Iodine Test For Starch Practical Experiment
Summary
TLDRThe iodine test is a biochemical method to differentiate certain polysaccharides like starch and glycogen from monosaccharides and cellulose. It involves using a diluted Lugol's iodine solution, which reacts with helical glucose chains in polysaccharides to produce distinct colors. Amylose in starch shows a blue-black color, while amylopectin gives an orange-yellow hue. This test is also used to confirm starch formation in plants during photosynthesis, with a negative result for cellulose due to its linear structure preventing iodine interaction.
Takeaways
- 🌟 The iodine test is a biochemical test used to detect and distinguish certain polysaccharides from monosaccharides and other types of polysaccharides.
- 🔍 A positive iodine test is indicated by the development of color, which varies depending on the type of polysaccharide: amylose turns blue-black, amylopectin orange-yellow, dextrin red, and glycogen reddish-brown.
- 📚 The iodine test was first described by J.J. Cullen and H.F. Gottierdo in 1814, and independently by F. Stromer in 1815.
- 🧪 The reagent used in the iodine test is a diluted form of Lugol's iodine, an aqueous solution of elemental iodine and potassium iodide.
- 🌿 Polysaccharides like starch, dextrin, and glycogen are made up of many alpha D-glucose units linked by alpha 1-4 glycosidic bonds, forming 3D spiral structures.
- 🍠 Starch is composed of two fractions: amylose (linear chain) and amylopectin (branched chain), with amylose forming a colloidal dispersion in hot water.
- 🧬 The iodine test works on the principle that polyiodide ions form colored absorption complexes with the helical chains of glucose residues in certain polysaccharides.
- 🌈 The color produced in the iodine test depends on factors like glucose chain length, temperature, presence of water-miscible organic compounds, and pH.
- 🌱 The iodine test is used in plant physiology to indirectly check if a plant is photosynthesizing by detecting the presence of starch.
- 🔬 The test involves adding a dilute iodine solution to various test samples and observing any color change, with starch turning blue-black, confirming its presence.
Q & A
What is the iodine test used for?
-The iodine test is a biochemical test used to detect and distinguish certain polysaccharides such as starch, dextrin, and glycogen from monosaccharides, disaccharides, and other polysaccharides like cellulose.
What is the difference between a positive and negative iodine test?
-A positive iodine test is indicated by the development of color depending on the type of polysaccharide, while a negative test shows no color change, retaining the original iodine color.
What colors do different polysaccharides produce in a positive iodine test?
-Amylose gives a blue-black color, amylopectin gives an orange-yellow color, dextrin gives a red color, and glycogen gives a reddish-brown color.
Who first described the iodine test?
-The iodine test was first described by J.J. Cullen and H.F. Gottierdo Clubri in 1814, and independently by F. Stormer in 1815.
What is the reagent used in the iodine test?
-The reagent used in the iodine test is a very diluted form of Lugol's iodine, also known as aqueous iodine.
Why was Lugol's iodine created?
-Lugol's iodine was created because elemental iodine is insoluble in water, and the addition of potassium iodide results in a reversible reaction that forms soluble polyiodide ions.
How does the iodine test work with starch?
-The iodine test works with starch because the polyiodide ions present in the iodine solution form colored absorption complexes with the helical chains of glucose residues in starch.
Why does starch appear blue-black with iodine, even though it contains both amylose and amylopectin?
-Starch appears blue-black with iodine because the amylose component forms a very dark blue-black complex that is intense enough to mask the orange-yellow color formed by amylopectin.
What is the role of charge transfer complexes in the iodine test?
-Charge transfer complexes are believed to be responsible for the color changes that occur in the iodine test. They form between the helical amylose and polyiodide ions, resulting in the transfer of charge that excites the electrons of the acceptor molecules, which then emit radiation in the visible spectrum.
Why does cellulose not give a positive iodine test?
-Cellulose does not give a positive iodine test because its beta-D-glucose units are linked by beta-1,4 glycosidic linkages, forming long linear chains instead of a helix, leaving no room for polyiodide ions to slip in and form colored complexes.
How can the iodine test be used in plant physiology experiments?
-The iodine test can be used in plant physiology experiments as an indirect test to check if a plant is photosynthesizing by confirming the presence of starch, which is formed as a reserved food material in the leaves of plants during photosynthesis.
