Hanna Instruments HI 4012 - Manual (Page 13)

13
This technique is called Known Addition. The method can
use an ideal sensor slope, but actual determined slopes at
the temperature of measurement should be used if known.
The volume and concentration of the added standard must
cause a mV change of at least 8 mV. This method is
preprogrammed in the Hanna HI 4222 pH/ISE/mV meter,
which simplifies the method greatly. The method works
well for samples with high ionic strengths.
Example: Lead ion determination in samples with concen-
trations less than 1 X 10
-3
M using known addition.
1. A 25 mL sample of unknown concentration (Vsample)
is placed in a clean plastic beaker with a lead sensor
with 1 mL of HI 4012-00 ISA (V
ISA=1 mL
) and 25 mL
of the methanol/formalin solution. The stable mV
value (mV 1) is recorded after the sample is mixed.
2. 5mL (Vstd) of 10 -1 M standard (Cstd) is added to the
beaker and the mV value increases as does the con-
centration.
∆
E is calculated as mV2-mV1. The un-
known lead concentration in the original sample
(Csample) can then be determined by the following
equation. Note: The methanol/formalin volume is
included into the volume calculations.
C
sample
=
(V
T
)10 ∆E/S - (V
S’
)
C
standard
V
standard
V
sample
V
S’
(V
sample
+V
standard
+V
ISA
+V
MeOH-Form
)= V
T
(V
sample
+V
ISA
+V
MeOH-Form
)= V
S’
3. The procedure can be repeated with a second stan-
dard addition to verify slope and operation of the
method.
Titration of Sulfate
A Lead electrode may be used as an indicator to follow
the progress and detect the endpoint of a precipitation
titration of sulfate
(with concentration >50 ppm) with
lead perchlorate standard. During the titration the sensor
follows the lead concentration while small additions of
lead perchlorate titrant are added. The lead reacts with