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Analysis of Anions by Chromatography (IC) and ion selective electrodes (ISE)

Prac Report

Aim-

The aims of the experiment are to calculate the amount of fluoride and chloride in a

range of samples including toothpaste Berocca and an unknown solution using Ion

Chromatography (IC) and ion selective electrodes (ISE). The experiment also aims to

estimate to limits of detection of each method.

Method-

Method is as per method described in lab manual.

Results-

A tables summarising the results obtained is shown below. The calculations are provided

with the attached sheets. The results will be discussed further in the results section.

Table 1:

Sample ISE ISE IC IC

[Cl-]

(ppm) [F

-] (ppm) [Cl

-]

(ppm) [F

-] (ppm)

Unknown 45±0.6 28.2±0.05 28±1 26±3

Toothpaste A 5±0.5 21.9±0.05 9±1 21±3

Berocca n/a n/a 151±1 n/a

Tap Water 14±0.5 1.2±0.05 20+/-3 0.4±3

Table.1 : Analysis of standard solution & sample for F&CL by using Ionic Chromatography.

Sample No Sample conce

(ppm)

F Ret.Time

(Min)

F Peak area (µS/min)

CL (peak area)

(µS/min

CL Ret.Time

(min) CL)

1 0 0.0491 2.62 0.097 3.75

2 0.1 0.3327 2.62 0.343 3.76

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3 10 3.906 2.62 3.070 3.75

4 25 9.720 2.63 8.117 3.74

5 30 11.110 2.63 9.574 3.73

6 40 14.521 2.64 13.038 3.72

7 50 17.563 2.64 16.308 3.71

8 Unknown

9.408 2.62 9.212 3.71

9 Tap Water 0.373 2.61 6.512 3.71

10 Toothpaste 7.647 2.61 3.004 3.73

11 Borroca

0.020 2.62 0.492 3.74

Table.1: Analysis of standard solution & sample for F&CL by using ISE.

Sample No Sample (ppm)

Log[Conce]

F (mV)

CL (Rmv)

1 0.1 -1 137.8 263.2

2 1 0 80.2 242.5

3 10 1 19.8 199.5

4 25 1.397940009 -4.3 171.3

5 30 1.477121255 -8.6 171.0

6 40 1.602059991 -17.4 161.9

7 50 1.698970004 -24.1 158.4

8 unknown ------ -7.8 164.0

9 Tap Water ----- 74.5 184.1

10 Toothpaste ------ -1.4 204.0

11 Borroca ------- a/n 105.8

Graph1: The calibration curve for fluoride (Concentration (ppm) vis Peak area (µS/Min))

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Graph2: The calibration curve for chloride (Concentration (ppm) vis Peak area (µS/Min))

Graph3: The calibration curve for Flouride (Log (ppm) vs ( Mv)

Graph4: The calibration curve for chloride ((Log (ppm) vs ( Mv))

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Discussion/Questions-

The results obtained with ISE and IC methods are shown in the tables and graphs above.

The calculated errors for the ISE method are significant larger for the chloride

analysis then for fluoride. The main reason for this can be attributed to the calibration

curve. The calibration curve for fluoride is very linear with an R2 value of

0.9997(graph.3). The calibration curve for chloride is not as linear with an R2 value of

0.9692(graph.4). The main error in the chloride calibration curve lies in the

measurements made at lower concentrations, the measurements at 0.1 ppm was below

the linear while the 1 & 10 ppm were above of the linear relationship (graph.4) . When a

trend line ( chloride) was fitted to all these points the line has an R2 value less than

would be obtained if these three points were not involved. The deviation from linearity

results in higher sx values obtained and hence a higher estimate for the precision is

obtained. The most likely reasons for the errors in the measurements at lower

concentrations of chloride was that the electrode was not given sufficient time to

equilibrate. Although the electrodes were allowed significant time (about 10 minets) to

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equilibrate before the measurements were taken the output never reached a steady

value as it was always changing slowly. This meant accurate measurements could not be

made as the instrument never reached a steady output. The other likely reason for the

errors in measurements of chloride concentration that the amount of solution (25.5 ml)

in electrode beaker was nearly enough only to cover the electrode heads in comparing

with the nearly double amount of solution (45ml) in fluoride electrode case.

