1. Extended Essay
Is there a relationship between the arsenic concentration in rice and the origin of the rice grain?
Name: Adam Costa
I.B. Candidate Number: 000534-062
School: DeLand High School
Date: 12-12-13
Supervisor: Dr. Monroe
Word Count: 3,846
2. 2
Abstract
Rice is grown under conditions that are conducive to the effortless absorption of water, and its
dissolved contents, into the rice plant’s membranes. Eventually, the water and its contents reach
the grain of the rice plant. Therefore, aqueous inorganic arsenic has the tendency to be readily
absorbed into the rice plant’s roots and stem along with the water that it is dissolved in. This
possibility leads to the questioning of there being a reasonable relationship between the arsenic
concentration in rice grain and the origin of the rice grain. The previously mentioned potential
for there to be a one-to-one correlation between the arsenic concentration found within rice
grains and the rice grains’ location of origin leads to the plausible hypothesis that the aqueous
inorganic arsenic in the irrigation waters for the rice crop will transcend the plants’ membranes,
until the concentration of inorganic arsenic in the irrigation waters and in the plants’ membranes
come to equilibrium. In the case of this investigation, the rice subspecies were altered between
each experimental trial (independent variable). The quantity recorded during this investigation
consists of the arsenic concentrations determined during this investigation’s experiment
(dependent variable). These experimentally yielded values were compared with arsenic
concentrations determined in professional field studies, with the intent of defining the
relationship between the arsenic concentrations found within the rice grains and the arsenic
concentrations found in the soil from where the rice grains originated from. The contrary to the
proposed hypothesis was discovered, as displayed by the accrued results: high arsenic
concentrations found within rice grains were associated with locations of origin that correlated
with low arsenic concentrations and vice-versa.
Word Count: 271
3. 3
Table of Contents
Introduction.................................................................................................................pgs. 2-3
Determination..............................................................................................................pgs. 3-4
Research Question......................................................................................................pg. 4
Hypothesis...................................................................................................................pgs. 4-5
Methodology...............................................................................................................pgs. 5-7
Calibration.......................................................................................................pgs. 5-6
Experiment......................................................................................................pgs. 6-7
Results and Analysis ...................................................................................................pgs. 7-8
Arsenic Concentrations in Rice Grain............................................................pg. 7
Arsenic Concentrations at the Locations of Origin of Rice Grains................pgs. 7-8
Conclusion...................................................................................................................pgs. 8-9
Evaluation and Further Investigation..........................................................................pgs. 9-10
Bibliography................................................................................................................pgs. 10-11
Appendix.....................................................................................................................pgs. 12-14
Appendix I......................................................................................................pg. 12
Appendix II ....................................................................................................pg. 13
Appendix III...................................................................................................pg. 14
4. 4
Introduction
Rice (genus Oryza. O. sativa) is one of the most consumed, if not the most consumed, grain,
nonetheless food product, in the world. For more than half of the world’s population rice is an
ever growing and permanent stable, with more than 90% of the world’s rice being consumed in
Asia. (4)(6) With such an enormous base of consumption, this grain has become increasingly
subsided by major countries. India provides an excellent example of a nation that has built on the
name of rice and has made it readily available to it poorest households.
With so much of the world relying on this grain, we call rice, the conditions and status of this
most important consumable must be kept in check. Given this fact, the world’s health and safety
organizations have taken measures to ensure the safe and secure growth, cultivation, and
distribution of rice. Among one of the analytical techniques employed by these international
health and safety organizations, and one that has surprised the world over, is the determination of
arsenic in all variants of rice.
Arsenic is known as a potent carcinogenic element, so it would be no surprise that international
health and safety organizations were to ensure that the world’s most consumed food stuff would
be free from cancer causing substances, specifically arsenic. With the inevitable discovery of
arsenic in rice, I say inevitable because of the conditions rice irrigation waters have been placed
under (tin mining; fertilizers and insecticides; poor sanitation; and natural evolution of inorganic
arsenic into water reserves, aquifers, and water tables.), came the increased coverage; analysis;
and research into the origins, interactions, and concentrations of arsenic in rice.
