Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 13)

Instruction Manual Lead Electrode
11
Titration of Sulfate
Titration of sulfate ion with lead perchlorate make use of the lead ion electrode as a sensitive
endpoint detector. Sulfate determinations by the gravimetric or turbidimetric methods are more
complicated and more time consuming than titration. Titration offers the same or greater precision
in solutions as dilute as 10
-4
M or 10 ppm sulfate ion. If present in amounts in excess of the
following,
NO
3
- ∼ 50 X SO
4
-2
Cl-
∼ 50 X SO
4
-2
HCO3- ∼ 100 X SO
4
-2
at pH 4,
the above ions will interfere with the titration. Phosphate and calcium must be absent.
The titrant is Eutech Lead Standard, 0.1M, Code no. EC-SCS-PB1-BT, and should be diluted to the
proper range for the expected concentration of the unknown. The methanol-formaldehyde solution
is used to dilute the unknown 1:1 before performing the titration.
The concentration of lead perchlorate titrant should be about 10 times greater than the expected
sulfate ion concentration of the unknown. Unknowns containing about 10
-3
M sulfate ion are ideal
for this titration method. If the sulfate samples are more dilute, the lead perchlorate titrant should
be correspondingly more dilute.
1. Prepare 0.01M lead perchlorate titrant by pipeting 100 ml of the 0.1M lead standard into a
one liter volumetric flask. Fill to the mark with distilled water, cap, and upend several
times to thoroughly mix the contents.
2.
Into a 150 ml beaker, pipet 50 ml of sample and 50 ml of methanol-formaldehyde solution.
Place the beaker on the magnetic stirrer and begin stirring at a constant rate.
3.
Fill a 50 ml burette with the lead perchlorate titrant. Position the electrode tips in the
solution about halfway between the center of the beaker and the beaker wall.
4.
Begin adding the titrant 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 ml
increments.
5.
Plot the milliliters of lead perchlorate added against the mV potential on standard
coordinate graph paper. The point of greatest potential change is the endpoint. (See Figure
4.)