The precision of the chloride measurements could be improved by allowing the

electrodes more time to stabilise in solution or by applying certain statistical tests to

justify rejecting the points at lower concentrations to give a more linear line and hence a

smaller sx value.

The sensitivity of ISE towards each analyte can be obtained from the slope of the

calibration curve. As the calibration curve plots the instrument response vs the log of

the concentration a more negative slope will mean a higher sensitivity towards the

particular analyte. the sensitivities of ISE towards Cl- and F- in units of mv/ log(ppm)

are -40.71 and –59.828 respectively(graph. 3&4). This indicates the lanthanum fluoride

(LaF3) electrode used to measure fluoride is more responsive to changes in fluoride

concentration than the (AgCl ) chloride electrode is to changes in chloride concentration.

The limits of detection of the chloride and fluoride ISE’s were not calculated.

The chloride ISE uses a reference electrode that consists of a double junction

silver/silver chloride cell. Reference electrodes are used to complete the redox reaction

and to provide a constant potential against which the potential for the electrode that

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responds to the analyte can be measured against. The silver/silver chloride reference

electrode consists of a silver wire coated with silver chloride and immersed in a solution

containing concentrated (KCl) saturated with silver chloride. The redox reaction in the

cell occurs as the (AgCl) coated on the wire receiving an electron causing chloride to go

into solution and vice versa. The fluoride concentration in the berocca solution was

found to be very low concentration as such the result is not reported.

During the analysis of fluoride with the ISE it was extremely important that the

pH of the solution was controlled. If the pH is greater than 8 hydroxide ions can

interfere with the measurement. The hydroxide ions can diffuse into the electrode and

cause the electrode to respond as if fluoride was present leading to incorrect

calculations of the fluoride content in the solution. At low pHs undissociated HF may

form which does not cause the a response in the elcetrode. It is for this reason that a

Total Ionic Strength Adjustment Buffer (TISAB) was added to all fluoride solutions to

be analysed by ISE. The TISAB contains some citrate to bind all Fe(III) ions and a pH

buffer to reduce the effect of hydroxide ions or HF on the measurement of fluoride in

the solutions.

The calibration curves for the IC data were very linear with R2 values of 0.9997

and 0.9974 for chloride and fluoride respectively (graph2&1). The precision obtained

for the fluoride measurements were mostly similar between the ISE and IC methods in

unknown and toothpaste samples . The errors for fluoride in IC were larger than those

obtained in ISE, the reason for this is due to the lower R2 value for the IC calibration

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curve than the ISE curve resulting in larger sx values being obtained. However the

result obtained was not excepted for Tap water because the floured concentration

from ISE was larger by triple than it obtained from IC.

The precision obtained for the chloride measurements were not similar between the

ISE and IC methods in unknown , toothpaste and tap water samples . The differences

between the measurements might be due to the lower R2 value for the ISE calibration

curve than the IC curve(graph.2&4) . Therefore , It can be said that the IC is more

accuracy method compare to ISE.

The limit of detection (LOD) of the IC method was not measured. In order to obtain

accurate results using a conductivity detector for IOC it is crucial a suppressor column

is used so that the conductivity of the electrolyte solution does not mask the

conductivity of the analytes. In this investigation a Na2CO3 resin is used. As fluoride and

chloride anions pass through then column they exchange with the carbonate anions in the

column. The released carbonate anions then travel through the column. If they did not

pass through a suppressor column as they reach the detector a huge conductivity will be

measured which will mask the signal of the analyte. Instead the carbonate passes

through the suppressor column where it reacts with the H2SO4 to produce the weak acid

H2CO3 which will have a much lower conductivity the carbonate anion thus allowing good

detection of the analytes.