These precursors led the determination of the final decision on how this research, experiment,
and paper were to be composed. A decided topic and question that was reasoned to be pertinent,
yet given limited coverage referring to heavy metals in rice and the risks associated with this was
resolved to be an evaluation of the relation between polished white rice grains and their region
(soil) of origin.
The method of determination of the concentrations of arsenic in rice samples boils down to
Beer’s Law. This law, in its essence, states that the transmission of light through an object is
dependent upon the absorption coefficient of an object or substance. The absorption coefficient is
based upon the molar absorptivity of the substance and the molar concentration of the substance.
An understanding of whether the arsenic content in the rice grain is directly correlated to the
arsenic content of its origin or not would allow for the comprehension of whether the arsenic
content in rice irrigation water truly matters, in the sense of rice contamination by heavy metals.
This knowledge would lead to an understanding of one of two possibilities. One such possibility
would be that the arsenic content of the rice grain is not correlated to the arsenic content of the
origin of the rice grain. In this case, rice species and genetic predispositions for arsenic uptake
would be taken into consideration as a determinate of how much arsenic would be concentrated
within a single grain of rice. The second of the two possibilities would be that the rice grain and
5. 5
origin arsenic concentrations would be of mirroring relation, and in this case, the origin of the
rice would be considered when regarding the concentrations of arsenic contained within a grain
of rice.
Determination
There are varying ways of determining the presence of arsenic in solutions, compounds, or
substances, such as the liberation of arsenic by acid digestion and analysis by flame Atomic
Absorption Spectroscopy, Silver Diethyldithiocarbamate Colorimetric analysis, Inductively
Coupled Plasma-Mass Spectrometry, and Hydride Generation Atomic Absorption analysis.
(10)(14)(17) In the case of this experiment, the Gutzeit reaction was chosen as the method of
choice when it came to the determination and analysis of inorganic arsenic species. The reason
for the decision to utilize the Gutzeit reaction for this experiment lies in how many iterations,
tests, and evaluations it has undergone from its initial usage in 1836 to its contemporary usage.
(13) Modern iterations of this reaction have become immensely more quantitative, accurate, and
require more readily available equipment and reactants.
This reaction begins with the evolution of hydrogen gas in a solution of aqueous inorganic
arsenic. In the case of this experiment, sodium borohyride and sulfamic acid were utilized for the
evolution of hydrogen gas. Given sodium borohyride and sulfamic acid’s tendency to favor
stronger bonds that transpose hydrogen, they are constituted along the lines of being strong acids,
which is ideal for the evolution of hydrogen gas for this experiment. Then, the aqueous arsenious
acid reacts with the evolved hydrogen gas to yield arsine gas. Following, the arsine gas reacts
with mercuric bromide. The original reaction employed silver nitrate as the reagent that would
react with the arsenuretted hydrogen, which led to this reaction becoming rather expensive to
conduct. An alternative reagent soon replaced silver nitrate (mercuric bromide), which yields a
colored compound ranging from bright yellow to black, depending on the concentration of
arsenious acid.
Production of hydrogen from sodium borohydride and sulfamic acid:
NaBH4 (s) + H3NSO3 (s) + 3H2O (l) 4H2 (g) + B(OH)3 (aq) + NaH2NSO3 (aq)
Production of arsine gas from arsenious acid:
H3AsO3 (aq) + 3H2 (g) AsH3 (g)+ 3H2O
H3AsO4 (aq) + 4H2 (g) AsH3 (g) + 4H2O
6. 6
Production of yellow-through-black color:
HgBr2 (s) + AsH3 (g) H2As–HgBr (s) + HBr (g)
Research Question
Is there a relationship between the arsenic concentration in rice and the origin of the rice grain?
