Chapter
4
|
pH, pH/ISE and pH/LogR Measurement Modules
Incremental Techniques (pH/ISE Module Only)
The V
ERSA
S
TAR
meter with pH/ISE measurement module offers the option for incremental
techniques when in the ISE measurement mode. Preset methods for single
known addition,
single known subtraction, double known addition, double known subtraction, single analate
addition and single analate subtraction are available.
·
When performing incremental techniques, the sample concentration must be known within
an order of magnitude so the increment may be correctly chosen and the sample and
standard must be volumetrically determined.
·
It is very important that any complexing agent is present in excess (at least 50 to 100
times) or not at all. The ratio of free to complexed ions must remain constant over the
addition. An indication that a complexing agent is present, but not in great enough quantity
to maintain a constant free
ion to complexed ion ratio, is an abnormally high or low slope. If
this is a suspected problem, add an excess amount of complexing or decomplexing agent
to the original solution in the beaker.
·
Electrode interferences should be at a minimum, since the effect of the interference might
change as the concentration of the ion of interest changes. This situation may also result in
high or low electrode slopes. Consult the electrode user manual for information on reducing
or eliminating some electrode interferences.
·
Known addition is a useful method for measuring dilute samples, occasional samples or
samples that contain an excess of complexing agent. Known addition can also be used to
verify the results of a direct measurement.
·
Known subtraction is useful when measuring ions for which stable standards do not exist.
Known subtraction can also be used as a quick version of a titration. When performing a
known subtraction, it is necessary to know the stoichiometric ratio between the standard
and sample.
·
Double incremental techniques calculate the electrode slope and sample concentration
simultaneously. The electrode slope is determined directly in the sample, which results in
greater accuracy for samples with complex matrices and greater analysis speed.
·
The Electrode Type must be set for the Ion
Selective Electrode (ISE) that will be used for the
analysis, since the selected Electrode Type
determines the expected slope
range and slope
direction (positive or negative). The Electrode Type
is set using the
ISE Mode Menu
.
48
|
Orion V
ERSA
S
TAR
Meter User Manual
Thermo Scientific
Chapter
4
|
pH, pH/ISE and pH/LogR Measurement Modules
Single Known Addition
In known addition, a standard of known concentration is added to a sample
of unknown
concentration. The total concentration should approximately double upon the addition of the
standard, therefore sample concentration should be known within an order of magnitude. Refer
to the following table to choose the correct standard concentration and volume of addition.
Amount of standard to be added per
100 mL of sample
Standard concentration compared to expected sample
concentration
1 mL
100
times more concentrated
5 mL
20 times more concentrated
10 mL
10 times more concentrated
1.
Prepare the ion selective electrode (ISE) according to the instructions in the electrode user
manual. Prepare all required solutions (ionic strength adjusters, standards, etc) and review
any special requirements outlined in the electrode user manual. Connect the ISE, and any
other electrodes to be used, to the module inputs on the selected channel.
2.
Determine the ISE slope as directed in the electrode user manual.
3.
In
the measurement mode, press the
channel key until the channel with the pH/ISE module
with ISE connected is shown. If needed, press the
mode key until
ISE Measurement is
shown as the measurement mode and use the
ISE Mode Menu
to set the Electrode Type.
4.
In the measurement mode, press the
setup key.
5.
Press the ◄ or ► key to highlight the channel setup menu that the ISE is connected with
(Channel 1, Channel 2, Channel 3 or Channel 4) and press the
f3 (Select) key.
6.
Press the ◄ or ►key to highlight Incremental Techniques and press the
f3 (Select) key.
7.
Press the
▲ or
▼ key to highlight Single Known Addition and press the
f3 (Select) key.
8.
Press the
f3 (Clear) key as needed to delete the displayed slope value. Use the numeric
keypad to enter the ISE slope value determined in step 2 and press the
f2 (Accept) key.
9.
Enter the sample volume using the numeric keypad and press the
f2 (Accept) key.
10.
Enter the total volume (sample volume plus added ISA volume) using the numeric keypad
and press the
f2 (Accept) key.
11.
Enter the standard concentration using the numeric keypad and press the
f2 (Accept) key.
12.
Rinse the ISE and any other electrodes in use with distilled water or rinse solution required
for that electrode. Blot the electrode dry with a lint-free tissue or shake the electrode, place
into the prepared sample and press the
f2 (Next) key.
a.
Follow all electrode rinsing and drying instructions or exceptions in the ISE user
manual. Do not wipe or rub the sensing element of the electrode.
13.
Wait for the mV value on the meter to stabilize and press the
f2 (Accept) key.
14.
Enter the standard volume using the numeric keypad and press the
f2 (Accept) key.
15.
Add the standard to the sample and press the
f2 (Yes) key.
16.
Wait for the mV value on the meter to stabilize and press the
f2 (Accept) key.
Thermo Scientific
Orion V
ERSA
S
TAR
Meter User Manual
|
49