Thermo Fisher Scientific Ion Selective Electrodes Cadmium - Manual (Page 12)

Cadmium Electrode Instruction Manual
12
Temperature Influences
Samples and standards should be at the same temperature, since the electrode potentials are
influenced by changes in temperature. A 1
o
C difference in temperature results in a 4% error at the
1.0X10
-3
M cadmium concentration. Because of the solubility equilibrium on which the electrode
depends, the absolute potential of the reference electrode changes slowly with temperature. The
slope of the electrode, as indicated by the factor "S" in the Nernst equation, also varies with
temperature. Table 3 gives values for the "S" factor in the Nernst equation for the cadmium ion.
TABLE 3: Temperature vs. Values for the Electrode Slope
Temp.(oC)
"S"
0 27.10
10 28.10
20 29.08
25 29.58
30 30.07
40 31.07
50 32.06
If changes in temperature occur, the electrodes should be re-calibrated. The temperature range for
the Eutech Cadmium Ion Electrode is 0
o
-80
o
C, provided that temperature equilibrium has occurred.
If the temperature varies substantially from room temperature, equilibrium times up to one hour are
recommended.
Electrode Response
Plotting the mV potential against the cadmium concentration on semi-logarithmic graph paper
results in a straight line with a slope of about 27 mV per decade. (Refer to Figure 1.)
The time needed to reach 99% of the stable electrode potential reading, the electrode response time,
varies from several seconds in highly concentrated solutions to several minutes near concentrations
of 1.0X10
-6
M cadmium. Below 10
-6
M, considerably longer response time can be expected. (Refer
to Figure 3.)