Conclusion:

Both ion chromatography and ion selective electrodes were successfully used to

measure the concentration of chloride and fluoride anions in a variety of solution.

However, IC has many advantages in operation than ISE .

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References:

1. Harris ,D C,2007, Quantitative chemical analysis,(7th Ed) W H Freeman.

2- Rundle, C C, 2000, A Beginners Guide toIon-Selective Electrode Measurements, http://www.nico2000.net/Book/Guide1.html 3- USEPA, 1996, Method 9214: Potentiometricdetermination of fluoride in aqueous samples with an

ion-selective electrode 3- USEPA, 1996, Method 9212: Potentiometric determination of chloride in aqueous samples with

an ion-selective electrode

No. of points log[Cl

-]

(ppm) y (mV) xy x^2 d d^2 6

-1 266.1 -266.1 1 -5.63746 31.78097

0 238.2 0 0 5.953512 35.4443

1 201.6 201.6 1 8.844485 78.22491

1.301029996 183.4 238.6089 1.692679 2.532452 6.413314

1.397940009 170.1 237.7896 1.954236 -6.94048 48.17022

1.698970004 160.4 272.5148 2.886499 -4.75251 22.58635

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

0 0 0 0

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0 0 0 0

0 0 0 0

0 0 0 0

Sums 4.397940009 1219.8 684.4133 8.533414 4.55E-13 222.6201

+/-

S.d. (y) 7.460229 D= 31.85861

m= -39.491 3.237534253

b = 232.2465 3.861003228

Measured y 191.1

Measured 1 times

Calc. X 1.041921

-0.205611627

Sample ISE IC

[Cl

-]

(ppm) [F

-]

(ppm) [Cl

-]

(ppm) [F

-]

(ppm) Unknown 20+/-4 27+/-1 20+/-3 27+/-5 Toothpaste

A 9+/-4 23+/-1 8+/-3 18+/-5 Toothpaste

B 8+/-4 15+/-1 6+/-3 13+/-4 Berocca 177+/-5 <LOQ 131+/-7 <LOD Tap Water 13+/-4 <LOQ 20+/-3 5+/-5

Method ISE IC Cl

- F- Cl

- F-

LOD (ppm) 2 3 0.8 0.3

LOQ (ppm) 6 9 2.54 1

y = -39.491x + 232.25 R² = 0.9738

0

50

100

150

200

250

300

-1.5 -1 -0.5 0 0.5 1 1.5 2

mV

log[Cl] (log ppm)

Cl Caibration Curve

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Sensitivity

Recoveries ISE IC

Cl- F- Cl

- F-

Berocca 11132% n/a 8240%

Toothpaste A n/a

55+/-2% n/a

43.6+/-12%

Toothpaste B n/a 41+/-3 n/a

35.9+/-10%

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lab manual

INTRODUCTION One measure of the health of a natural water system is the level of dissolved salts (ionic

substances) it contains. Although conductivity measurements can be used, the concentration

of individual ions in natural water systems is of considerable environmental interest. For

example high concentrations of chloride contribute to salinization of the water. High

concentrations of sulfate ion may indicate that the body of water is receiving significant

quantities of runoff in which contains sulfuric acid (such as that from acid sulfate soils).

High concentrations of nitrate and phosphate ions may indicate that the body of water is

receiving run-off from lawns or agricultural land which often are treated with high levels of

fertilizer. These nutrients stimulate the growth of algae which in turn contribute to

eutrophication (oxygen depletion) of the body of water and subsequent death of aquatic

animals. Fluoride is added to drinking water and toothpaste to inhibit dental caries; it is also

present in effluents from many industrial processes, e.g. the manufacture of fluoro-

polymers like Teflon.

There are many different methods of anion analysis. Two of the most widely used in industry

today are Ion Chromatography (IC) and Ion Selective Electrodes (ISEs).

