Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 11)

Instruction Manual Nitrate Ion Electrode
9
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 the solubility equilibrium on which the electrode depends, the absolute potential of the
reference electrode changes slowly with temperature. The slope of the nitrate electrode, as indicated by
the factor "S" in the Nernstian equation, also varies with temperature. Table 4 gives values for the "S"
factor in the Nernst equation for the nitrate ion.
The operating range of the nitrate ion electrode is 0
o
-40
o
C, provided that temperature equilibrium has
occurred. If the temperature varies substantially from room temperature, equilibrium times up to one
hour are recommended.
TABLE 4: Temperature vs. Values for the Electrode Slope
Temp (
o
C) "S"
0 54.20
10 56.18
20 58.18
25 59.16
30 60.15
40 62.13
50 64.11
Electrode Response
Plotting the mV potential against the nitrate 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 in highly concentrated solutions to several minutes near the detection
limit. (Refer to Figure 2.)