Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 10)

Low Level Lead Determination (using a pH/mV meter)
This procedure is recommended for solutions with lead concentrations of less than 1.0X10
-6
M. If
the solution is high in ionic strength, but low in lead ion concentration, use the same procedure, but
prepare a calibration solution with a composition similar to the sample.
1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water. This low level ISA
(1.0M NaClO
4
) is added at the rate of 1 ml low level ISA to each 100 ml of solution. The
background ionic strength will be 1.0X10
-2
M.
2.
Dilute 1ml of 0.1M standard to one liter to prepare a 1.0X10
-4
M solution for measurements
in moles per liter. Prepare a 10 ppm standard solution by diluting 1 ml of the 1,000 ppm
standard to 100 ml for measurements in ppm. Standards should be prepared fresh daily.
Plastic lab-ware is recommended to avoid absorption of lead on the beaker walls.
3.
Add 50 ml of distilled water, 50 ml of methanol-formaldehyde solution, and 1 ml of low
level ISA to a 150 ml plastic beaker. Place the beaker on the magnetic stirrer and begin
stirring at a constant rate.
4.
Place the electrode tips in the solution. Assure that the meter is in the mV mode.
5.
Add increments of the 1.0X10
-4
M or 10 ppm standard as given in Table 2 below.
6.
After the reading has stabilized, record the mV reading.
TABLE 2: Step-wise Calibration For Low Level Lead Measurements
Added Concentration
Step Pipet Volume (ml) ppm M
1 A 0.1 1.0X10
-2
1.0X10
-7
2 A 0.1 2.0X10
-2
2.0X10
-7
3 A 0.2 4.0X10
-2
4.0X10
-7
4 A 0.2 6.0X10
-2
6.0X10
-7
5 A 0.4 1.0X10
-1
9.9X10
-7
6 B 2.0 2.9X10
-1
2.9X10
-6
7 B 2.0 4.8X10
-1
4.8X10
-6
Pipet A = 1 ml graduated pipet
Pipet B = 2 ml pipet
Solutions: additions of 10 ppm or 1.0X10
-4
M standard to 100 ml of solution prepared in
Step 3 above
7. On a semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis) as in Figure 1.
8.
Rinse the electrodes and blot dry.
9.
Measure out 50 ml of the sample into a 150 ml plastic beaker. Add 50 ml of methanol-
formaldehyde solution and 1 ml of low level ISA. Place the beaker on the magnetic stirrer
and begin stirring at a constant rate. Lower the electrode tips into the solution. After the