Thermo Fisher Scientific Ion Selective Electrodes Silver - Manual (Page 20)

Silver/Silver Sulfide Electrode Instruction Manual
20
5. If using jeweller's rouge, place a cotton ball on the 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 silver nitrate solutions is 1M. In the presence of other ions, the
upper limit of detection is above 1.0x10
-1
M silver, 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 1M and 1.0x10
-1
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 and Figure 2 for a comparison of the theoretical response to the actual response at
low levels of sulfide and silver.
pH Effects
Silver reacts with hydroxide ions to form a precipitate of Ag
2
O, in ammonia-free basic solutions.
By keeping all solutions slightly acidic, this can be avoided. Adjust the pH of silver solutions below
8, if necessary, with 1M HNO
3
.
Bisulfide ion (HS
-1
) and hydrogen sulfide (H
2
S) result when hydrogen ion complexes sulfide ion.
Larger amounts of sulfide ion are complexed as the pH is lowered. The use of SAOB in all samples
containing sulfide maintains a fixed level of S-
2 ions, since the free sulfide ion (S
-2
) exists in only
very basic solutions. In the acid range, sulfide is chiefly in the form of H
2
S, while in the pH range
6-12, almost all the sulfide is in the HS- 1 form.
Electrode Life
A silver/sulfide ion electrode will last six months in normal laboratory use. On-line measurements
might shorten operational lifetime to several months. In time, the response time will increase and
the calibration slope decreases to the point calibration is difficult and electrode replacement is
required.
Electrode Storage
The silver/sulfide electrode may be stored for short periods of time in 1.0x10
-2
M silver (or sulfide)
solution. For longer storage (longer than two weeks), rinse and dry the sensing pellet and cover the
membrane tip with any protective cap shipped with the electrode. The reference portion of the
combination electrode (or the outer chamber of the reference electrode) should be drained of filling
solution, if refillable, and the rubber insert placed over the filling hole.