Thermo Fisher Scientific Ion Selective Electrodes Cadmium - Manual (Page 6)

Cadmium Electrode Instruction Manual
6
Units of Measurement
Cadmium ion concentrations are measured in units of parts per million, moles per liter, equivalents
per liter, or any other convenient concentration unit. Table 1 indicates some of these concentration
units.
TABLE
1: Concentration Unit Conversion Factors
ppm Cd
+2
moles/liter
1124.0 1.0X10
-2
112.4 1.0X10
-3
11.2 1.0X10
-4
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. The temperature of both sample solutions and of
standard solutions should be the same.
Direct Measurement of Cadmium (using a pH/mV meter)
1. By serial dilution prepare three standard solutions from the 0.1M, 1,000 ppm or 100 ppm
standard. The resultant concentrations should be 10
-2
, 10
-3
, and 10
-4
M or 1,000, 100 and 10
ppm 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 100 ml of 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. After the reading has stabilized, record
the mV reading.
3.
Place 100 ml of 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 with
distilled water, blot dry and immerse the electrode tips in the solution. After the reading
has stabilized, record the mV reading.
4.
Place 100 ml of 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 with distilled water, blot dry and immerse the electrode tips in the solution.
After the reading has stabilized, record the mV reading.
5.
Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against
concentration (log axis). Extrapolate the curve down to about 1.0X10
-5
or 1 ppm. A typical
calibration curve can be found in Figure 1.