Direct potentiometric measurements using ISEs provide a rapid, convenient and relatively

inexpensive method for determining a number of anions (and cations). ISEs are available for

measuring more than 20 different cations (e.g., Ag+, Na

+ , K

+, Ca

2+, Cu

2+) and anions (e.g., F

-,

Cl-, S

2-, CN

-). These probes are widely used inside and outside of laboratories. They are

readily adapted to automatic and/or continuous recording of analytical measurements within

labs, within industrial plants where monitoring is needed, and in environmental settings. Page

11

IC is particularly useful in the analysis of mixtures of cations or mixtures of anions. By using

an ion exchange medium (either cation or anion) as the stationary phase in what amounts to a

liquid chromatography system, separation of the ions in solution can be achieved. This

permits IC to carry out multi-analyte determinations on a single sample, which has almost

made it the method of choice for anion analysis.

In this practical you will apply both of these methods to the analysis of chloride and fluoride

in several different water samples, as well as in the determination of the fluoride and

chloride contents of toothpaste. Page 12

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SAMPLES

Prior to undertaking the analysis with either method, you will need to collect and prepare

the required samples. Once prepared, you should use the same samples in both weeks.

Collect your unknown in a rinsed 250.0 mL volumetric flask, and make up to volume. In

addition, collect a sample of tap water. To prepare your toothpaste sample:

1. The amount of toothpaste required will depend on the manufacturer – refer to the

approximate weights provided for each brand. Accurately weigh the required amount (usually

1.5 – 2.0 g) of toothpaste into each of two 100 mL plastic digestion tubes. Add about 50 mL

of water to each vessel.

2. Heat the mixture for 10 minutes. Cool the solution, quantitatively transfer the liquid to a

100.0 mL volumetric flask and dilute to volume.

3. With the remaining beaker, stir well and quantitatively transfer the liquid to a 100.0 mL

volumetric flask and dilute to volume.

4. Prepare the “Borrocca samples as per “typical” serve described on the side of the

container.

EXPERIMENTS Carry out the ISE and IC experiments in your groups in the order determined by the lab

schedule. Complete the calculations required for each method.

REPORTING Your report should provide all calibration graphs and estimates of uncertainties in the

determinations carried out with both methods.

You should present your results for each sample in a comparison table, and provide a

discussion of the relative merits of each analytical method. Don’t forget to consider the

commonly used analytical figures of merit (such as accuracy, precision, selectivity, limit of

detection) in your discussion. You should also comment on any differences you observed

between the two toothpaste sample preparation methods, as well as the impact of Fe3+

on

your results. In addition, your discussion should consider the following technical questions:

1. The reference electrode required for a chloride ISE is a double junction silver/silver

chloride electrode. What is this electrode and why would it be needed?

2. Why is pH control important in the case of fluoride determination by ISE?

3. Why would a sulfide (Ag2S) ISE have less interference than the other silver halide based

ISEs?

4. Try to assign all of the anion peaks you observe in the ion chromatogram of the

toothpaste and Borocca™ sample.

5. In IC, explain the role of the suppressor used in conjunction with the conductivity

detector.

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PART A ION SELECTIVE ELECTRODES The fluoride ISE consists of a single crystal of lanthanum fluoride that acts as an insoluble

membrane. Ionic conduction is the mechanism by which this membrane responds to changes

in the activity of fluoride ion in a solution. Lattice defects in the crystal allow small ions with

a low formal charge (F-) a degree of mobility within the membrane. It is this shift in charge

centres which give rise to the membrane potentials.