This question was formed through conducted research and professional consultation. Initial
questions were composed with resource availability in mind. Further questions strove to
investigate limitedly covered topics. Consequently, reactants and apparatuses that were not
readily available and additionally had infeasible cost projections were acknowledged, and these
questions were quickly omitted from consideration. Research question alternatives and thorough
professional opinion were considered, resulting in the terminal research topic utilized for the
basis of this investigation. Further research allowed for a narrowed and definitively worded
research question with which could be sufficiently investigated; had international relevance; yet
was sparsely covered, in the sense of direct experimental conduction.
Hypothesis
The outcome of this investigation is predicted to be found as a positive correlation between the
concentration of arsenic found within the rice grains and the concentration of arsenic found at the
location of the rice grains’ origin. With the understanding that rice plants are submerged as a
condition of healthy growth and desirable grain yield, it can be concluded that the roots and
stems of the submerged plants would absorb the water that submerges the plants, as a part of the
plants’ method of producing food for their furthered growth and survival. Under the
circumstances of this investigation, the irrigation waters for these rice plants would be laden with
dissolved inorganic arsenic, and consequently, the plants would unintentionally take in the
aqueous inorganic arsenic. Additionally, given that the inorganic arsenic would have been at a
higher concentration within the irrigation water than the water within the plants, the inorganic
arsenic would traverse the semi-permeable membranes of the plants through osmosis, thus
bringing inorganic arsenic concentrations within and on the exterior of the plants to equilibrium,
one-to-one ratio. The dissolved heavy metal compound would travel up the roots and stems of
the plants and reach the plants’ grain (rice). The inorganic arsenic would enter the rice husks and
starchy fleshes. At this stage, the arsenic would become surrounded and contained by the soft,
sticky Amylopectin of the rice’s starchy flesh, thus remaining in the rice grain, until excited
(boiled) out of the rice grain. With this understanding, it can be concluded that the inorganic
arsenic concentrations within the plant, tantamount to that of the concentration of inorganic
arsenic in the rice grain, would have a relative correspondence to the concentration of inorganic
arsenic in the irrigation water from the rice grain’s place of origin.
7. 7
Methodology
Calibration
Sodium arsenate “salt” Na2HAsO4 * 7H2O was used as the compound by which the arsenator,
provided by an arsenic testing kit, used for this experiment was calibrated. 312.01 g. of this salt
is one mole of this salt, while 74.92 g. of 312.01 g. of this salt is one mole of arsenic in this
compound. A 10 mM. solution was created by measuring 0.312 g. of the salt with an electric
scale, accurate to the milligram, and adding the measured salt to a 100 mL. volumetric flask
containing 100 mL. of distilled water. The resulting solution had a concentration of
approximately 7.5 𝑥 105
𝜇𝑔𝐿−1
. of arsenic.
The solution of 7.5 x 105 𝜇𝑔𝐿−1
. was diluted 15 times to 5.0 x 104 𝜇𝑔𝐿−1
. by taking 66.67 𝜇𝐿. of
the 7.5 x 105 𝜇𝑔𝐿−1
. solution and adding it to a 1 mL. flat-bottom centrifuge tube that contained
933.33 𝜇𝐿. of distilled water. 20, 40, 60, and 80 𝜇𝐿. of the 1 mL., 5.0 x 104 𝜇𝑔𝐿−1
. stock solution
were used to create four working calibration solutions with concentrations of 20, 40, 60, and 80
𝜇𝑔𝐿−1
. of arsenic respectively. The 20, 40, 60, and 80 𝜇𝐿. of the stock solution were added to the
100 mL. sample flask provided by the test kit, which contained 50 mL. of distilled water before
each individual addition of the calibration samples (Appendix I). The calibration proceeded by
following the procedure provided by the test kit.
The hydrogen sulfide filter provided was placed into the hole at the bottom of the provided bung.
Care was given in wiping the grasping tips of the provided stainless-steel forceps with a towel
before usage. The red filter slide from the kit was filled with one piece of the red slide filter
paper, while wearing latex gloves and using the stainless-steel forceps to position the filter paper
onto the outline of the filter paper on the slide, to prevent the leakage of evolved sample gas. The
slide was promptly slid into the top slot of the bung. The black filter slide was also filled with
one piece of filter paper through the same method used with the red slide, but the filter paper
used was particular to the black slide: the black slide filter paper, which was impregnated with a
high concentration of mercuric bromide.
