Iodine Test For Starch Practical Experiment

Summary

The 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

00:00

🧪 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.

05:02

🌿 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.

Keywords

💡Iodine Test

The Iodine Test is a biochemical assay used to differentiate between certain types of carbohydrates, specifically polysaccharides like starch, dextrin, and glycogen, and simpler sugars like monosaccharides and disaccharides. In the context of the video, a positive iodine test is indicated by the development of color, which varies based on the polysaccharide type. This test is central to the video's theme as it sets the stage for explaining how different carbohydrates react with iodine.

💡Polysaccharides

Polysaccharides are complex carbohydrates composed of long chains of monosaccharide units. They are a key focus of the iodine test, as the video explains how different polysaccharides react differently to iodine. For example, amylose (a component of starch) produces a blue-black color, while cellulose does not react positively, illustrating the specificity of the iodine test for certain polysaccharides.

💡Amylose

Amylose is a type of carbohydrate and a component of starch that consists of linear chains of glucose units. It is highlighted in the video as producing a blue-black color when it reacts with iodine, indicating the presence of amylose. This reaction is crucial for the iodine test's ability to detect starch.

💡Amylopectin

Amylopectin is another component of starch, which differs from amylose in that it has branched chains of glucose units. The video mentions that amylopectin produces an orange-yellow color when it reacts with iodine, contrasting with the blue-black color of amylose.

💡Dextrin

Dextrin is a polysaccharide that results from the partial hydrolysis of starch. The video script notes that dextrin gives a red color upon reaction with iodine, which is a distinct result that helps differentiate it from other carbohydrates in the iodine test.

💡Glycogen

Glycogen is a polysaccharide that serves as the primary form of glucose storage in animals. In the video, it is mentioned that glycogen produces a reddish-brown color when subjected to the iodine test, which is another specific reaction that aids in identifying this molecule.

💡Lugol's Iodine

Lugol's Iodine is a solution of elemental iodine and potassium iodide in water, which is used as the reagent in the iodine test. The video explains that this solution was first made by Jean Lugol in 1829. It is crucial for the iodine test as it contains the iodine that reacts with certain polysaccharides to produce color.

💡Charge Transfer Complexes (CT Complexes)

Charge Transfer Complexes are discussed in the video as the mechanism behind the color change observed in the iodine test. When polyiodide ions interact with helical structures like amylose, they form CT complexes that result in the transfer of charge, leading to the emission of light in the visible spectrum and thus the color change.

💡Glycosidic Bonds

Glycosidic bonds are the chemical linkages that join monosaccharide units together to form polysaccharides. The video explains that the type of glycosidic bond (alpha 1-4 or alpha 1-6) and the resulting 3D structure of the polysaccharide influence how it reacts with iodine in the test.

💡Cellulose

Cellulose is a polysaccharide composed of long, linear chains of glucose units linked by beta-1,4 glycosidic bonds. The video contrasts cellulose with starch, noting that while both are polysaccharides, cellulose does not give a positive iodine test due to its linear structure, which does not allow for the formation of charge transfer complexes with iodine.

💡Photosynthesis

Photosynthesis is the process by which plants convert light energy into chemical energy, storing it as glucose, which can then be converted into starch. The video mentions that the iodine test can be used as an indirect test to check if a plant is photosynthesizing by detecting the presence of starch in the leaves.

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