Liu Yining Thesis
Liu Yining Thesis
Liu Yining Thesis
IN AQUEOUS SOLUTION
by
A THESIS
IN
CHEMISTRY
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
ACKNOWLEDGEMENTS
Dasgupta, my dear mentor and advisor. Without his continuous guidance and
not have been possible. I would like also thank Dr. Dimitri Pappas and Dr. Carol
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
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
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS...…………………………………..….………...……...…… ii
ABSTRACT………………………………………………………………….…….….. viii
LIST OF TABLES…………………………………..………...….………...……............. x
CHAPTER
I. INTRODUCTION: IODINE...………….……..............................……………….. 1
1.4.1 Goiter.......................................................................................... 3
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Texas Tech University, Yining Liu, August, 2007
1.7 References............................................................................................9
2.1 Introduction....................................................................................... 11
2.3.3 Measurements…………………………………........................... 16
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2.5 Conclusion……………..….............................................................. 23
2.6 References....................................................................................... 24
3.1 Introduction....................................................................................... 38
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Salt Solution…...……………………………....………....…51
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Salts…………………………………………………………..54
3.5 Conclusion………………………..…………………………...………….55
3.6 References………………………………..………………………………56
IV. CONCLUSIONS...........................................................................................73
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ABSTRACT
Sufficient daily dietary iodine (I2) intake is necessary for the production of
termed “Iodine Deficiency Disorders” (IDD) may develop. Iodate and Iodide are
the only two forms in which iodine is added to table salt. Iodide is used in the US
but iodization has never been mandatory and iodine content of table salt has
never been determined independently. Potassium iodide (KI) added to table salt
may oxidize and then sublime at ambient humidity and temperature. Further
additives are sometimes added to salt, including silica or calcium silicate (to
across the US. The iodine content of the salt samples was measured by ICP-MS
salt program in Asian countries because iodate is the iodization vector for salt in
Asia. Iodate is also naturally formed and the content of iodate in natural deposits
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determine iodate concentration in five Chilean Caliche samples and eight table
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LIST OF TABLES
2.2 Lab controlled Relative Humidity (RH) by Change the Density of H2SO4
System….…….......................................................................................... 59
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LIST OF FIGURES
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
2.9 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.
…………………………………………………………………………………...37
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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
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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
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LIST OF ABBREVIATIONS
ABS Absorbance
A Ampere
oC Degree celsius
cm Centimeter
DI Deionized water
Hz Hertz
I Iodine
IC Ion Chromatography
L Liter
mg Miligram
mL Mililiter
MS Mass spectrometry
PC Personal Computer
s Second
TH Thyroid Hormones
μA Microampere
μL Microliter
μg Microgram
V Volt
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CHAPTER I
INTRODUCTION: IODINE
burnt sea sponge in the treatment of goiter. Such treatment reduced its size and
ash with concentrated sulfuric acid. The name “iode” was proposed by J. L.
the Greek word “ioeides” and reflects its most characteristic property: the color
violet.3
Iodine is a bluish-black, lustrous solid metal (solid density 4.93 g/cm3 at 300
element in the periodic table. It has an atomic number 53 and an atomic weight
vapor (gas density 11.2 g/L, 1 atm) with an irritating odor. Its electronic
possible.3-6
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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
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
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
Chilean Caliche Nitrate bed is rich in iodine (~0.02-1 wt% I) in the form of
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
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Today, iodine is well known as an essential trace element required for the
amounts and is stored in the thyroid gland. The thyroid gland removes iodine
iodide after ingestion in food or water. When iodine intake is not adequate, the
The only clearly known need for iodine is for the formation of thyroid
Deficiency Disorders” (IDD). Iodine deficiency has the potential to increase the
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.
thyroid stimulating hormone (TSH). TSH stimulates the thyroid and its growth.
Ordinary and endemic goiters are termed “nontoxic” and can be treated with
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thyroxine (T4) and triiodothyronine (T3). Insufficient levels of these two thyroid
hormones during early life may result in abnormal development. The brain and
handling the element, as skin contact can cause lesions and the vapor is highly
resulting in low iodine concentrations in food products. One of the best and least
Iodine is added to salt in the form of potassium iodide (KI) or potassium iodate
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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
Both the World Health Organization (WHO) and the US Food and Drug
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
Press report in 2004, the tolerable Upper Intake iodine level for adults (UL) is
1,100 μg/day.
