Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 15)

Instruction Manual Lead Electrode
13
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to
±2% can be obtained if the electrode is calibrated
frequently. Factors such as temperature fluctuations, drift, and noise limit reproducibility.
Reproducibility is independent of concentration within the electrode's operating range.
Interferences
A surface layer of silver metal may be formed by strongly reducing solutions. A layer of silver salt
may be deposited on the membrane if high levels of ions forming very insoluble salts are present in
the sample. Proper performance can be restored by polishing. See the section entitled
Electrode
Response
for proper polishing procedure.
The lead ion electrodes do not respond to anions or to most cations. The electrode membrane is
poisoned by solutions containing copper, mercury, and silver. These ions must be absent from the
solution.
If the level of ferric or cadmium ion is less than the level of lead ion, no interference occurs. If the
level of ferric or cadmium ion is more than the level of lead ion, interferences will be present,
resulting in false readings. The ferric ion interference is eliminated by pH adjustment to above pH 4
by the addition of NaOH.
Precipitation and Complexation
Sulfide, phosphate, hydroxide, and other ions precipitate insoluble lead salts. The level of lead ion,
the level of 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 lead ion. The total lead concentration, the concentration of the complexing
species, the solution pH, and the ionic strength all determine the extent of complexation.
Complexation reduces the free lead ion concentration and, since the electrode responds only to free
lead ions, a false reading results.
Temperature Influences
Samples and standards should be within ±1
o
C of each other, since 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 lead ion 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 lead ion.
TABLE 3: Temperature vs. Values for the Electrode Slope
Temp(
o
C) S
0 27.10
10 28.09
20 29.08
25 29.58