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

Sodium Electrode Instruction Manual
8
Rb
+1
1.1X10
4 ppm
1.1X10
5 ppm
--
NH
4
+1
1.8X10
3
ppm 1.8X10
4 ppm
--
Ag
+1
0.0001 ppm 0.001 ppm 0.01 ppm
Tl
+1
4.5X10
3 ppm
4.5X10
4 ppm
--
Temperature Influences
Samples and standards should be at the same temperature, since electrode potentials are influenced
by changes in temperature. A 1
o
C difference in temperature results in a 2% error at the 10
-3
M level.
Because of 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 4 indicates the variation of
theoretical slope with temperature.
Provided that temperature equilibrium has occurred, the sodium ion electrodes can be used at
temperatures from -5
o
to 70
o
C. Room temperature measurements are recommended, since
measurements at temperatures markedly different from room temperature may require equilibrium
times up to one hour. The electrode should not be used at temperatures above 70
o
C, since damage to
the membrane may result.
TABLE 4: Temperature vs. Values for the Electrode Slope
Temperature
(
o
C) "S"
0 54.20
10 56.18
20 58.16
25 59.16
30 60.15
40 62.13
50 64.11
Electrode Response
Plotting the electrode mV potential against the sodium concentration on semi-logarithmic paper
results in a straight line with a slope of about 56 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 one minute or less for sodium concentrations above 1.0X10
-5
M to several minutes near
the detection limit. (Refer to Figure 2.)