The Seliwanoff's test distinguishes between aldose and ketose sugars. It involves heating the sugar with resorcinol and hydrochloric acid. Ketoses are more rapidly dehydrated than aldoses under these conditions. The dehydrated ketose will react with resorcinol to produce a deep cherry red color, indicating a positive test. Aldoses may produce a faint pink color. Fructose and sucrose give a positive test since sucrose contains fructose. The test exploits the difference in reactivity between aldehyde and ketone functional groups.

1. 9/24/13 Benedict
Benedict's Test for Reducing Sugars
The Benedict's test allows us to detect the presence of reducing sugars (sugars with
a free aldehyde or ketone group). All monosaccharides are reducing sugars; they
all have a free reactive carbonyl group. Some disaccharides have exposed
carbonyl groups and are also reducing sugars. Other disaccharides such as
sucrose are non-reducing sugars and will not react with Benedict's solution.
Starches are also non-reducing sugars. The copper sulfate (CuSO4) present in
Benedict's solution reacts with electrons from the aldehyde or ketone group of the
reducing sugar to form cuprous oxide (Cu2O), a red-brown precipitate.
CuSO4 Cu++ + SO4--
2 Cu++ + Reducing Sugar Cu+
(electron donor)
Cu+ Cu2O (precipitate)
The final color of the solution depends on how much of this precipitate was
formed, and therefore the color gives an indication of how much reducing sugar
was present.
Increasing amounts of reducing sugar
green orange red brown
Return to Lab 3 Home Page
Return to 170 Course Outline Page
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www.biosci.ohiou.edu/introbioslab/Bios170/170_2/benedict.htm 1/1

2. 9/24/13 Carbohydrates - Barfoed's Test
Barfoed's Test
Shows positive test for:
Reducing monosaccharides
Reactions:
Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a
reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same
reaction, but do so at a slower rate.
How to perform the test:
One ml of a sample solution is placed in a test tube. Three ml of Barfoed's reagent (a solution of cupric
acetate and acetic acid) is added. The solution is then heated in a boiling water bath for three minutes.
A positive test is indicated by:
The formation of a reddish precipitate within three minutes.
a negative test (left) and a positive test (right)
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/barf/barfoed.htm 1/2

3. 9/24/13 Carbohydrates - Barfoed's Test
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/barf/barfoed.htm 2/2

4. 9/24/13 Starch - Iodine
Carbohydrate
MiniTopics
Mono-Carbohydrates Di, poly-
Carbohydrates Elmhurst College
High Fructose Corn
Syrup Sweeteners Starch - Iodine Test Chemistry Department
Blood Glucose Test Sugar and Tooth Decay Virtual ChemBook
Click for larger image
Starch - Iodine
Starch:
Plants store glucose as the polysaccharide starch. The cereal
grains (wheat, rice, corn, oats, barley) as well as tubers such as
potatoes are rich in starch.
Starch can be separated into two fractions--amylose and
amylopectin. Natural starches are mixtures of amylose (10-
20%) and amylopectin (80-90%).
Amylose forms a colloidal dispersion in hot water whereas
amylopectin is completely insoluble. The structure of amylose
consists of long polymer chains of glucose units connected by
an alpha acetal linkage.
Starch - Amylose - ( Graphic Image in new window) shows a
very small portion of an amylose chain. All of the monomer
units are alpha -D-glucose, and all the alpha acetal links
connect C # 1 of one glucose to C # 4 of the next glucose.
Starch - Amylose - Chime in new window
Starch Coil or Spiral Structure:
As a result of the bond angles in the alpha acetal linkage,
amylose actually forms a spiral much like a coiled spring. See
the graphic on the left which show four views in turning from a
the side to an end view.
Starch Coil - Chime in new window
Chemical Test for Starch or Iodine:
Amylose in starch is responsible for the formation of a deep blue
color in the presence of iodine. The iodine molecule slips inside
of the amylose coil.
Iodine - KI Reagent: Iodine is not very soluble in water,
therefore the iodine reagent is made by dissolving iodine in water
in the presence of potassium iodide. This makes a linear triiodide
ion complex with is soluble. The triiodide ion ion slips into the coil
of the starch causing an intense blue-black color.
Starch Test: Add Iodine-KI reagent to a solution or directly on
a potato or other materials such as bread, crackers, or flour. A
blue-black color results if starch is present. If starch amylose is
www.elmhurst.edu/~chm/vchembook/548starchiodine.html 1/2