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Voluntary fortification of salt with iodine was introduced in 1924 and resulted
still not mandatory in the US. Potassium iodide (KI) is used as the iodization
buffers, such as sodium bicarbonate (NaHCO3) and drying agents such as silicon
table salt to prevent iodide sublimation. Anti-caking agents are normally added
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.
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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
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1.7 References
http://healthy-information.naturalhealthdoc.net/IODINE'S-INFORMATION/IODINE
-What-Is-Iodine-Why-Iodine__protected~.htm (5/12/2007).
http://www.radiochemistry.org/periodictable/elements/53.html (5/12/2007).
resources/radiation/pdf/iodine.pdf (5/12/2007)
periodic/i.html (6/11/2007)
7. Vogt, R.; Sander, R.; Glasow, V.R.; Crutzen, J.P. Journal of Atmospheric
8. Baker, A.R.; Thompson, D.; Campos, M.L.A.M.; Parry, S.J.; Jickells, T.D.
10. Gildeffer, B.S.; Petri, M.; Biester, H. Atmos. Chem. Phys. 2007, 7, 2661-2669.
12. Edmonds, J.S.; Morita, M. Pure & Appl. Chem. 1998, 70, (8) 1567-1584.
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15. Pearce, E.N.; Pino, S.; He, X.; Bazrafshan, H.R.; Lee, S.L.; Braverman, L.E. J.
17. WHO document, Iodine status worldwide: WHO Global Database on Iodine
18. WHO document, Iodine status worldwide: WHO Global Database on Iodine
20. US Department of Health and Human Services, Food and Drug Administration
21. Food and Nutrition Board Institute of Medicine 2001 Dietary reference in-takes.
22. Aquaron, R. Iodine content of non iodized salts and iodized salts obtained
from the retail markets worldwide. 8th World Salt Symposium: Hague, the
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CHAPTER II
2.1 Introduction
research is to determine if iodized salt in the US contains the level of iodine that it
purportedly does. We also wanted to determine if iodine is lost from the salt over
matrices, including drinking water, surface water, saline water, sewage and
with bromine water and then titrated with Phenylarsine Oxide (PAO) or Sodium
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The applicable concentration range is 2-20 mg/L; the range of iodide that would
be obtained by dissolving a real iodized salt sample without getting problems from
chloride does not correspond well to this. The execution of the method is
onerous: while details are not given here, some eighteen different reagents are
needed to carry out the method. To avoid interference, visible excess of CaO
This method is not likely the best choice to determine iodide in large number of
salt samples.
coupled with different detectors for the determination of iodide. Dionex Corp.
sulfide, iodide and other common anions in water that form insoluble silver salts.
Detection Limit was as low as 5 μg/L iodide with 10 μL sample. Bichsel reported
Iodide was determined as IBr2- at 249 nm, which was formed after the IC
0.1 μg/L iodide. Dudoit and Perganits reported a IC method with conductivity
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possible to measure iodide at the sub-μg/L level. ICP-MS is one of the most
reliable and sensitive methods to measure iodine. Gilfedder et al.5 used ion
precipitation (rain and snow) collected from various locations. Their results
suggest that iodate may not be the most common iodine species. Haldimann et
al.6 measured iodine content of various food groups in the Swiss market using
I-129 as an internal standard. These authors also looked at the iodide catalyzed
reaction (Sandell-Kolthoff reaction) between Ce4+ and As3+ for the determination
whether or not the salt is iodine fortified. 7 Another Catalytic reaction, the
Moxon-Dixon method, involves the loss of color from the Fe3+-SCN- complex due
to the slow reduction of Fe3+ to Fe2+by NO2-; this is a process that is catalyzed by
used as an internal standard6,7,9 but some maintain that the isobaric 127IH2+ poses
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The ICP–MS used was an X Series II ICP-MS with nebulizer peltier cooling
option (Thermo Electron Corporation). Samples were introduced to the ICP via
software was used to optimize the ICP-MS operating parameters, control the auto
sampler and acquire the mass data. The optimized operating parameters and
All chemicals used were analytical reagent grade and deionized (DI) water
An internal standard was added to all samples and used to quantify iodide.
The internal standard was made from Germanium (IV) Oxide (Strem Chemicals,
99.999%). A 3700 ppm stock solution was made by dissolving 0.37g GeO2 in
working solution.