The arsenator was turned on and the black slide was slid into the slot of the arsenator, once the
screen read, “insert slide.” Then, the flask filled with 50 mL. of the sample was filled with one
sachet of the A1 powder provided. The black slide was then expeditiously transferred from the
arsenator to the bottom slot of the bung, since the timer for the reaction began on the screen of
the arsenator once the black slide was removed from the slot. Then, the A2 tablet provided was
dropped into the sample flask as the bung sealed the opening of the flask and contained the
reaction. The recommended wait time for this reaction to occur was 20 minutes.
After the passage of these 20 minutes, the bung was removed from the flask and the black slide
was removed from the bottom slot of the bung and was placed into the slot of the arsenator
where the filter paper was then analyzed by the arsenator. Moments later the arsenator gave a
reading of the concentration of arsenic in the sample through U.V.-vis. spectroscopy. The
8. 8
wavelength emitted from the diode of the provided arsenator ranged from 290 nm. to 320 nm..
(16) The reaction product (H2As–HgBr) of this experiment has wavelength absorption values
ranging from 230 nm. to 340 nm.. (11)
The 50 mL. flask containing the reacted sample was then emptied into a waste beaker, the filters
from the slides were emptied into the plastic waste bags provided, one A1 powder sachet and one
A2 tablet were set out, and the sample flask was cleaned and rinsed with distilled water in
preparation for next test. The calibration samples were tested from the 20 𝜇𝑔𝐿−1
. to 80 𝜇𝑔𝐿−1
.,
and a reliability curve was created from the data recorded (Appendix II) (Appendix III).
Experiment
Approximately 40 g. of each rice type, Jasmine, Texmati, and Basmati, were measured
individually using an electric balance accurate to the milligram. Each rice sample was given its
own 600 mL. beaker and was poured into it. 300 mL. of distilled water was then added to each
beaker, using a 100 mL. graduated cylinder. A ring stand with one ring secured 13 cm. from the
top and another ring covered with an iron wire mesh was secured 10 cm. below the first ring was
positioned near a Bunsen burner fuel supply and was utilized for the placement of the previously
mentioned 600 mL. beakers.
One of the three beakers was selected and placed on the wire mesh on the bottom ring of the ring
stand, a watch glass was placed on the rim of the beaker, and a Bunsen burner was connected to
the fuel supply near the ring stand and placed to the side of the stand. The fuel supply was turned
on, the Bunsen burner was ignited using a striker, the intensity of the flame was adjusted to give
a clean, blue flame, and the Bunsen burner was promptly placed underneath the beaker placed on
the ring stand. The water was given 10 minutes to come to a boil, the rice was given 15 minutes
to cook as well as 2 minutes to cool down, resulting in a total heating time of 27 minutes.
During the course of the heating, the rice was stirred twice using a glass stirring rod once 5
minutes into the 10 minute waiting period for the water to come to a boil and once 7 minutes and
30 seconds into the 15 minute rice cooking time. Upon removal of the glass stirring rod from the
water after stirring, special attention was given to the removal of as much water from the stirring
rod as possible by tapping the stirring rod on the sides of the beaker, to prevent the measured
contents of the beaker from being removed from the beaker and skewing the results.
After the rice had finished the cooking, the Bunsen burner’s fuel supply was cut off and the rice
was given 2 minutes to cool. Then, the watch glass was tilted over the beaker to return the
evaporated water that condensed on the watch glass to the beaker. The water from the beaker
was then drained into a 400 mL. beaker while trying to prevent sample rice from entering the 400
mL. beaker. The rice was disposed of in a plastic waste bag that was set to the side. 50 mL. of
the extracted water was poured into a 100 mL. sample flask that the arsenic test kit provided, and
the experiment proceeded by following the procedure provided by the kit. The provided
9. 9
procedure was previously mentioned and can be referenced in the methodology of the calibration
for this experiment.
