DETERMINATION OF IODIDE AND IODATE IN AQUEOUS SOLUTION by YINING LIU, B.Sc. A THESIS IN CHEMISTRY Submitted to the Graduate Faculty of Texas Tec University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Purnendu K. Dasgupta Chairperson of the Committee Dimitri Pappas Co-Chair of the Committee Carol Korzeniewski Member of the Committee Accepted John Borrelli Dean of the Graduate School August, 2007
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DETERMINATION OF IODIDE AND IODATE
IN AQUEOUS SOLUTION
by
YINING LIU, B.Sc.
A THESIS IN
CHEMISTRY
Submitted to the Graduate Faculty of Texas Tec University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Purnendu K. Dasgupta Chairperson of the Committee
Dimitri Pappas Co-Chair of the Committee
Carol Korzeniewski Member of the Committee
Accepted
John Borrelli Dean of the Graduate School
August, 2007
Copyright 2007, Yining Liu
Texas Tech University, Yining Liu, August, 2007
ii
ACKNOWLEDGEMENTS
I would like to express my deepest appreciation to Dr. Purnendu K.
Dasgupta, my dear mentor and advisor. Without his continuous guidance and
encouragement, my achievements towards a Master’s degree in Chemistry would
not have been possible. I would like also thank Dr. Dimitri Pappas and Dr. Carol
Korzeniewski for their assistance and valued comments on my thesis work.
I would like to express my great appreciation to those people in our group,
especially to Dr. Kalyani Martinelango, Dr. Qingyang Li, Dr. Takeuchi Masaki and
Mr. Jason V. Dyke. Those people gave me a lot of advice and guidance to assist
me to finish my research projects.
Last but not least, I would like to thank my family for their continuous support
and understanding of my studying thousands miles away from home. I love you
all. Finally, my deepest love would be expressed to my wife, Xia Wei, thank you
to be always along with me, patient and supportive. It’s my pleasure to enjoy the
great journey of life with you.
Texas Tech University, Yining Liu, August, 2007
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS...…………………………………..….………...……...…… ii
ABSTRACT………………………………………………………………….…….….. viii
LIST OF TABLES…………………………………..………...….………...……............. x
LIST OF FIGURES…………………………………….....……..…..……………….… .xi
LIST OF ABBREVIATIONS...………………………..…………….……….…..…. …xiv
CHAPTER
I. INTRODUCTION: IODINE...………….……..............................……………….. 1
3.4 Iodate Determined in the Table Salt...........................................................60
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LIST OF FIGURES
2.1 A closed system is designed in order to control relative humidity (RH)......28 2.2 Iodized salt loses iodine when the environment is humid. This graph shows
the iodine decay in the lab controlled humidity of 40% -90%......... 29 2.3 Iodine in dry salt decays when heated for 5 minutes at 200oC..................30 2.4 Iodine decays slightly in the presence of light……………..........................31 2.5 Homogeneity of iodine in 4 iodized salt samples………….........................32 2.6 Iodide concentration in collected iodized salt samples in US. RDA,
Recommended Daily Allowance; 45% RDA = 45 mg/kg iodide in salt (based on 1.5 g per serving, RDA=150 mg/kg).........................................33
2.7 Iodine concentration in salt samples from 37 states in US.........................34 2.8 Iodine content in 21 brands of newly purchased salt samples………....…35 2.9 1st and 2nd salt sample sent from 47 salt providers in US………..………36 2.10 1st, 2nd and 3rd salt samples sent from 24 salt sample providers in US.