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reference. Potassium iodide (KI, Baxter Scientific Products, AR) was used to
prepare iodide standard. A 1000 mg/L stock iodide standard was prepared by
prepared by diluting 25 g solid NaCl to a 500 mL final volume with DI water. The
iodide standards were prepared by spiking various amounts of the 1000 mg/L
iodide stock solution into 10 mL of the 5% NaCl solution. Six iodide standards
Iodized salt samples were sealed in a plastic zip lock bag and wrapped in
aluminum foil and stored in a freezer (-20oC) until the time of analysis. Sample
stand for 8 hours in a dark location to allow the salts to fully dissolve and then they
were filtered through a 0.45 μm Nylon syringe filter (FisherBrand) to remove any
The samples and standards injected and analyzed by the ICP-MS were
This was done by adding 9.8 mL DI water to 0.1 mL of the 5% salt solution in a 15
mL culture tube. All samples and standard were then spiked with 0.1 mL of the
3.7 mg/L Ge internal standard solution to give a final concentration of 7 μg/L 72Ge
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2.3.3 Measurements
Samples and standards were loaded into the auto sampler rack and the
automated analysis procedure was initiated. A peristaltic pump built into the
ICP-MS was used to prime the sample into the nebulizer at 1.6 mL/min for 45
seconds and then continuously aspirate the sample into the nebulizer at 0.8
sampling was complete, the auto sampler probe was washed in DI water for 1
minute.
The calibration of the iodide standards showed excellent linearity over the
calibration range (R2=0.9985). The limit of detection (LOD, 3σ, where σ is the
standard deviation of the blank) routinely obtained by the ICP-MS was 0.047 μg/L.
Iodide recovery was measured by means of spike recovery. The measured and
recovered iodide content was generally in good agreement and the overall
method was the used to determine iodide content in salt samples collected within
the US.
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iodide (KI) is less stable than potassium iodate (KIO3) because it can be oxidized
exposure to light and heat. Storage conditions will affect iodine stability in table
salt.
“Wegmans” iodized salt was used to study the humidity effect, dextrose was
added in as stabilizing agent from the production point. Humidity was controlled
In the closed system in Figure 2.1, 20 grams of iodized salt were placed on a
watch glass, which is located on a 25 mL glass beaker. All of them were placed
in a 500 mL glass beaker, which contained 50 mL sulfuric acid in it, and the beaker
was sealed with plastic wrap. The real-time humidity was monitored by
Hygrometer (Extech, RH-45400). The relative humidity (RH) in the three closed
500 mL glass beakers was measured as 90.1%, 81.8% and 67.6% at about 25 oC
room temperature. The results confirm the significant effect of ambient humidity
on iodine stability (Figure 2.2). Iodine in table salt was not detectable after six
weeks storage at RH 90%. The iodide content of salt decreased from 75 mg/kg
by to about half (39 mg/kg) at RH 67.6% and to 1/5 the original content (15 mg/kg)
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at 81.9% after the same length of time (6 weeks). Note that the rate of loss was
by the time 80% RH is reached. The deliquescence point of NaCl is 75% RH,
and our observations thus indicate that actual uptake of liquid water may be
Four iodized salt samples were purchased from local grocery stores
(Lubbock, TX). The four brands of iodized salt used in this experiment, Salt
sense, Rich food, Morton and Hain sea iodized salt were all added with dextrose
as stabilizing agent from the production point. About 5 grams of each salt was
placed in an oven preheated to 200 oC for 5 minutes. This thus simulated dry
heating to which iodized salt may be exposed to during cooking. The iodine
respective concentrations (Figure 2.3). The two samples with the highest
original iodine concentrations lost 10- 20% of their iodine during heating.
“Wegmans” iodized salt was used to study the light effect. Each of 20
grams of salt was stored exposed to ambient air (mean RH over the period was
36 %) with or without room fluorescent lighting, which was kept on 24 hours a day.
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mg/kg after 42 days exposure to ambient air. Iodine in the salt sample stored in
ambient air and under light lost only slightly more iodine than the sample stored
without light (Figure 2.4). The iodine decay rates were very similar in both
Voluntary fortification of salt with iodine was introduced in 1924 and resulted
not mandatory in the US Iodine is added to salt in the form of potassium iodide
(KI) or potassium iodate (KIO3) either as a dry solid powder (dry mixing) or in
water dissolved with salt solutions (spray mixing or drip feed mixing) at the point
60-100 mg/kg to salt that is sold in the US market labeled as “Iodized Salt”.
