Thermo Fisher Scientific Ion Selective Electrodes Copper - Manual (Page 18)

18
table top and flatten it using the bottom of a beaker.
6. Put 1-2 drops of distilled or deionized water in the
center of the cotton pad.
7. Add a small amount of jeweller's rouge to the damp
cotton.
8. Continue with Steps 3 and 4 above.
Limits of Detection
The upper limit of detection in pure cupric nitrate solutions is
0.1M. In the presence of other ions, the upper limit of
detection is above 1.0x10
-2
M copper, but two factors influence
this upper limit. Both the possibility of a liquid junction
potential developing at the reference electrode and the salt
extraction effect influence this upper limit. Some salts may
extract into the electrode membrane at high salt concentrations,
causing deviation from the theoretical response. Either dilute
samples between 0.1M and 1.0x10
-2
M or calibrate the electrode at 4
or 5 intermediate points.
The lower limit of detection is influenced by the slight water
solubility of the electrode pellet. Refer to Figure 1 for a
comparison of the theoretical response to the actual response at
low levels of cupric ion. Neutral solutions containing free
cupric ions can be measured down to 1.0X10
-8
M (0.0006 ppm).
Extreme care must be taken with measurements below 1.0X10
-5
M (0.6
ppm) to avoid contamination of samples.
pH Effects
Figure 5 shows how the electrode responds to cupric ion in sol-
ution at various pH levels.
At a high pH, free cupric ion precipitates with hydroxide ion,
thereby reducing the cupric ion concentration. The maximum pH at
which cupric concentration can be measured without interference
from hydroxide is given by a solid line to the left of the lined
area on Figure 5. Within this lined area, cupric ion combines
with hydroxide to form Cu(OH)
2
. Since only free cupric
concentration can be measured with the cupric ion electrodes, a
false reading results.