The boiled 300 mL. from all of the samples individually yielded approximately 150 mL. of test
solution, which allowed for the test to be performed three times per rice sample. The test for the
rice sample that was first tested was then completed two more times, and the other rice samples
were tested in this same fashion until all of the tests were completed and all of the results were
recorded.
Results and Analysis
Arsenic Concentrations in Rice Grain
Arsenic concentrations in the Jasmine rice grain were found to be the highest in comparison to
the other two rice grain samples, at a mean value of 10.33 𝜇𝑔𝐿−1
. with a 40.006 𝑔. rice sample.
The arsenic concentration in the Texmati rice grain was found to be an intermediate
concentration, at a mean value of 9.33 𝜇𝑔𝐿−1
. with a 40.005 𝑔. rice sample. The arsenic
concentration in the Basmati rice grain was found to be the lowest in comparison to the other two
rice grain samples, at a mean value of 3.66 𝜇𝑔𝐿−1
. with a 39.998 𝑔. rice sample.
Arsenic Concentrations (𝜇𝑔𝐿−1
.)
Texmati (40.005 g.) Basmati (39.998 g.) Jasmine (40.006 g.)
1 11 4 11
2 11 4 12
3 6 3 8
Mean 9.33 3.66 10.33
Figure 1: Illustrated arsenic concentrations from experimental trials of the three rice grain
samples as well as the sample masses and mean values.
Arsenic Concentrations at the Locations of Origin of Rice Grains
The arsenic concentrations within the Evangeline aquifer in Texas were found to result in a mean
value of 8.03 𝜇𝑔𝐿−1
.. (3) The Evangeline aquifer resides along the southeast coast of Texas,
running along the coast’s entirety. (3) The location where Texmati rice is grown resides due east
of Houston, Texas: Beaumont, Texas which is located on the southeastern border between Texas
and Louisiana as well as the northeastern section of the Evangeline aquifer. (12) The arsenic
concentrations within shallow wells from aquifers of the Buddhamonthon sub-district of the
Nakhon Chaisi district, Nakhon Pathom province and lower Chao Phraya Basin were found to
result in a mean value of 11 𝑚𝑔𝐿−1
. or 0.011 𝜇𝑔𝐿−1
. (7) The Nakhon Chaisi district resides due
west of Bangkok; the lower Chao Phraya Basin resides within the Bangkok city limits (207 km.
radius). (1) The primary location of Jasmine rice growth resides on the southeastern portion of
10. 10
the Chao Phraya river basin in the Chachoengsao province of Thailand’s central region: Nariang
sub-district. (2) The arsenic concentrations within shallow aquifers and major canals in the
district of Punjab, India were found to result in a mean value of 14.06 𝑝𝑝𝑏. or 𝜇𝑔𝐿−1
.. (5)The
Punjab district of India is the primary region of Basmati rice growth. (9)
Conclusion
Based upon the results recorded during the conduction of this investigation’s experiment and the
values recorded during professional field studies, it can be concluded that the developed
hypothesis for this investigation was incorrect. With regard to the results, the results should not
be analyzed isolated from one another. The ratios between the experimental values and field
study values should be formed and appropriately compared. The numerical arsenic
concentrations found during the experiment of this investigation were determined under the
intention of achieving concentrations that were discernible by the experimental apparatuses.
Varying masses of rice as well as varying timespans of boiling would yield varying
concentrations of arsenic, given that the solvent (water) remained a constant volume, thus an
ideal sample mass and boiling timespan had to be determined. This ideal mass and timespan was
established through experimentation with the objective of determining the sample mass and the
timespan of boiling that would yield arsenic concentrations discernible by the experimental
apparatuses, whereupon the ideal sample mass was found to be 40 grams with the ideal timespan
resulting to be 27 minutes.