Stabilizing agents such as sodium thiosulfate (Na2S2O3) and Dextrose are added
companies, among them are INQIUM, FRANMAR and IODINEX in Chile and
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Iodine is added to table salt by dry or wet methods. There are no reports in
marketed iodized salt. To study this, four brands of iodized salt were purchased
at local grocery stores. Five salt samples of 1.25 g each were taken from each of
the 4 new salt containers from the very top, very bottom, and three more evenly
spaced depth settings. Figure 2.5 shows the homogeneity (or lack thereof) of
iodine concentrations in each of the 4 brands of iodized salt tested. The results
obviously indicate that iodine distribution homogeneity differs from one sample to
basis of the analysis of a single can, it is clear that the iodine was not uniformly
newly purchased table salt samples collected across 42 states in the US.
samples from the top when they newly purchased a can/box. The sample was
put in a Zip-lock bag with care taken to not leave too much air in the bag, and then
wrapped in Al foil prior to sending us, along with brand, batch and date of
purchase information. Our volunteers not only took the trouble to do this, all
expenses for this enterprise was borne by them and we thank them for this.
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Upon arrivals the samples were catalogued, acknowledged and immediately put
iodine per serving size (1.5 g). That is, the iodine concentration should be
that more than half the samples had iodine concentrations below the 45 mg/kg
standard. Of the received 94 first iodized salt samples from 37 states, the mean
iodide concentration was 43.96 ± 20.50 mg/kg. Concentrations ranged from 2.78
± 0.30 mg/kg to 149.97 ± 0.87 mg/kg. The sample with the lowest iodine
concentration was purchased in Washington (WA) and the highest came from
course the caveat that the number of samples studied for a given brand is not
2.4.2.5 Does Iodine Content Decay Over a Period of Time Under Actual Use
Conditions?
storage, especially under humid conditions, this does not directly answer the
question whether iodine loss similarly occurs during actual use since an average
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can of salt typically lasts several months in an average household. The humidity
practices of how a container of salt is kept (spout open, spout closed, etc.) may
differ greatly. Note that if we got our daily iodine requirement solely from salt, it
will require a family of three 2.5 months to finish a 737 g container of salt
our request, several of our volunteers not only sent a ~ 5 g sample aliquot when
they first purchased a new container of salt, they similarly sent additional sample
aliquots when the container was approximately half empty and sent a final sample
when the container was nearly empty. The dates were noted and recorded.
Our data showed it required between 17 and 225 days for the salt container to be
half emptied (112.83±53.25 days) and between 38 and 349 days from the
Forty-seven salt providers sent us the two salt samples obtained when the
salt container was just purchased and when half empty. Eleven of the 47 second
samples had less than 15% the iodine present in the first samples (Figure 2.9).
Twenty-four providers sent third samples from their containers. Ten of these
premature to conclude that iodide decay during the period of time in household
storage condition because previous experiment showed that the iodide added in
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2.5 Conclusion
iodine in iodized table salt in the US where the iodization vector is iodide. Iodine
loss from salt may occur, especially when stored under high humidity. Additionally
our experiments suggest that more iodine may be lost during cooking.
in different iodized salt samples sold in US markets. More than half of the
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2.6 References
(6/16/2007).
2. Cheng, J.; Jandik, P.; Avdalovic, N. Anal. Chim. Acta. 2005, 536, 267-274
6. Haldimann, M.; Alt, A.; Blanc A.; Blondeau, K. Journal of Food Composition and
8. Perring, L.; Basic-Dvorzak, M.; Andrey, D. Analyst, 2001, 126 (7), 985-988.
10. Bienvenu, P.; Brochard, E.; Excoffier, E.; Piccione, M. Canadian Journal of
103-109.
12. Yamada, H.; Kiriyama, T.; Yonebayashi, K. Soil Science and Plant Nutrition,
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13. Poluzzi, V.; Cavalchi, B.; Mazzoli, A.; Alberini, G.; Lutman, A.; Coan, P.; Ciani,
I,; Trentini, P.; Ascanelli, M.; Daovoli, V. Journal of Analytical Atomic Spectrometry,
14. Nobrega, JA.; Genlinas, Y.; Krushevska, A.; Barnes, RM. Journal of Analytical
15. Chemistry of the Elements, Green wood, N.N.; Earnshaw, A. 2nd Ed.;
16. http://www.ceecis.org/iodine/08_production/00_mp/prod_iod_stability.pdf
(5/12/2007).
