Instruction Manual
Lead Electrode
5
The temperature of the standard solutions and of the sample solutions should be the same and below
80
o
C. About a 4% error in the slope will occur for each 1oC difference in temperature.
Interferences should be absent. If they are present, use the procedure found in the
Interferences
and
Electrode Response
sections to remove them.
Units of Measurement
Lead concentrations are measured in units of ppm as lead, moles per liter, or any other convenient
concentration unit. Table 1 indicates some concentration units and conversion factors.
TABLE 1: Concentration Unit Conversion Factors
ppm
Pb
+2
M
ppm SO
4
-2
20.7
1.0X10
-4
9.6
207.0
1.0X10
-3
96.0
2070.0
1.0X10
-2
960.0
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 ISA. The temperature of both sample solution and standard
solution should be the same.
Direct Measurement of Lead (using a pH/mV meter)
1.
By serial dilution, prepare 10
-2
M, 10
-3
M, and 10
-4
M or 100 ppm and 10 ppm standards,
from the 0.1M or 1,000 ppm standards. Prepare standards with a composition similar to
the samples if the samples have an ionic strength above 0.1M.
2.
Place 50 ml of the 10
-4
M or 10 ppm standard in a 150 ml beaker and add 50 ml of
methanol-formaldehyde solution. Place the beaker on the magnetic stirrer and begin
stirring at a constant rate. Add 2 ml of ISA. 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 50 ml of the 10
-3
M or 100 ppm standard in a 150 ml beaker and add 50 ml of
methanol-formaldehyde solution. Place the beaker on the magnetic stirrer and begin
stirring. Add 2 ml of ISA. 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 50 ml of the 10
-2
M or 1,000 ppm standard in a 150 ml beaker and add 50 ml of
methanol-formaldehyde solution. Place the beaker on the magnetic stirrer and begin
stirring. Add 2 ml of ISA. 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.