Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 12)

Titration
Titration is a very accurate determination of total lead or sulfate ion concentration. This method
makes use of the electrode as an endpoint detector. The endpoint break is enhanced by the use of
methanol-formaldehyde solution added to samples to reduce the solubility of the product formed
during titration.
Titration of Lead
The method outlined in this section makes use of the lead ion electrode as a highly sensitive
endpoint detector for lead-containing sample. The titrant used is EDTA. The sample concentrations
should be above 1.0X10
-3
M lead ion. If the samples contain lower lead concentrations, the titration
will not be as accurate and the EDTA titrant must be diluted correspondingly.
EDTA complexes lead as well as other cations. The sample pH can be adjusted to eliminate
unwanted ion complexes. Masking agents may be added in some cases.
1.
Prepare a 0.01M EDTA titrant by adding 3.772 grams of reagent-grade Na
2
EDTA·2H
2
O to
a 1 liter volumetric flask containing 500 ml of methanol-formaldehyde solution. Swirl the
flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and
upend the flask several times to mix the solution.
2.
Fill a 50 ml buret with the EDTA solution. Pipet 50 ml of the sample into a 150 ml beaker
and add 50 ml of methanol-formaldehyde solution. Place the beaker on a magnetic stirrer,
and begin stirring at a constant rate.
3.
Position the electrode tips in the solution about halfway between the center of the beaker
and the beaker wall.
4.
Begin adding the EDTA in 0.5 ml to 1.0 ml increments, followed by smaller increments
down to about 0.1 ml to 0.2 ml increments as the potential change increases. Record the
mV potential after each addition. Continue the additions several milliliters past the
endpoint until little change is noted in the mV reading even when adding 0.5-1.0
increments.
5.
Plot the milliliters of EDTA added against the mV potential on standard coordinate graph
paper. The point of greatest potential change is the endpoint. The lead ion concentration
from the unknown is calculated as follows:
Vt Mt
M
Pb
+2
= ⎯⎯⎯⎯
V
Pb
+2
where:
M
Pb
+2 = concentration of lead ion in the sample (moles/liter)
Vt = volume of EDTA added at endpoint
Mt = EDTA concentration (moles/liter)
V
Pb
+2 = volume of unknown sample (50 ml)