17. http://www.micronutrient.org/Salt_CD/4.0_useful/4.1_fulltext/pdfs/4.1.1.pdf
(5/12/2007).
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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)
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Plastic Wrap
Iodized Salt
Watch Glass
25 mL Glass
Beaker
Sulfuric Acid
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.
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80
Effect of Moisture
90% RH
80% RH
65% RH
40% (RH in our lab, Lubbock,TX)
Iodide concentration (mg/kg)
60
40
20
0 10 20 30 40 50
Time (day)
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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60
Original content
After heating
Iodide content (mg/kg)
40
20
0
Salt Sense Richfood Morton Hain
Sea Salt
Figure 2.3 Iodine in dry salt comparison before and after being heated for 5
minutes at 200 oC.
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80
Light Effect
No Light
With Light
Iodide Concentration (mg/kg)
60
40
20
0 10 20 30 40 50
Time (day)
Figure 2.4 Iodine decays slightly in the presence of light, under room temperature
(22oC) and humidity (RH=40%)
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Average (SD)
80
Average (SD)
Average (SD)
60
Average (SD)
Iodide content, mg/kg
40
20
0
Hain Rich Wegman's Salt
Sea Food Sense
Salt
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160
120
Iodide content (mg/kg)
80
45% RDA
level
40
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)
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120
80
45% RDA
level
40
0
Salt purchased from 37 States in US
Figure 2.7 Iodine concentration in salt samples from 37 states in US
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100
80
Iodide Concentration (mg/kg)
60
40
20
0
21 Brands of iodized salt purchased in US
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160
1st sample
2nd sample
Iodine Concentration (mg/kg)
120
80
40
0
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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250
2nd sample
3rd sample
150
100
50
0
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
3.1 Introduction
the latter also serving as a virtual reference electrode. The peak height
operated directly in a flow injection analysis (FIA) system and also in conjunction
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
electrode, while the working electrode was at virtual ground, being connected
converter but inverted the sign of the signal; the second stage (1/2 TL072CN)
To acidify the iodate sample flow stream prior to detection, it will thus be
water:
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Texas Tech University, Yining Liu, August, 2007
H2O → 2 H+ + ½ O2 + 2e …(2)
It would have been possible to use a merging stream of acid. However, this
NAFION membrane into the flow stream. Although the penetration of sulfate
NAFION ®
Naf ion (R)
(CF2-CF2)x (CF2-CF)y
O
F3 C C F
CF2
CF2
SO3 -
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
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Texas Tech University, Yining Liu, August, 2007
Donnan Forbidden ion penetration rate through small diameter ion exchange
membrane tube had been studied two decades ago.4 An ion similarly charged
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
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
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
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Texas Tech University, Yining Liu, August, 2007
The voltage output from the homemade amperometric detection system was
model A22m.
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Texas Tech University, Yining Liu, August, 2007
sensitive (the detection limit is in the μg/L level for most anions) and the
Chilean Caliche soil samples. Iodate is a hard ion with a low charge density.
difficult to separate iodate from other poorly retained ions, most notably fluoride.
Under most IC conditions, fluoride and iodate elute virtually together, almost with
the same sample is also necessary, analysis time is greatly prolonged. Gradient
elution protocols with a long re-equilibration time become essential, making the
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Texas Tech University, Yining Liu, August, 2007
The reduction current peak height is directly related to the iodate concentration in
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
Suppressor. The conductivity measurement of all the anions was then carried
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
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Texas Tech University, Yining Liu, August, 2007
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
column. Three solutions were prepared for each solid Caliche sample because
All chemicals were analytical reagent (AR) grade. The standard iodate
solutions and acid reagent were prepared with DI water (Millipore, 18.3 MΩ). A
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
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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Texas Tech University, Yining Liu, August, 2007
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
voltage of 250 mV was then fixed to the amperometric detector electrodes for
iodate detection.
Sodium hydroxide eluent runs through the IC system during the first 8
minutes. 35 mM KOH eluent runs for the remaining 15 minutes until the last
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
response fits a nice linear relation with concentration (Figure 3.5) and the best-fit
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Texas Tech University, Yining Liu, August, 2007
The detection limit (based on S/N=3) was 17.6 μg/L. The relative standard
deviation was 2.13 % for 10 repeated injections of a 500 μg/L iodate standard.
for the iodate anion relative to fluoride whereas both anions respond in
conductivity detection. The iodate signal is overlapped with that of fluoride in the
detector output (red line) selectively responds to iodate. There are minor
apparent responses to the other anion peaks which are present in very large
inactive ions (chloride, nitrate, and sulfate) because the simple two-electrode
detection system does not have any additional background electrolyte in the
system and the large increase in solution conductance reduces the solution
resistance.