Outlines
🧪 Understanding the Iodine Test
The iodine test is a biochemical method used to differentiate certain polysaccharides like starch, dextrin, and glycogen from other carbohydrates. It relies on the formation of colored complexes with polyiodide ions present in the iodine solution. Amylose, a component of starch, produces a blue-black color, while amylopectin, another starch component, yields an orange-yellow hue. Dextrin and glycogen show red and reddish-brown colors, respectively. The test was first described in the early 19th century and uses a diluted form of Lugol's iodine, an aqueous solution of iodine and potassium iodide. The test is based on the interaction of polyiodide ions with the helical structures of glucose residues in certain polysaccharides, leading to the formation of charge transfer complexes and the resulting colors. Factors such as glucose chain length, temperature, and pH can affect the color development.
🌿 Iodine Test in Plant Physiology
The iodine test is also used in plant physiology to indirectly assess photosynthesis by checking for the presence of starch, a product of this process. Starch is stored in plant leaves and can be detected using the iodine test. The video script describes an experiment using various test samples, including glucose, sucrose, starch, and cellulose, to demonstrate the test's positive and negative results. Starch turns blue-black upon contact with iodine, confirming its presence, while other substances like cellulose do not react, showing the test's specificity. The experiment involves adding test samples to tubes, applying iodine solution, and observing color changes. The difference in reaction between starch and cellulose is attributed to the helical structure of starch, which allows polyiodide ions to form colored complexes, whereas cellulose's linear structure does not.
Mindmap
Keywords
💡Iodine Test
💡Polysaccharides
💡Amylose
💡Amylopectin
💡Dextrin
💡Glycogen
💡Lugol's Iodine
💡Charge Transfer Complexes (CT Complexes)
💡Glycosidic Bonds
💡Cellulose
💡Photosynthesis
Highlights
The iodine test is used to detect and distinguish certain polysaccharides from monosaccharides and disaccharides.
Polysaccharides like starch, dextrin, and glycogen give different colors when tested with iodine.
Amylose in starch gives a blue-black color, while amylopectin gives an orange-yellow color.
Dextrin produces a red color, and glycogen gives a reddish-brown color in the iodine test.
The iodine test was first described in 1814 by J.J. Cullen and H.F. Gottierdo, and independently by F. Stromer in 1815.
Lugol's iodine, used as a reagent, is a diluted form of an aqueous iodine solution.
Lugol's iodine was first made in 1829 by French physician Jean Lugol.
Polysaccharide molecules form 3D spiral structures due to alpha-1,4 glycosidic bonds.
Starch is composed of linear amylose and branched amylopectin.
The iodine test is based on the formation of colored absorption complexes with helical chains of glucose residues.
Triiodide ions in iodine solution form complexes with certain polysaccharides, producing visual colors.
Amylose in starch forms a deep blue or blue-black color in the presence of iodine.
The color of the starch-iodine complex can be detected even at low iodine concentrations.
The iodine test is used in plant physiology to check if a plant is photosynthesizing.
Starch is formed as a reserve food material in plants during photosynthesis.
The iodine test can be performed using a variety of test samples, including glucose, sucrose, starch, and cellulose.
Cellulose does not give a positive iodine test due to its linear structure that does not allow polyiodide ions to form colored complexes.
The iodine test is a popular method for identifying the presence of starch in various food items.
Transcripts
the iodine test is a biochemical test to
detect and distinguish certain
polysaccharides such as starch dextrin
and glycogen from monosaccharides
disaccharides and polysaccharides such
as cellulose
a positive iodine test is reflected by
the development of color which depends
on the type of polysaccharide
amylose gives blue black color
amylopectin gives orange yellow color
dextrin gives a red color
while glycogen gives a reddish brown
color the iodine starts test was first
described by j.j cullen an hf gottierdo
clubri in 1814
and independently by f stromer in 1815
the reagent used in the iodine test is a
very diluted form of lugol's iodine also
known as aqueous iodine
lugol's iodine was first made in 1829 by
the french physician jean lugol
it's an aqueous solution of elemental
iodine and potassium iodide iodine on
its own is insoluble in water addition
of potassium iodide results in a
reversible reaction of an iodide ion
with iodine to form a tri iodide ion
which further reacts with an iodine
molecule to form a penta iodide ion and
so on make sure to check out my short
video on how to prepare iodine solution
by clicking on the link given in the
screen right now or the link in the
description below
polysaccharide molecules such as starch
dextrin and glycogen are comprised of a
large number of alpha d glucose units
that are bound together by alpha 1 4
glycosidic bonds or alpha acetyl
linkages with occasional alpha 1 6
glycosidic linkages that result in
branched chains except in a milos as a
result of the bond angles in the alpha
acetyl linkages these molecules form
branched or unbranched 3d spiral
structures much like a coiled spring
starts can be separated into two
fractions the linear chain amylose and
the branched chain amylopectin natural