3.2 Ion Chromatography Coupled Amperometric Detection………..……....….62 3.3 Applied voltage on the working electrode was scanning with 500 μg/L
iodate standards (Triplet injections). Signal to Noise ratio (S/N) was calculated when applied voltage was increased from100 mV to 700 mV in 50 mV steps. The error bars represent ±1 standard deviation. At 250-300 mV the detection reaches maximum sensitivity…………………………….63
3.4 Typical system output for iodate standards concentrations (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of response iodate standards range from 0 to 1500 μg/L……….……...…….64
3.5 Calibration of iodate standards: 0 – 1500 μg/L, where y and x respectively
represent signal output and iodate concentration…………………….…....65 3.6 In SCIC chromatogram, iodate signal is overlapped by that of fluoride
because both of them have conductivity response. The first peak is fluoride, iodate elutes as a shoulder. The amperometric detection gives iodate a selective current signal. Gradient eluent protocol: 6 mM NaOH Eluent is running in the IC system in the first 8 minutes. After that the Eluent concentration is increased to 35 mM in two minutes. 35 mM NaOH is running for the next 15 minutes until the last anion, Perchlorate, is running out………………………………………………………………………….……66
System (a). Schematic diagram of FIA system (b). NAFION Device: The carrier stream is acidified when it passes through the 20 cm long NAFION tube. Flow rate of sulfuric acid is 0.1 mL/mi (Detailed dimension information of NAFION tube was discussed in 3.2.1.2. ………………………...…….....67
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3.8 Applied voltage on the working electrode was scanning with 2 mg/L iodate
standard solution (triplet injections). Signal to Noise ratio (S/N) was calculated when applied voltage was increased from 50 mV to 800 mV (50 mV step). At 300 mV the detection reaches maximum sensitivity……………………………………………………………………..…68
3.9 Flow rate of 1% NaCl carrier was studied in the range from 0.2 ml/min to
2.0 mL/min. Both of the signal peak height and background noise decreases as the flow rate increases. At 1.5 mL/min flow rate, S/N of 1 mg/L iodate standard reaches the maximum…….…………..…….……….69
3.10 Sample injection volumes are studied in the range from 100 μL to 1000
μL. 500μL is selected to be the optimal injection volume………...........….70 3.11 Typical system output for iodate standards: Concentration (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of response iodate standards range from 0 to 2000 μg/L…………………….71
3.12 Calibration of iodate standards: 0 – 2000 μg/L, where y and x respectively
represent signal output and iodate concentration…………………………..72
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LIST OF ABBREVIATIONS
ABS Absorbance
A Ampere
AR Analytical reagent grade
oC Degree celsius
cm Centimeter
DI Deionized water
FDA Food and Drug Administration
FIA Flow Injection Analysis
Hz Hertz
I Iodine
IC Ion Chromatography
ICP/MS Inductively Coupled Plasma Mass Spectrometry
IDD Iodine Deficiency Disorder
LOD Limit of detection
L Liter
mg Miligram
mL Mililiter
MS Mass spectrometry
N.D. Not detected
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PC Personal Computer
RDA Recommend Daily Allowance
s Second
TH Thyroid Hormones
THS Thyroid Stimulating Hormone
μA Microampere
μL Microliter
μg Microgram
USEPA United States Environmental Protection Agency
v/v Volume in the volume
V Volt
WHO World Health Organization
Texas Tech University, Yining Liu, August, 2007
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CHAPTER I
INTRODUCTION: IODINE
1.1 Historical Discovery
The ancient Chinese people recognized the effectiveness of seaweed and
burnt sea sponge in the treatment of goiter. Such treatment reduced its size and
caused its disappearance. However, there was no knowledge of iodine or iodine
deficiency available at that time1. Iodine was discovered by a French scientist,
Bernard Courtois, in 1811.1,2 Courtois obtained the element by treating seaweed
ash with concentrated sulfuric acid. The name “iode” was proposed by J. L.
Gay-Lussac in 1813.2 The word iode/iodine (element symbol I) is derived from
the Greek word “ioeides” and reflects its most characteristic property: the color
violet.3
1.2 Physical and Chemical Properties
Iodine is a bluish-black, lustrous solid metal (solid density 4.93 g/cm3 at 300
K) found throughout the environment in a stable form, I-127. It is a Group 7
element in the periodic table. It has an atomic number 53 and an atomic weight
of 126.9045 g /mol. Iodine sublimes at room temperature in to a blue-violet
vapor (gas density 11.2 g/L, 1 atm) with an irritating odor. Its electronic
configuration, [Kr] 5s24d105p5, suggests the valence of +1,+3, +5… are
possible.3-6
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Iodine is found throughout the environment as the stable isotope, I-127.
I-131 and I-129 are two common radioactive forms of iodine. The radioisotope
I-131 is used often in clinical medicine. It has a half life of 8 days while I-129 has
a much longer half life of 15 million years.5,6
1.3 Occurrence, Production and Uses
Iodine is found in inorganic forms in ground water and soil. The form in
which iodine compounds are found is mainly decided by the matrices in which
they occur. Organic iodine in the seawater is transformed in the biogeochemical
cycle eventually to iodate (IO3-) in the atmospheric aerosol and deposited on land
via rain. Iodate, a soluble oxidation product is often considered to be the only
stable species of iodine converted to the aerosol phase7 and it is the dominant
form of inorganic iodine in precipitation.8 Some very recent work, however,
questioned the relative importance of iodate domination.9-10 It is known that the
Chilean Caliche Nitrate bed is rich in iodine (~0.02-1 wt% I) in the form of
Laurarite, Ca(IO3)2 and Dietzeite, 7Ca(IO3)2.8CaCrO3.2 Studying the
environmental occurrence of iodate helps us understand the transport and
chemical influence of iodine oxides in the troposphere, including the destruction
and depletion of ozone.11 It is generally held that iodide and iodate are the only
iodine species in natural water, with total iodine equaling the combined
concentrations of iodate and iodide.12
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1.4 Health Importance of Iodine
Today, iodine is well known as an essential trace element required for the
synthesis of thyroid hormones (TH). Iodine is present in the body in minute
amounts and is stored in the thyroid gland. The thyroid gland removes iodine
from the circulating bloodstream. Iodine normally enters the bloodstream as
iodide after ingestion in food or water. When iodine intake is not adequate, the
thyroid may not be able to synthesize sufficient amounts of thyroid hormones to
meet one’s physiological needs.