The fluoride electrode is one of the best ion-selective electrodes available. The electrode

has a Nernstian response to 10-5

M F- (0.2 ppm) and selectivity of at least 1000:1 for F

- over

all other anions except OH-. Free fluoride concentrations are, however, subject to

interference through complexation by metal ions. Page 13

The response of the ISE to the F- ion concentration in solution at 298K is (ideally) given by

the Nernst equation:

pFKvE0592.0+= Ev = electrode potential (V) vs a reference electrode

K = cell constant (V)

pF= -log [F-], where [F

-] is in mol L

-1

The chloride ion electrode works on a similar principle, using a silver halide-silver sulfide

membrane. The membrane actually responds to the silver concentration (or activity) in

solution,

E = Eo + (RT/F) ln a

Ag +

but due to the low solubility of silver chloride, the silver concentration is dependent on the

chloride concentration:

K sp

= aAg

+ aCl-

E = Eo+ (RT/F) ln K

sp/ a

Cl-

E = E o- (RT/F) ln a

Cl-

Hence the measurements can be directly related to the chloride concentration. In its more

usual form, at 298K:

pClKvE0592.0+= where these symbols were defined above.

The chloride ISE is susceptible to interference from other ions. This is due to the

insolubility of other ionic silver compounds, so these anions also govern the activity of silver

ions in solution and thus the response of the membrane.

For ISE measurements, temperature and ionic strength of the samples and standards should

be the same. Recall that the electrode responds to the activity of the analyte, not

specifically the concentration. The activity and concentration are related by the activity

coefficient γ, which is dependent on the ionic strength of the solution. The ionic strength is

made identical in all solutions by addition of a known amount of some form of Ionic Strength

Adjuster (ISA). The Page 14

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ionic strength of the analyte solution will also influence the rate and extent of interaction

of the analyte ions with the membrane of the ISE.

Several methods of getting anion concentration can be employed:

o a direct reading is simple and quick, but the instrument is only calibrated against a single

standard. It is inherently error-prone, as it assumes a Nernstian response, although is useful

for the routine analysis of a large number of samples.

o a standard addition procedure, which involves adding a standard of known concentration to

a sample solution. From the change in electrode potential before and after addition, the

original sample concentration is determined. Standard addition is also a useful method for

measuring occasional samples. Any convenient concentration unit can be used for direct

measurement and standard addition.

o the most time-consuming, but perhaps most accurate, involves the preparation of a

calibration curve - a plot of mV vs log([F-]) or log([Cl

-]) for a series of standards.

In this part of the practical exercise the concentration of F- and Cl

- in various samples is

estimated by using a calibration curve.

A potentiometric method is dependant upon ionic adsorption equilibria being established at

the electrode (crystal/solution) interface. This can take a little time so wait for the

response to stabilise before you take a reading. This will normally take about 30 s or so

(What does this mean about the measurement bandwidth?).

EXPERIMENT Each pair or individual should carry out both the chloride and fluoride measurements.

REAGENTS

Total Ionic Strength Adjustment Buffer (TISAB) for F-

Ionic Strength Adjuster (ISA) for Cl- (5M NaNO

3)

Chloride stock solutions (1000 mg/L)

Fluoride stock solutions (1000 mg/L)

Fluoride stock containing Fe3+

APPARATUS

Orion pH/mV meter

Magnetic stirrer

Fluoride ion selective electrode

Chloride ion selective electrode

Reference electrode

Double junction reference

electrode

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C A U T I O N: Do not touch the ISE ion sensitive membrane. Rinse it

with deionized water between measurements and then with a small volume

of the new solution. Do not wipe it dry.

PROCEDURE

You will use the Nernstian relationship to draw up a calibration curve to

estimate the concentration of fluoride and chloride your samples. Since the

ISE response is logarithmic you should prepare standards that are 0.1, 1, 10,

20, 30, 40, and 50 ppm F- and Cl-.

CALIBRATION CURVE FOR FLUORIDE ANALYSIS

1. All readings should be done in solutions which are 1:1 TISAB/sample. Pipette

20 mL of your first standard (lowest concentration) and 20 mL TISAB into a

clean, dry beaker.

2. Analyse using the Orion ion selective electrode placing the electrodes in the

stirred solution. Record the mV reading.

3. Repeat for each of the standard solutions and for your unknown sample and

tap water. Rinse electrode with distilled water between samples.

4. Now measure the 25 ppm fluoride standard that contains Fe3+

(check with

your demonstrator about concentrations). Repeat the measurement procedure

using 20 mL of this solution, and compare your result with the iron-free 20

ppm fluoride reading.

