Thermo Fisher Scientific Ion Selective Electrodes Silver - Manual (Page 8)

Silver/Silver Sulfide Electrode Instruction Manual
8
Mercury cannot be present in silver samples. Since HgS and Hg
2
S are insoluble, no mercury will be
present in sulfide samples. Other interference should be absent. If they are present, use the
procedures found in the
Interference
and
Electrode Response
sections to remove them.
Units of Measurement
Silver and sulfide concentrations are measured in units of parts per million, equivalents per liter,
moles per liter, or any other convenient concentration unit. Table 1 indicates some of the
concentration units.
TABLE 1: Concentration Unit Conversion Factors
ppm
S
-2
ppm Ag
+1
N(S
-2
) M(Ag
+1
)
321.0 1079.0 2.00X10- 2 1.00X10- 2
100.0 --- 6.23X10- 3 3.12X10- 3
32.1 108.0 2.00X10- 3 1.00X10- 3
--- 100.0 1.84X10- 3 9.27X10- 4
3.2 10.8 2.00X10- 4 1.00X10- 4
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should
be made the same by adjustment with SAOB for all sulfide solutions and with ISA for all silver
solutions. The temperature of both sample solution and standard solution should be the same.
Direct Measurement of Silver (using a pH/mV meter)
1.
By serial dilution of the 0.1M or 1,000 ppm standards, prepare 10
-2
, 10
-3
, and 10
-4
M or 100
and 10 ppm silver standards. Add 2 ml of ISA per 100 ml of standard. Prepare standards
with a composition similar to the samples if the samples have an ionic strength above
0.1M.
2.
Place the most dilute solution (1.0X10
-4
M or 10 ppm) on the magnetic stirrer and begin
stirring at a constant rate. After assuring that the meter is in the mV mode, lower the
electrode tips into the solution. When the reading has stabilized, record the mV reading.
3.
Place the midrange solution (1.0X10
-3
M or 100 ppm) on the magnetic stirrer and begin
stirring. After rinsing the electrodes with distilled water, blot dry and immerse the
electrode tips in the solution. When the reading has stabilized, record the mV reading.
4.
Place the most concentrated solution (1.0X10-
2
M or 1,000 ppm) on the magnetic stirrer
and begin stirring. After rinsing the electrodes with distilled water, blot dry and immerse
the electrode tips in the solution. When the reading has stabilized, record the mV reading.