The only clearly known need for iodine is for the formation of thyroid
hormones. Insufficient thyroid hormone synthesis results in hypothyroidism and
a range of functional and developmental abnormalities collectively termed “Iodine
Deficiency Disorders” (IDD). Iodine deficiency has the potential to increase the
prevalence of goiter and increases the risk of intellectual deficiency.
1.4.1 Goiter
The name “goiter” refers to those patients with a greatly swollen thyroid,
when the diet is deficient in iodine, the thyroid gland may become very large.
The pituitary attempts to increase iodide trapping by increasing its excretion of
thyroid stimulating hormone (TSH). TSH stimulates the thyroid and its growth.
Ordinary and endemic goiters are termed “nontoxic” and can be treated with
iodine supplementation. However, “toxic” goiters, such as Graves’s or
Basedow’s disease, are caused by autoimmune problems. Improving iodine
intake does not help such patients.13
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1.4.2 Neurological Disorder
Iodine deficiency leads to reduced production of the two thyroid hormones
thyroxine (T4) and triiodothyronine (T3). Insufficient levels of these two thyroid
hormones during early life may result in abnormal development. The brain and
neurological system may be severely affected14 as T4 and T3 hormones are
essential for pre- and postnatal brain development. 14-15 Congenital
hypothyroidism results in mental retardation, ataxia, spasticity and deafness.14 If
TH insufficiency occurs in early pregnancy, the offspring display problems in
visual attention and visual processing. If TH insufficiency occurs after birth,
language and memory skills are most predominantly affected.13-16
1.4.3 Hazard and Toxicity
Although iodine is essential for proper nutrition, care is needed when
handling the element, as skin contact can cause lesions and the vapor is highly
irritating to the eyes and mucous membranes.4,6
1.5 Prevention of IDD Worldwide
Iodine deficiency is a major threat to the health and development of people
worldwide. Iodine deficiency is common when the environment is poor in iodine,
resulting in low iodine concentrations in food products. One of the best and least
expensive methods of preventing IDD is supplementation of table salt with iodine.
Iodine is added to salt in the form of potassium iodide (KI) or potassium iodate
(KIO3) either as a dry solid or as a sprayed aqueous solution at the point of
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production. In more than 100 countries throughout the world, the iodine content
of the food supply is supplemented by adding iodine to table salt.17-18 These salt
iodization programs have been very successful in improving thyroid health status
in populations where salt iodization programs have been in effect of several years
have been overwhelming. The number of countries with high prevalence of
iodine deficiency has decreased from 110 in 1993 to 54 in 2003.17
1.5.1 Iodized Intake Regulation
Both the World Health Organization (WHO) and the US Food and Drug
administration (FDA) suggest a Recommended Daily Allowance (RDA) of 130
μg/day for adolescents and adults and 65 μg/day for school age children.17-20 In
2002, the WHO revised the recommended daily iodine intake for pregnant women
to 200 μg/day in consideration of the fact that iodine requirements increase during
pregnancy to provide for the needs of the fetus.20 In 2001, the Institute of
Medicine (IOM) released detailed RDA values for iodine for groups of people of
varying ages (Table 1.1).21 According to the US National Academy of Sciences
Press report in 2004, the tolerable Upper Intake iodine level for adults (UL) is
1,100 μg/day.
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Table 1.1 Suggested daily iodine intake by IOM
Group Iodine intake (μg/day)
Age 0-6 months 110 (AI*)
Age 7-12 months 130 (AI)
Age 1-8 yr 90 (RDA**)
Age 9-13 yr 120 (RDA)
Age ≥ 14 yr 150 (RDA)
Pregnant woman 220 (RDA)
Lactating woman 290 (RDA)
AI*, Adequate Intake RDA**, Recommended Daily Allowance The RDA is the intake of a nutrient expected to meet the needs of 97-98% of healthy individuals. The AI is an approximation of the dietary intake when there is not enough evidence to determine the RDA, which always exceeds the RDA.21
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1.5.2 Iodized Salt in United States
Voluntary fortification of salt with iodine was introduced in 1924 and resulted
in a virtual elimination of endemic goiter in the US. However, salt iodization is
still not mandatory in the US. Potassium iodide (KI) is used as the iodization
vector rather than iodate (KIO3).22 KI is normally added at a concentration of
60-100 mg/kg. Stabilizing agents such as sodium thiosulfate (Na2S2O3), pH
buffers, such as sodium bicarbonate (NaHCO3) and drying agents such as silicon
dioxide (SiO2) or calcium silicate are added at concentration of 0.04% or 0.05% to
table salt to prevent iodide sublimation. Anti-caking agents are normally added
in concentrations of 1-1.5 %.23
1.6 Objectives of Present Work
We wanted to develop a fast interference-free method to determine iodide in
salt when iodide is used as the iodization vector. Studying the iodide
concentration in many salt samples collected from across United States helps us
understand how the storage conditions affect the iodide sublimation from salt.