Since the dilution with TISAB applies to all solutions you are to analyse there

is no need to take this dilution into account. For example, 50 mL 100ppm F- +

50 mL TISAB can still called 100ppm F- when recording the result.

CALIBRATION CURVE FOR CHLORIDE ANALYSIS

1. For each of the standards and samples take a 25.00mL aliquot and add

0.5mL of the ISA (for Cl-).

2. Place electrodes into the standard with the lowest concentrations and stir

using the magnetic stirrer. Wait until there is a stable reading. Repeat this

for all the other standards (in the order of increasing concentration), and

then the samples.

CALCULATIONS Based upon results obtained above, and the Nernst equation (as given), draw

calibration curves with the appropriate axes and estimate the concentration

of fluoride and chloride in the samples tested; use these curves to provide an

estimate of the measurement precision of your results.

REFERENCES • Rundle, C C, 2000, A Beginners Guide to Ion-Selective Electrode

Measurements, http://www.nico2000.net/Book/Guide1.html

• USEPA, 1996, Method 9214: Potentiometric determination of fluoride in

aqueous samples with an ion-selective electrode

• USEPA, 1996, Method 9212: Potentiometric determination of chloride in

aqueous samples with an ion-selective electrode

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• Thermo-Orion ISE manuals Reproduction rights obtained Feb 2007

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PART B ION CHROMATOGRAPHY Example ion chromatogram illustrating the separation of anions commonly found in water

samples

BEFORE YOU COME TO THE LABORATORY

Ion chromatography systems have several components (injection valve, guard

column, supressor, etc.) that in our case are integrated into a single unit. You

should be familiar with the function of each of these components prior to the

laboratory session. Refer to your 214 chromatography notes, or a relevant

text.

EXPERIMENT

REAGENTS

Chloride stock solutions (1000 mg/L)

Fluoride stock solutions (1000 mg/L)

Samples and unknowns

APPARATUS

Dionex ICS-90 ion chromatography system

Note down column, eluent and injection loop details

Caution: ALL solutions must be filtered using a 0.45 μm membrane filter

as they are injected into the ion chromatography system.

PROCEDURE

1. Familiarise yourself with the operation of the ion chromatography system.

Ask one of the demonstrators to help you can identify the following

components:

o The eluent reservoirs (record the nature of the eluent solution)

o The injection loop (note its volume) and valve

o The pump

o The chromatography column (make sure you write down the column type)

o The suppressor

o The conductivity detector

2. Using one of your sample solutions, fill the injection loop using a syringe

that has a 0.45 μm filter attached. Start the pre-loaded chromatography

method.

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3. While the first chromatogram is developing, prepare six combined standard

solutions with 0.1, 1, 10, 20, 30, 40 and 50 mg/L concentrations by making

suitable dilutions of the anion stock standard.

4. Examine your chromatogram and identify the anions responsible for each

peak. You may carry out additional injections (eg. of single component

solutions) if this helps

5. Continue to inject your group’s samples and standards into the

chromatography system. Make sure you wait until the injection loop is full and

the last chromatogram has fully developed before you start the next run.

6. Using the chromatogram for the 0 ppm standard, make an estimate of the

noise in the baseline.

CALCULATIONS Tabulate the areas of each of the peaks in your sample and standard

chromatograms. Use the results for your standards to generate a calibration

curve for each anion, using linear regression to generate an equation for your

response function. Use the regression equations to determine the

concentration of each anion in your samples, along with estimates of the

measurement precision.

Calculate the concentration Limit of Detection for each anion that you can

achieve with this method. Is the sensitivity the same for each anion?

REFERENCES

1. Harris, D C, 2003, Quantitative Chemical Analysis, (6th

Ed), W H Freeman

2. Dionex ISC-90 Manual

3. Dionex IC Application notes: http://www1.dionex.com/en-

us/by_technique/ic/anion/lp5008.html Copyright © 2006 by Sidney Harris