Chilean Caliche samples were made available by Dr. Jason Rech from the
Department of Geology, Miami University of Ohio. Table 3.2 shows the relevant
data. Iodate concentrations in 13 solutions made from five Chilean Caliche soil
dilution factor, the range of iodate in solid soil samples was 215.72 ± 4.74 mg/kg
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Texas Tech University, Yining Liu, August, 2007
sample solutions with various amount of iodate. The measured and recovered
iodate concentration was generally in good agreement and the overall recovery of
iodide was found to be 93.78% ± 0.78%. The IC-CD data ids always higher than
cannot calculate the area of the iodate peak accurately when it is overlapped with
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Texas Tech University, Yining Liu, August, 2007
In the past few years, several analytical methods and techniques have been
Flow Injection Analysis (FIA) systems are inherently simple and represent a
fast and inexpensive means for the determination of iodate in table salt. Xie and
determination of iodate and iodide in table salt10; the method cannot distinguish
designed to connect with a FIA system for the determination of iodate in table
salt11. They used a single line system with acidic iodide solution as a carrier.
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Texas Tech University, Yining Liu, August, 2007
The formed triiodide was reduced to iodide and the process was monitored
The sample throughput was 35/hour. A limit of detection of 0.5 mg iodate/L was
reached in 1.0 % (w/v) salt solution, equal to an LOD of 50 mg iodate /kg salt.
However, some iodized salts may have iodate concentrations lower than this.
LOD was reported to be 8 x10-8 M. Tian and Chen13 et. al. similarly reported on
was 5 x 10-7 M. Tian et al.14 cast an organic gel film containing LixMoOy and
practical approach will result from such involved method of electrode fabrication.
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Texas Tech University, Yining Liu, August, 2007
Injection Analysis (FIA) system to determine iodate in table salt solution; table salt
virtual reference. The resulting current peak is linearly proportional to the iodate
detection limit of below 10 μg/L is reached; the linear r2 over this concentration
range is 0.9961. Iodate is the main form of supplementary iodine in table salt
sold in Asian countries and is the preferred iodization vector recommended by the
determine iodate in 24 samples of iodized salt taken in triplicate from eight solid
salt samples obtained variously from India, China, Thailand and Australia.
Eight iodized table salt samples were obtained from providers in India,
China, Thailand and Australia. Three subsamples were taken from each solid
salt container, respectively from top, middle and bottom. Each of three 0.5
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Texas Tech University, Yining Liu, August, 2007
All chemicals were analytical reagent grade and were used without further
purification. All solutions were prepared with deionized (DI) water (Milliipore,
18.3 MΩ). A standard stock solution containing 2 g/L iodate was prepared by
water to give a final volume of 250 mL. The stock solution was diluted with 1 %
NaCl solution to make 2 mg/L, 1 mg/L, 500 μg/L, 250 μg/L, 100 μg/L and 50 μg/L
iodate standards.
The arrangement is typical of a FIA system (Figure 3.7 a). The carrier
solution (1% w/v reagent grade NaCl) was peristaltically pumped (Dynamax RP-1,
Rainin Inc.) peristaltic pump at a flow rate of 1.5 mL/min and samples were
injected with a 6-port distribution valve. A fixed 500 μL sample volume was then
injected into the stream. The flow stream was acidified to pH ~1 by passing it
amperometric detector cell (same NAFION tube and detector cell devices
through a Teflon tube jacket (flow rate = 0.1 mL/min). In the present case, the
membrane and thus far more acid is introduced compared to when water is
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Texas Tech University, Yining Liu, August, 2007
The optimal applied voltage was determined by varying the applied voltage
NaCl in triplicate under the same FIA conditions described above. The signal to
noise ratio reached a maximum at 300 mV (Figure 3.8). A voltage of 300 mV was
3.3). A series of 1 mg/L iodate standards were injected into the system and the
signal to noise ratio (S/N) was monitored. The best S/N was observed at a flow
rate of 1.5 mL/min when the flow rate was varied from 0.2 to 2.0 mL/min (Figure
3.9).