We analyzed all archived salt samples, stored in the dark at -20 °C, by ICP-MS.
In chapter II, we report the ICP-MS instrumentation and method setup for iodide
determination in iodized salt solution. Chapter II also discusses how the storage
environment affects iodide loss and the iodide concentrations of salt samples
collected in 37 US states.
We also wanted to develop a simple, fast, interference-free, detection
scheme for aqueous iodate in different matrices. When connected in a
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post-column configuration in ion chromatography (IC), the analyzer can work
selectively detect iodate without interference from fluoride. When used in a
flow-injection analysis (FIA) system, it can determine iodate in a sample made
from table salt that is iodized with iodate. Chapter III of this dissertation
describes the water-phase amperometric detection of iodate, and how it has been
adapted for a post-IC column system and FIA system. The conclusions are
summarized in Chapter IV.
The experiments described in Chapter II were conducted at the University of
Texas at Arlington, Arlington, Texas. The experiments described in Chapter III
were conducted at Texas Tech University, Lubbock, Texas.
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1.7 References
1. Rosenfeld, L. Journal of Chemical Education, 2000, 77, 984-987.
2. Chemistry of the Elements, Green wood, N.N.; Earnshaw, A. 2nd Ed.;
Expansion Chamber Pressure: 1.9 (mbar) Analyzer Chamber Pressure: 3.6 x 10-7 (mbar) Nebulizer Back Pressure: 2.1 (bar)
Software: Thermo PlasmaLab (version 2.5.5.290) Data Acquisition Parameters: Mode: Peakjump Sweeps: 800 Dwell Time: 10 (ms) Mass Separation: 0.02 (amu) Elements Monitored: 127I, 72Ge, 74Ge ________________________________________________________________
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Table 2.2 Lab Controlled Relative Humidity (RH) by Change the Density of H2SO4 Solution Stored in the Closed System. (Handbook of Chemistry and Physics, 55th Ed. CRC press) Density of H2SO4 solution Theoretical RH (%) Measured RH (%) (g/ml) 1.00 100 90.1 1.20 80.5 81.8 1.30 58.3 67.6
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Figure 2.1 A closed system is designed in order to control relative humidity (RH). 20 grams of Iodized salt was placed on a watch glass on a 25 mL glass beaker. All of them were placed in a 500 mL glass beaker, which contained 50 mL sulfuric acid. The RH is controlled by changing sulfuric acid concentration in the closed system, which is sealed with plastic cover.
500 mL Glass Beaker
Watch Glass
25 mL Glass Beaker
Plastic Wrap
Iodized Salt
Sulfuric Acid
(Air humidity is controlled in the closed system.)
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0 10 20 30 40 50Time (day)
0
20
40
60
80Io
dide
con
cent
ratio
n (m
g/kg
)Effect of Moisture90% RH80% RH65% RH40% (RH in our lab, Lubbock,TX)
Figure 2.2 Iodized salt loses iodine when the environment is humid. This graph shows the iodine decay in the lab controlled humidity of 40% - 90%. Under room temperature (22oC)
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0
20
40
60Io
dide
con
tent
(mg/
kg)
Original contentAfter heating
Salt Sense Richfood Morton HainSea Salt
Figure 2.3 Iodine in dry salt comparison before and after being heated for 5 minutes at 200 oC.