Consequently, the exact arsenic concentrations from the rice grain’s location of origin would not
numerically coincide with the arsenic concentrations experimentally yielded. The previously
mentioned issue with numerical values is the reasoning behind the usage of ratios as a means of
analyzing the correlation between experimental and field arsenic concentrations. The ratio
correlate between the experimental arsenic concentrations and the arsenic concentrations found
at the rice grain’s regions of origin are as follows: 0.26 (Basmati/ Punjab, India), 1.16 (Texmati/
Beaumont, Texas), 939.09 (Jasmine/ Nakhon Pathom, Thailand). The previously mentioned
ratios signify the relation with which the experimental and field values were related to one
another with respect to the rice grain’s region of origin (𝑒𝑥.
𝐵𝑎 𝑠𝑚𝑎𝑡𝑖
𝑃𝑢𝑛𝑗𝑎𝑏,𝐼𝑛𝑑𝑖𝑎
). The Basmati relation
communicates Basmati rice as yielding a lesser amount of arsenic with relation to the grain’s
region of origin. Intermediately, the relation between the Arsenic concentrations found within the
Texmati rice grain and its location of origin correlated as being the ratio of most relation.
Regarding the experimental ratio values of the Jasmine rice grain and its respective region of
origin, it is seen that this relation expresses the most deviation between the experimentally
yielded and field yielded values given that this ratio results in an excessively large number which
,based upon the method with which experimental and field values were related, is indicative of a
vastly higher valued experimental yield with reference to the yielded field value. Undoubtedly,
the arsenic-concentration ratio relations between the Basmati rice grain and its location of origin
11. 11
as well as the Jasmine rice grain and its location of origin have no association to one another,
thus nullifying the composed hypothesis for this investigation.
Evaluation and Further Investigations
The results and conclusion of this experiment have spurred the possibility of various secondary
questions. With the understanding that the rice grain of the same species (Aromatic)
demonstrated no correlation between the arsenic contained by the rice grain and their location of
origin, questions can arise regarding how the individual genetic composition of the rice grain’s
subspecies could contribute to a predisposition for heavy metal absorption through predefined
chemical compositions of the plants’ various limbs and yielded grain. Given that rice grows with
shallow roots and thrives on surface water (canals) it would be suggested to solely utilize field
values that referenced canals and rivers. Unfortunately, this investigation lacked devotion to field
values exclusively referencing canal values on the basis that this information was not readily
available, in comparison to the greater availability of values regarding shallow wells and
aquifers. Whether particular rice grain species have a predilection to absorb one heavy metal
over another, cobalt; zinc; or iron uptake preferred over arsenic due to the geometric orientation
in space of certain chemical species or the charge of one chemical species over another, would
be intriguing to question and determine, if further investigations were to be conducted.
Additionally and as a given, it would be beneficial to the conductors of furthered investigations
on this topic to utilize more sensitive and less crude methods of heavy metal concentration
quantification. The quantifying apparatuses for this conducted investigation served their purpose
well but involved qualitative and more error prone methods of quantifying the determination of
heavy metal concentrations, such as a chromatic scale of sample yields where experimentally
yielded yellow through black color intensities would be evaluated with the sample yields,
whereupon the sample yields had concentration values ascribed to them. The experimentally
yielded yellow through black intensities would be given the comparatively intense sample yield’s
concentration value.
Amid instances that involved experimentally yielded color intensities below the provided sample
yields (< 10𝜇𝑔𝐿−1
.), the provided arsenator quantified arsenic concentrations accurate to
the 2.0 𝜇𝑔𝐿−1
. with an analytical variability of 3%. (8)(15) Given that the methods and
apparatuses of quantification for this investigation were scantly accurate at lower values of
concentration, with regard to the amount of accuracy that can be obtained with methods such as
Atomic Absorption Spectroscopy, Silver Diethyldithiocarbamate Colorimetric analysis, and
Hydride Generation Atomic Absorption analysis, it can be concluded that the values acquired
during this investigation should be taken with great scrutiny and furthered investigation with
more sensitive equipment would be of utmost pertinence and importance. Furthermore, the
significance of the mass deviation of the rice samples used for this investigation should also be
acknowledged. Given that this investigation involves quantities of miniscule proportion, it would
12. 12
be essential to determine how much of a significance a one or an eight milligram deviation has
on the prevalence of yielded arsenic concentrations of micrograms.
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