Thermo Fisher Scientific Ion Selective Electrodes Copper - Manual (Page 16)

Instruction Manual Copper Electrode
16
the precipitated ion, and the pH of the sample determine
formation of a precipitate.
A wide variety of species, including acetate, ammonia, amino
acids, citrate, cyanide, and EDTA, form complexes with cupric
ion. The total cupric concentration, the concentration of the
complexing species, the solution pH, and the ionic strength all
determine the extent of complexation. Complexation reduces the
free cupric ion concentration and, since the electrode responds
only to free cupric ions, a false reading results.
Temperature Influences
Samples and standards should be within
∀1
o C of each other, since
the electrode potentials are influenced by changes in
temperature. A 1
o C difference in temperature results in a 4%
error at 1.0X10
-3
M cupric ion concentration. Because of the
solubility equilibria 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 cupric ion.
TABLE 3:
Temperature vs. Values for the Electrode Slope.
Temp. (
o
C) 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 electrode(s) should be re-
calibrated.
The temperature range for the Cupric 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 cupric ion concentration on
semi-logarithmic graph paper results in a straight line with a
slope of about 27 mV per decade. Refer to Figure 1.