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0 10 20 30 40 50Time (day)
0
20
40
60
80Io
dide
Con
cent
ratio
n (m
g/kg
)Light Effect
No LightWith Light
Figure 2.4 Iodine decays slightly in the presence of light, under room temperature (22oC) and humidity (RH=40%)
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0
20
40
60
80Io
dide
con
tent
, mg/
kg Ave
rage
(SD
)
Ave
rage
(SD
) Ave
rage
(SD
)
Ave
rage
(SD
)
HainSeaSalt
Rich Food
Wegman's SaltSense
Figure 2.5 Homogeneity of iodine in 4 iodized salt samples
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0
40
80
120
160Io
dide
con
tent
(mg/
kg)
94 new purchased iodized salt in 37 states of US Fig 2.6 Iodide concentration in collected iodized salt samples in US. RDA, Recommended Daily Allowance; 45% RDA = 45 mg/kg iodide in salt (based on 1.5 g per serving, RDA=150 mg/kg)
45% RDA level
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0
40
80
120
160Io
dide
Con
cent
ratio
n (m
g/kg
)
Salt purchased from 37 States in US Figure 2.7 Iodine concentration in salt samples from 37 states in US
45% RDA level
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0
20
40
60
80
100Io
dide
Con
cent
ratio
n (m
g/kg
)
21 Brands of iodized salt purchased in US Figure 2.8 Iodine content in 21 brands of newly purchased salt samples
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0
40
80
120
160Io
dine
Con
cent
ratio
n (m
g/kg
)
1st sample2nd sample
1st salt sample vs. 2nd salt sample Figure 2.9 1st and 2nd salt samples sent from 47 salt providers in US.
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0
50
100
150
200
250Io
dide
Con
cent
ratio
n (m
g/kg
) 1st sample2nd sample3rd sample
1st samples vs. 2nd samples vs. 3rd samples Figure 2.10 1st, 2nd and 3rd salt samples sent from 24 salt sample providers in US.
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CHAPTER III
AN AMPEROMETRIC IODATE ANALYZER
FOR AQUEOUS SAMPLES
3.1 Introduction
This chapter describes a simple and selective amperometric detection
system for the determination of iodate (IO3-) in aqueous solution. Iodate is
reduced at a stainless steel working electrode with a platinum auxiliary electrode,
the latter also serving as a virtual reference electrode. The peak height
response is directly related to the iodate concentration. This detector was
operated directly in a flow injection analysis (FIA) system and also in conjunction
with Ion Chromatography (IC) system. It provides a simple and sensitive
approach to measuring iodate in solution with different matrices.
3.2 Experimental Section
3.2.1 Instrument Setup
3.2.1.1 Amperometric Detector Cell
An amperometric detector cell (Figure 3.1 a), was composed of a 2.5 cm
long stainless steel tube (i.d. 0.5 mm, o.d. 0.75 mm, Small Part Inc.), functioning
as a working electrode. One end of the stainless steel tube was inserted into a
Teflon tube (0.8 mm i.d., 1.4 mm o.d. and 1.5 cm long, Zeus products). The
platinum counter electrode was 1 mm in diameter and was inserted through the
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wall of the Teflon tube and epoxied in place. The distance between the two
electrodes was 1 mm. The other end of stainless steel tube was connected to a
Teflon tube (0.71mm i.d. and 1.30 mm o.d.) and both inserted into a flexible PVC
tube (0.74 mm i.d., 2.45 mm o.d., and ~0.5 cm long, Cole-Parmer). Referring to
Figure 3.1 b, the power source for the electrodes was a 9 V battery, connected
across a 300 KΩ potentiometer. The negative terminal of the battery was
connected to the electronic system ground (GND). The slider of the
potentiometer, providing variable positive potential was connected to the counter
electrode, while the working electrode was at virtual ground, being connected
through a current to voltage converter (a low-noise JFET operational amplifier, 1/2
TL072CN) to ground. The first stage of the amplifier functioned as a 1 V/μA I →V
converter but inverted the sign of the signal; the second stage (1/2 TL072CN)
merely corrected the sign; both stages had a time constant of 1 s.
3.2.1.2 NAFION Tube & Acid Penetration
The reduction of iodate is facilitated in an acid medium:
IO3- + 6 H+ + 6e → I- + 3 H2O …(1)
To acidify the iodate sample flow stream prior to detection, it will thus be
beneficial to add acid so that the reduction can be efficiently accomplished at a
lower applied voltage. In addition, having a finite concentration of H+ defines the
reference potential at the counter electrode through the electrolytic breakdown of
water:
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H2O → 2 H+ + ½ O2 + 2e …(2)
It would have been possible to use a merging stream of acid. However, this
would involve sample dilution and necessitate an additional pump. We designed
instead a device to allow acid to be introduced without a pump and without
volumetric dilution. The scheme involves the introduction of sulfuric through a
NAFION membrane into the flow stream. Although the penetration of sulfate
through a negatively charged perfluorosulfonate NAFION membrane is
Donnan-forbidden, this barrier is overcome if a large concentration gradient exists
across the membrane.
NAFION ionomers were developed and produced by DuPont Company in
the early 70’s as Proton-Exchange Membrane. This material is generated by
copolymerization of a perfluorinated vinyl ether comonomer with TFE
(tetrafluoroethlene).1-3 Below is the chemical structure of NAFION:
(CF2-CF2)x (CF2-CF)y
O
CF3C F
CF2
CF2
SO3-
Nafion (R)
The sulfonate groups (-SO3-) facilitate the electrostatic binding of cations.