The sample injection volume was varied from 100 μL to 1000 μL by altering
the length of the sample injection loop (Figure 3.10). Increased injection volume
causes an increase in signal (and S/N) until an injection volume of 500 μL and
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Texas Tech University, Yining Liu, August, 2007
The calibration curve for iodate was obtained under optimum conditions as
determined above. The amperometric signal was linear with the iodate
concentration in the range of 50-2000 μg/L in 1% salt solution. Figure 3.11 shows
data traces for the optimized instrument for an iodate calibration series with each
(Figure 3.12.). The detection limit (3 times the noise level) was 7.7 μg/L (4.4 x
10-8 M, the best reported to our knowledge) and the relative standard deviation
Three replicates of each of the eight iodized table salt samples were
analyzed under the same conditions as the standards. The iodate concentration
was calculated from the calibration equation. The relative standard deviation
(RSD) of the three sample solutions prepared from each solid salt was calculated
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Texas Tech University, Yining Liu, August, 2007
3.5 Conclusion
system was developed for the determination of iodate in solution. This method
utilizes the reduction of iodate in an acidic medium under optimum applied voltage
samples and in iodized table salt samples from India, China, Thailand and
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Texas Tech University, Yining Liu, August, 2007
3.6 References
1. Kenneth, A.; Mauritz, R.; Moore, B. Chem. Rev. 2004, 104, 4535-4585.
3. Seger, B.; Vinodgopal, K.; Kamat, P.V. Langmuir, 2007, 23, 5471-5476.
4. Dasgupta, P. K.; Bligh, R. Q.; Lee, J.; D’Agostino, V. Anal. Chem. 1985, 57,
253-257.
11. Jakmunee, J.; Grudpan, K. Anal. Chim. Acta. 2001, 438, 299-304.
12. Chen, L.; Tian, X.; Tian, L.; Liu, L.; Song W.; Xu, H. Analytical and
13. Chen, L.; Liu L.; Tian, L.; Lu, N.; Xu, H. Sensor and Acuators B-Chemical,
14. Tian, L.; Chen, L.; Liu, L. ; Lu, N.; Xu, H. Analytical and Bioanalytical
16. Sun, D.; Zhu, L.; Zhu, G. Analytica Chimica Acta, 2006, 564, 243-247.
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Texas Tech University, Yining Liu, August, 2007
(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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Texas Tech University, Yining Liu, August, 2007
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Texas Tech University, Yining Liu, August, 2007
70
60
Signal / Noise
50
40
30
20
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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Texas Tech University, Yining Liu, August, 2007
0.48
1500 ppb
0.46
0.44
Detector Output (V)
1000 ppb
0.42
500 ppb
0.4
250 ppb
100 ppb
0.38 50 ppb
0.36
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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Texas Tech University, Yining Liu, August, 2007
0.48
Y = 5.923*10-5 * X + 0.3706
R2 = 0.9998
0.46
0.44
Output (V)
0.42
0.4
0.38
0.36
0 400 800 1200 1600
Concentration (μg/L)
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Texas Tech University, Yining Liu, August, 2007
4 0.4
Iodate
Fluoride
2 0.36
0 0.32
-1
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Texas Tech University, Yining Liu, August, 2007
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Texas Tech University, Yining Liu, August, 2007
1000
600
Signal/Noise
400
200
0
0 200 400 600 800
Applied Voltage (mV)
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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Texas Tech University, Yining Liu, August, 2007
250
150
100
50
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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Texas Tech University, Yining Liu, August, 2007
280
240
Signal/Noise
200
160
120
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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Texas Tech University, Yining Liu, August, 2007
0.25
2000
0.2
Detector Output, V
0.15 1000
0.1
500
250
0.05 100
50
0
0
0 2000 4000 6000 8000
Time (s)
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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Texas Tech University, Yining Liu, August, 2007
0.1
Y = 1.010*10-4*X + 0.0296
R2 = 0.9961
0.08
Detector Output (V)
0.06
0.04
0.02
-0.02
0 400 800 1200 1600
Concentration (μg/L) 7
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Texas Tech University, Yining Liu, August, 2007
CHAPTER IV
CONCLUSIONS
detector gives a good selective response for iodate without interference from
fluoride. When used in a Flow Injection Analysis system, the detector gives very
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.
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Texas Tech University, Yining Liu, August, 2007
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