starches are mixtures of amylose and
amylopectin amylose forms a colloidal
dispersion in hot water whereas
amylopectin is completely insoluble
iodine test is based on the principle
that polyiodide ions mainly the
triiodide ions present in iodine
solution forms colored absorption
complexes with the helical chains of
glucose residues of certain
polysaccharides
the tri iodide and penta iodide ions
formed are linear and slip inside the
helix structures which then produces
visual colors upon contact with these
polysaccharide molecules
the amylose component in starch is
responsible for the formation of a deep
blue or blue black color in the presence
of iodine while amylopectin produces an
orange yellow hue
dextrins form a red chromogen while
glycogen produces reddish brown color
upon contact with iodine
the amilos component of starch forms a
very dark blue black complex with iodine
the color is intense enough to be able
to effectively mask the orange yellow
color formed by amylopectin this is why
starch in general appears to produce
only blue black color with iodine the
color of the starch complex is so deep
that it can be detected visually even
when the concentration of the iodine is
as low as 20 micromolar at 20 degrees
celsius
further the resulting color depends on
certain factors such as the length of
the glucose chains temperature
presence of water miscible organic
compounds like ethanol
and ph
the color changes that occur are
believed to be caused by so-called
charge transfer complexes or ct
complexes
after the polyiodide such as triiodide
is inside of the polysaccharide helix
such as amylose a charged transfer
complex is formed between the two in
this chart's transfer complex the
helical amylose acts as a charged donor
and the polyiodide as an acceptor this
transfer of charge between the two
entities excites the electrons of the
acceptor molecules in this case the
polyiodide when the electrons from the
acceptor molecule return to their ground
state they give off electromagnetic
radiation that is in the uv visible
spectrum in the case of the amylose
component in start this radiation
corresponds to a deep blue color it
takes 40 glucose molecules to form a
complex helix around the polyiodide
molecule
the iodine test is also popularly used
in plant physiology experiments as an
indirect test to check if a plant is
photosynthesizing
starch is formed as reserved food
material primarily in the leaves of
plants during photosynthesis the
formation and presence of starch in such
plants is confirmed or ruled out using
the iodine test do check out my
photosynthesis related experiment videos
to know more about iodine test and
photosynthesis
click on the links given in the
description below
in this video we'll be demonstrating
positive and negative iodine tests using
a variety of test samples
to perform this experiment we'll need
the following
test samples consisting of one percent
glucose one percent sucrose one percent
starch
cellulose sample in the form of a small
ball of cotton and distilled water as
control sample
you may also consider any starchy food
items such as a cut piece of potato
white bread boiled rice etc
point one normal aqueous iodine solution
or leukocyadine diluted about five times
with distilled water
four clean dry identical test tubes five
graduated droppers or pipettes of one or
2 ml capacities
test tube stand test tube holder
to begin the test
take the 4 labeled test tubes in the
test tube stand
using different droppers or pipettes add
one ml each of the test samples in the
tubes
also take one ml of distilled water in
the fourth tube this will be the
negative control for our experiment also
take the cellulose cotton sample
now add a few drops each of the dilute
iodine solution to each of the four
tubes and also to the cellulose sample
observe the change in color in the
samples
you'll notice that the test tube
containing the start solution shows a
rapid development of a deep blue black
color all the other samples including
the control test tube retain the
original iodine color similarly starchy
food stuffs like potato bread and boiled
rice all give a blue black coloration
with iodine
this shows that iodine gives a color
reaction only with certain
polysaccharides
such as starch
both starch and cellulose are
polysaccharides the main reason why
starch shows a positive iodine test
while cellulose does not is because as
explained previously at the start of
this video the alpha d-glucose monomer
units of starch are linked together by
alpha-1 for glycosidic bonds which
results in a 3d helical arrangement of
the glucose units the polyiodide ions
can slip inside the helix and eventually
produce visible color through the
formation of charge transfer complexes
on the other hand polysaccharides such
as cellulose have their constituent beta
d-glucose units linked together by
beta-1 for glycosidic linkages which
gives rise to long linear straight
chains of glucose units instead of a
helix so there is no room for polyiodide
and ions to slip into and form colored
complexes the result is that there is no
significant chemical interaction between
the iodine solution and the cellulose
structures hence a negative iodine test
so this is all about the iodine test for
polysaccharides such as starch click on
the end screen card shown on the screen
right now along with the various links
given in the description below to watch
my other videos on iodine solution
preparation protocol and also my other
biochemical test videos thanks for
watching
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