Cations can exchange through those active sites. For example, the film can be
saturated with protons (H+) when immersed in acid solution.3 Permitted and
NAFION ®
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Donnan Forbidden ion penetration rate through small diameter ion exchange
membrane tube had been studied two decades ago.4 An ion similarly charged
as the membrane matrix (cation exchange membrane, sulfonate group) is
retarded as referred to Donnan Forbidden. However, the barrier to the forbidden
ion is not sufficient to completely eliminate its penetration when the difference of
the concentrations across the membrane is high enough.4 That is, the sulfate
ion can penetrate the membrane wall to the other side as sulfuric acid if the
membrane contains sulfuric acid on one side and water on the other and the
sulfuric acid concentration is high enough.
A NAFION acid penetrating device (Figure 3.2 b) was made by inserting a
20 cm long NAFION tube (0.60 mm i.d., 0.80 mm o.d., www.Permapure.com) into
a Teflon tube jacket (1.5 mm i.d., 2.3 mm o.d. and 25 cm long, Zeus Products).
Each of the two ends of NAFION tube was inserted into another two Teflon tubes
(1.30 mm o.d., 0.72 mm i.d., and 10 cm long of each). One connected to the
amperometric detector and the other one is to the iodate sample solution inlet.
Each of the two ends of Teflon tube jacket was connected with Teflon Tee (~2.0
mm i.d., Ark-Plas Products), in which the sulfuric acid flowed in and out. All the
Tee-Tube and Tube-Tube connections were naturally tube-size-fitted and fortified
by epoxy in places. The iodate sample carrier stream flows (1 mL/min) through
in the NAFION tube and 1 M sulfuric acid flows countercurrent by gravity (~0.1
mL/min) in the Teflon jacket tube and out of the NAFION tube. The carrier
stream was thus acidified through the device; the effluent pH was measured to be
~2.0 (Φ71 pH meter, Beckman Corp.).
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3.2.1.3 Data Acquisition (Figure 3.1 b)
The voltage output from the homemade amperometric detection system was
acquired by a data acquisition card (PC-CARD-DAS16/12AO, Measurement
Computing Inc., Middleboro, MA) housed in an IBM laptop personal computer
model A22m.
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3.3 Result and Discussion part I:
Determination of Iodate in Chilean Caliche Soil
3.3.1 Standard Detection Method
Present USEPA approved detection method for anion analysis in water is
based on Suppressed conductometric ion chromatography (SCIC) (EPA method
300.1).5 Anions in solution are separated on an IC column and determined after
ion exchange suppression by a conductivity detector.6,7 The method is very
sensitive (the detection limit is in the μg/L level for most anions) and the
conductivity signal corresponds to the anion concentration.
3.3.2 SCIC on Determining Iodate in Caliche Samples
The SCIC method is generally reliable for the determination of anions in
Chilean Caliche soil samples. Iodate is a hard ion with a low charge density.
As such on most ion exchange columns it is very poorly retained. It is thus
difficult to separate iodate from other poorly retained ions, most notably fluoride.
Fluoride is a common ion in many samples, including Chilean Caliche.
Under most IC conditions, fluoride and iodate elute virtually together, almost with
little or no retention on the column and thus constitutes a mutual interference.
Separation is possible on specialized high capacity columns with very low
concentration eluents but if the measurement of other strongly retained anions in
the same sample is also necessary, analysis time is greatly prolonged. Gradient
elution protocols with a long re-equilibration time become essential, making the
analysis of large number of samples very time consuming.
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3.3.3 Improved Iodate Amperometric Detection
Iodate is a reducible anion while fluoride is not. Here, we have taken
advantage of the electrochemical reducibility of iodate to perform selective
detection using a simple flow-through two-electrode amperometric detector cell.
The reduction current peak height is directly related to the iodate concentration in
samples. The amperometric detection system described above was connected
in the IC system after the conductivity detector.
3.3.4 Detector Interface to the Ion Chromatography System
Figure 3.2 (a) shows the general schematic outline of the IC system and the
placement of the amperometric detector. Sample injection volume was 200 μL.
Anions in the Chilean Caliche sample solution were eluted by a KOH eluent at a
flow rate of 1 mL/min and separated on a Dionex 4 mm IonPac® AS16 column
and then suppressed in Dionex ASRS Ultra II 4mm Anion Self-Regenerating
Suppressor. The conductivity measurement of all the anions was then carried
out by a conductivity detector integral to the ICS 2000 system. Software
Chromeleon Client (version 6.60) was used to optimize the ICS2000 system
operating parameters, control the sample injection value, suppressor and gradient
concentration eluent and acquire the conductometric data. The details of the IC
separation system parameters were shown in Table 3.1.
