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C30F Analytical Chemistry Experiments
Ion Selective Electrode - chloride

Determination of chloride in seawater

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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



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