typical calibration curve for iodide can be found in Figure 1.