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3.3.5 Preparation of Samples and Reagents
3.3.5.1 Sample Preparation
Chilean Caliche samples were extracted into DI water. Ten mL DI water
was added to a 50 mg aliquot of Caliche sample, which was then shaken well and
decanted. This was repeated an additional 4 time so that all the soluble ions
were dissolved in 50 mL of solution. The extract was then filtered through 0.45
μm nylon syringe filters (FisherBrand) prior to injection on to the IC separation
column. Three solutions were prepared for each solid Caliche sample because
these powdered ore sample is inherently not homogeneous.
3.3.5.2 Chemicals and Reagents
All chemicals were analytical reagent (AR) grade. The standard iodate
solutions and acid reagent were prepared with DI water (Millipore, 18.3 MΩ). A
standard stock solution of 2 g/L iodate was prepared by dissolving 0.6143 g
potassium iodate (MCB Chemicals) in DI water to give a final volume of 250 mL.
The stock solution was further diluted to obtain iodate standard solutions, ranging
from 50 μg/L to 1 mg/L.
3.3.6 Optimization of Detection System
3.3.6.1 Optimization of Applied Voltage
Voltage scanning was used to study and optimize iodate detection sensitivity.
The applied voltage on the working electrode was increased in 50 mV step from
100 mV to 800 mV to find the optimum signal to noise ratio for detection of iodate.
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At each applied voltage, 500 μg/L iodate standard solution was then injected and
the current measured three times. The current signal (peak height) to noise ratio
reaches a maximum at an applied voltage of 250 mV (Figure 3.3). An applied
voltage of 250 mV was then fixed to the amperometric detector electrodes for
iodate detection.
3.3.6.2 Gradient Eluent Protocol (Table 3.1)
Sodium hydroxide eluent runs through the IC system during the first 8
minutes. After this point the eluent concentration is increased to 35 mM in two
minutes. 35 mM KOH eluent runs for the remaining 15 minutes until the last
anion, perchlorate, exits from the detector.
3.3.7 System Response
3.3.7.1 Calibration and Determination of Iodate
The calibration curve for iodate was obtained under an applied voltage of
250 mV. It was found that the amperometric signal is linear with iodate
concentration in the range studied: 50-1500 μg/L. Figure 3.4 shows the typical
amperometric detector response (triplicate injection peaks). The signal
response fits a nice linear relation with concentration (Figure 3.5) and the best-fit
Table 3.2. Iodate Concentration in 13 Chilean Caliche Samples Solutions
Sample IC-CD (μg/L) AD (μg/L)
Sp3#9-a 1247.6±14.01 850.90±41.77
Sp3#9-b 1256.1±18.95 777.81±59.46
Sp3#9-NEW N.A. 617.04±44.45
Sp4#2-a N.A. 428.60±3.17
Sp4#2-b N.A. 215.72±4.74
Sp5#1-a N.A. 776.22±62.08
Sp5#1-b 1231.8±182.29 804.294±32.20
Sp5#1-NEW N.A. 264.26±3.76
Sp6#4-a 354.2±11.46 250.11±28.55
Sp6#4-b 1355.9±1.67 728.40±71.69
Sp6#4-N N.A. 1023.23±27.85
Sp7#3-a N.A. 1409.65±48.26
Sp7#3-b N.A. 954.57±8.64
*N.A. sample was not analyze
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Table 3.3 Optimum Condition of the Flow Injection Amperometric Detection System Parameter Studied range Optimum condition Applied voltage (mV) 0-800 300 Flow rate (mL/min) 0.2-2.0 1.5 Sample loop volume (μL) 100-1000 500 Measurement base -- Time based Working Electrode -- Stainless Steel tube, 0.5 mm ID Gain (V/μA) 1
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Table 3.4. Iodate Determined in the Table Salt Sample Found iodate content in salt (mg/kg) (1) RSD (2)
(mean ± sd) Thailand (I) 104.2 ± 4.218 4.22% India 86.2 ± 4.548 5.28% China (I) 57.4 ± 6.409 11.16% China (II) N.D.(3) Thailand (II) N.D. Thailand (III) 88.21 ± 2.276 2.60% Australia (I) 99.62 ± 5.809 5.83% Australia (II) N.D. (1) Iodate content was recalculated from ppb (1% salt solution) to mg/kg (in solid state) (2) Relative standard deviation of iodate concentration in the three solutions made from each salt sample, which indicates the homogeneity (or lack thereof) of the iodate distribution. (3) Not Detectable, samples in which iodate cannot be detected, samples were not specifically labeled as iodized.
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0 200 400 600 800Applied Voltage (mV)
20
30
40
50
60
70Si
gnal
/ N
oise
Figure 3.3 Applied voltage on the working electrode was scanned with 500 μg/L iodate standards (Triplet injections). Signal to Noise ratio (S/N) was calculated when applied voltage was increased from 100 mV to 700 mV in 50 mV steps. The error bars represent ±1 standard deviation. At 250-300 mV the detection reaches maximum sensitivity.
