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ISE - Chloride
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Determination of chloride in sea water by ISE
Introduction
The operation of the chloride ion selective electrode (ISE) to be used
in this experiment is based on the response of a disc of sensing
element, which is sealed into one end of an epoxy tube (Baily 1980).
The tube is filled with an internal filling solution, which contacts
the inner surface of the sensing element, as well as an internal
electrode which provides a stable electrode potential against which the
potential of the sensor element is measured. An external reference
electrode is also connected in series with the chloride ISE, and may be
incorporated into the ISE with a salt bridge connecting the ISE and
reference electrodes.This arrangement, where the ISE and external
reference electrodes are housed in a single electrode body is refered
to as a combination electrode. (Orion Chloride Combination Electrode
Instruction Manual 1990)
Theory
When the electrode is placed in a solution containing chloride ions, an
electrode potential develops across the seneing element, the magnitude
of which depends on the chloride ion activity in the test solution.This
potential is measured with a digital mV meter or an ISE meter, and
follows the Nernst equation as follows:
E = E0 ±S/N log A where;
E =measured electrode potential
Eo = reference electrode potential (constant)
A = chloride ion activity in test solution
S = slope of the electrode (usually 56-60 mV / decade concentration
change)
n = charge on ion
For chloride ion, n = 1, so that slope is negative and equation for the
chloride ISE reduces to
E = E0 - S log A
where S = slope of the
calibration curve (theoretically 59.5mV / decade concentration
Standard calibration curve:
A plot of E vs log A will produce a calibration curve, which will be
linear over a range of analyte concentrations, the range depending on
the performance of the electrode. Non-linear responses and longer
equilibration times are usually observed at low chloride
concentrations.
The chloride ion activity (A) in solution, is dependent on the ionic activity of the solution and may be related to the total free chloride ion concentration (Cf) by the ionic activity coefficient (γ) according to the following equation: A = γCf
Ionic activity coefficients (γ) are variable and depend heavily on
total ionic strength of the solution, which may be calculated
as:
Ionic strength = 0.5 ∑CiZi²
where Ci = concentration of ion i and ZI = charge
on ion i
If the ionic strength of the test solution can be maintained at a high
and constant level for both standard and sample solutions, the activity
coefficient of the solutions can be correspondingly held constant.
Under these conditions, the activity of the chloride ion can be
directly propportional to the chloride ion concentration in
solution.
For this purpose, a constant volume of Ionic Strength Adjuster (ISA),
which is a neutral, concentrated, non-interfering salt solution (e.g.
5M sodium nitrate used in this experiment) is added to similar volumes
of both samples and standards.
A plot of E vs log C of standard chloride solutions thus provides a
calibration curve, from which chloride concentrations of samples may be
interpolated, based on their respective potentiometric
responses.
Standard addition for determination of chloride in
samples:
The potentimetric response (Ei) of the ISE in a sample
solution may be obtained for interpolation from a standard calibration
curve as previously described.
If a measured volume (Vs) of a chloride standard solution of
concentration (Cs) is added to a known volume
(V0)of the sample solution, then a new response
(Ef) is obtained, due to the standard added. The change of
chloride concentration (CΔ) in the sample solution is
proportional to the change (ΔE = ef - Ei) in
potentiometric response (Moody and Thomas 1971).
The original sample concentration (C0) may thus be
calculated from the following equation:
C0 = CΔ [1/(antilog ΔE/S) - 1]
Procedure
Preparation of chloride standard solutions in distilled water (no
ISA included)
A stock standard 1M chloride solution will be provided, as well as a 5M
ISA solution.
Into 100ml volumetric flasks, pipette 10ml of 1M chloride solution,
make to volume with distilled water and mix well. This provides
your 10-1M chloride standard solution in distilled water.
Into a second 100ml volumetric flask, pipette 10 ml of the 10-1M
chloride standard solution, make to the mark with distilled water and
mix well. This provides your 10-2M chloride standard solution in
distilled water.
Continue with this serial dilution of the chloride standards to prepare
10-3M, 10-4M, and 10-5M standard solutions
respectively in distilled water.
Preparation of chloride calibration standards (ISA
included)
Into 100ml volumetric flasks, pipette 10ml of 1M chloride solution, 2
ml of ISA solution,make to volume with distilled water and mix well.
This provides your 10-1M chloride standard calibration
solution in distilled water.
Into a second 100ml volumetric flask, pipette 10 ml of the chloride
standard solution made up in distilled water only.Then add 2 ml of ISA
solution, make up to the mark with distilled water and mix well. This
provides your 10-2M chloride standard calibration
solution.
Continue with this serial dilution of the distilled water chloride standards, including ISA, to prepare your 10-3M,
10-4M, and 10-5M standard calibrationsolutions
respectively.
Determination of chloride in sample provided:
(1) Sample preparation:
A sample of seawater will be provided. Pipette out triplicate 1 ml
volumes into 100ml volumetric flasks, add 2 ml of ISA to each, make to
volume with distilled water and mix thoroughly.Prepare a sample blank
by adding only the ISA solution and making up to volume.
(2) ISE measurements:
When not in use, the chloride ISE will be kept immersed in distilled
water.
Before placing the electrode in a test solution, remove the electrode
from its storage container, rinse with distilled water and gently wipe
free of water with a strip of clean absorbent tissue. Be careful not to
scratch the sensor membrane surface.
Measurements will be made in 100 ml beakers in which solutions wil be
stirred using a magnetic stirrer.Pour approximately 50 ml of each
solution into a beaker, insert a small magnetic stirring bar into the
solution, and place the beaker in the center of the magnetic stirring
plate. Adjust the speed of rotation of the stirring magnet until the
solution is steadily stirred, without creating a vortex.
(3) Standard chloride solutions:
Place the electrode into the10-5M chloride solution, start a
stop watch and observe the potentiometric (mV) response, until a stable
reading is obtained. Note this value and the time taken to achieve it.
Remove the electrode from the test solution, rinse with distilled water
and dry with a tissue as before.
Replace the 10-5 standard calibration solution with the
10-4 chloride standard calibration solution and repeat the
process, followed by the other standards in increasing order of
concentration, recording the respective responses and equilibration
times.
(4) Sample solutions
Also, immediately after obtaining the initial response (Ei), add to
each of the sample solution a calculated volume of an appropriate
standard (see demonstrator), allow to mix into the sample solution and
record the new equilibrated value(Ef) and time.
Calculations
Standard calibration curve:
Plot log concentration of chloride standard solutions vs the
corresponding mV values.
Use your calibration curve to determine the chloride concentrations of
sample solution. Using the dilution factors and atomic mass of
chloride, calculate the chloride concentration of the seawater sample
in parts per thousand (ppt).
Standard addition
Using the standard equation above, calculate the chloride sample
concentrations. Convert to ppt concentrations as above.
Exercises
(1) Are the results obtained by the two methods significantly different
at the 95% confidence level?
(2) Explain the reasons for the difference between results obtained by
calculation procedures above.
(3) Suggest three other applications of the chloride
ISE.
Bibliography
1. Bailey PL (1980) Analysis with Ion-Selective Electrodes, Second
Edition, Heyden (London UK) 80-124
2. Moody GJ andThomas JDR (1971), Selective Ion Sensitive Electrodes,
Merrow (Bath, UK) 50-55
3. Orion Combination Chloride Electrode Instruction Manual, Model 94-17B
(1990) Orion Research Inc.(Mass., USA) 1-30
delloyd infolab
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