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

Silver/Silver Sulfide Electrode Instruction Manual
18
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors like temperature fluctuations, drift, and noise limit reproducibility. Reproducibility is
independent of concentration within the electrode's operating range.
Interference
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. Performance may be restored by polishing. See section
Electrode Response
for proper
polishing procedure.
All silver samples must be free of mercury. Sulfide samples will not have mercury present due to
the extreme insolubility of HgS and Hg
2
S. Biological samples and protein in food interferes with
silver measurements, but the protein interference can be removed by acidifying to pH 2-3 with 1M
HNO
3
.
Complexation
Total concentration (Ct) whether sulfide or silver ions, consists of free ions (Cf) and complexed or
bound ions (Cc) in solution:
Ct = Cf + Cc
Since the electrode only responds to free ions, any complexing agent in the solution reduces the
measured concentration of ions. Silver ions complex with many species, notably cyanide,
thiosulfate, ammonia and chelants such as EDTA.
Sulfide ions form complexes with hydrogen ions (HS
-1
and H
2
S). Sulfide ions also form soluble
complexes with elemental sulfur (S
2
-2
, S
3
-2
, S
4
-2
, etc.) and tin, arsenic, and antimony ions.
Temperature Influences
Samples and standards should be within ±1oC of each other, since electrode potentials are
influenced by changes in temperature. 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 each ion.
TABLE 3: Temperature vs. Values for the Electrode Slope
Temp (oC) S
-2
Ag
+1
0 27.1 54.2
10 28.1 56.2
20 29.1 58.2
25 29.6 59.2
30 30.1 60.1
40 31.1 62.1