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0 2000 4000 6000Time (s)
0.36
0.38
0.4
0.42
0.44
0.46
0.48D
etec
tor O
utpu
t (V
)
50 ppb100 ppb
250 ppb
500 ppb
1000 ppb
1500 ppb
Figure 3.4 Typical system output for iodate standards concentrations (μg/L) are indicated on top of each triplicate set. The graph shows magnified view of response iodate standards range from 0 to 1500 μg/L.
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0 400 800 1200 1600Concentration (μg/L)
0.36
0.38
0.4
0.42
0.44
0.46
0.48O
utpu
t (V)
Y = 5.923*10-5 * X + 0.3706R2 = 0.9998
Figure 3.5 Calibration of iodate standards: 0 – 1500 μg/L, where y and x respectively represent signal output and iodate concentration.
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0 200 400 600 800 1000Time (s)
-1
0
1
2
3
4
Con
duct
omet
ric S
igna
l (μS
)
0.32
0.36
0.4
A
mpe
rom
etric
Sig
nal (μA
)
Fluoride
Iodate
Figure 3.6 In SCIC chromatogram iodate signal is overlapped by that of fluoride because both of them have conductivity response. The first peak is fluoride, iodate elutes as a shoulder. The amperometric detection gives iodate a selective current signal. Gradient eluent protocol: 6 mM KOH eluent is running in the IC system in the first 8 minutes. After that the eluent concentration is increased to 35 mM in two minutes. 35 mM KOH is running for the next 15 minutes until the last anion, Perchlorate, is running out.
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0 200 400 600 800Applied Voltage (mV)
0
200
400
600
800
1000S
igna
l/Noi
se
Applied Voltage vs. S/N
Figure 3.8. Applied voltage on the working electrode was scanned with 2 mg/L iodate standard solution (triplet injections). Signal to Noise ratio (S/N) was calculated when applied voltage was increased from 50 mV to 800 mV (50 mV step). At 300 mV the detection reaches maximum sensitivity.
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0 0.4 0.8 1.2 1.6 2Flow rate (mL/min)
0
50
100
150
200
250Si
gnal
/ N
oise
Flow rate vs. S/N
Figure 3.9 Flow rate of 1% NaCl carrier was studied in the range from 0.2 ml/min to 2.0 mL/min. Both of the signal peak height and background noise decreases as the flow rate increases. At 1.5 mL/min flow rate, S/N of 1 mg/L iodate standard reaches the maximum.
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0 200 400 600 800 1000Sample volume (μL)
120
160
200
240
280S
igna
l/Noi
seSample injection volume vs. S/N
Figure 3.10 Sample injection volumes are studied in the range from 100 μL to 1000 μL. 500μL is selected to be the optimal injection volume.
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0 2000 4000 6000 8000Time (s)
0
0.05
0.1
0.15
0.2
0.25D
etec
tor O
utpu
t, V
050
100
250
500
1000
2000
Figure 3.11 Typical system output for iodate standards: Concentration (μg/L) are indicated on top of each triplicate set. The graph shows magnified view of response iodate standards range from 0 to 2000 μg/L
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0 400 800 1200 1600Concentration (μg/L)
-0.02
0
0.02
0.04
0.06
0.08
0.1D
etec
tor O
utpu
t (V)
Y = 1.010*10-4*X + 0.0296R2 = 0.9961
7 Figure 3.12 Calibration of iodate standards: 0 – 2000 μg/L, where y and x respectively represent signal output and iodate concentration.
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CHAPTER IV
CONCLUSIONS
The amperometric detector presented here for the determination of aqueous
iodate has substantially greater selectivity and sensitivity than a conductivity
detector. The method utilized the electrochemical reducibility of iodate ion in
acid medium under applied voltage. When connected to an IC system, this
detector gives a good selective response for iodate without interference from
fluoride. When used in a Flow Injection Analysis system, the detector gives very
sensitive response to iodate in a table salt matrix.
In the study of iodide stability in iodized table salt, we have confirmed the
loss of iodine from salt under humid conditions and high temperature. Based on
the analysis of many samples from providers across the US, a large fraction of
salt samples do not contain the amount of iodine stated on the labels.
Texas Tech University, Yining Liu, August, 2007
PERMISSION TO COPY
In presenting this thesis in partial fulfillment of the requirements for a master’s
degree at Texas Tech University or Texas Tech University Health Sciences Center, I
agree that the Library and my major department shall make it freely available for research
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Agree (Permission is granted.)
____________________________________________________ Student Signature Date Disagree (Permission is not granted.) _____Yining Liu _______________________06/28/2007_____