5. 9/24/13 Starch - Iodine
not present, then the color will stay orange or yellow. Starch
amylopectin does not give the color, nor does cellulose, nor do
disaccharides such as sucrose in sugar.
Iodine Test: When following the changes in some inorganic
oxidation reduction reactions, iodine may be used as an indicator
to follow the changes of iodide ion and iodine element. Soluble
starch solution is added. Only iodine element in the presence of
iodide ion will give the characteristic blue black color. Neither
iodine element alone nor iodide ions alone will give the color
result.
www.elmhurst.edu/~chm/vchembook/548starchiodine.html 2/2

6. 9/24/13 www.homebrewtalk.com/wiki/index.php/Iodine_test
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Iodine test
From HomeBrewTalk Wiki
An iodine test uses the reaction between starch and iodine to check for the completion of the starch converted in the mash. It is not required to
perform such a test, but it is seen as good practice as it would alert the brewer of problems during the mashing process. Reasons for a mash to fail
to convert could be improper temperature (faulty thermometer), completely inadequate mash pH or insufficient diastatic power of the malts (large
amounts of adjuncts, decoction mashing).
When mixing starch or large chained dextrines and iodine a color reaction will appear. The darker and more intense the color is, the longer the chains
and/or higher the concentration of starch is. [Noonan, 1996]:
"Blue-black indicates the presence of native starch (amylose); deep mahagony/red-brown evidences gelatinized starch (amylose fragments and
large a-limit dextrins), faint red simple a-limit dextrins. A faint mahagony to violet-reddish reaction denotes a mix of small dextrines. Total mash
saccrification (a solution of some small a-limit dextrins with maltotriose, maltose and simple sugars) causes no change in the yellow color of
iodine"
How to perform an iodine test
An iodine test requires a white saucer, a small bottle of iodine with a dropper, and a small sample of liquid from the mash tun.
Let the liquid drip onto the saucer, being careful not to include any solid material such as grain husks, which could give a false positive reading.
Then add a drop or two of iodine. If there is still unconverted starch in the mash, the wort will quickly turn dark blue or black.
Iodine is a poison: regardless of the color, do not return the sample to the mash tun.
Iodine test on chalk
Another elegant way of performing an iodine test is to test for iodine on chalk. Take a piece of side walk chalk and drip one drop of mash liquid onto
it. This can easily be done with the probe of the thermometer or the mash paddle. Now add a few drops of iodine and observe the reaction. To test
again, with a knife cut off of the piece of chalk that was used for the test so you have a clean surface again. The following pictures of an iodine
test on chalk were taken during a mash of Briess Pale Malt at 153 *F (67 *C):
0 min
Searching for iodine test?
The iodine reaction shortly after dough-in. Note the strong dark blue to purple
reaction.
The iodine reaction 10 min later. As you can see the reaction is significantly less, but there are still many long chained dextrines left.
www.homebrewtalk.com/wiki/index.php/Iodine_test 1/3

7. 9/24/13 www.homebrewtalk.com/wiki/index.php/Iodine_test
10 min
20 min
30 min
At this point the reaction is only faint and the mash is almost converted. Because of
the high diastatic power of the pale malt and a relatively high mash temperature the
conversion was done in less than 30 min. However, the mash was done for 60 min
since the mash temperature and time were chosen to achieve a targeted
fermentability.
No significant reaction left after 30 min.
The same after 50 min.
www.homebrewtalk.com/wiki/index.php/Iodine_test 2/3

8. 9/24/13 www.homebrewtalk.com/wiki/index.php/Iodine_test
50 min
Sources
[Noonan, 1996] Gregory J. Noonan, New Brewing Lager Beer. Brewers Publications, USA, 1996
Categories: Beer Beer brewing process Beer chemistry Mashing
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9. 9/24/13 Carbohydrates - Bial's Test
Bial's Test
Shows positive test for:
Pentoses
Reactions:
The test reagent dehydrates pentoses to form furfural. Furfural further reacts with orcinol and the iron ion
present in the test reagent to produce a bluish product (reaction not shown).
How to perform the test:
Two ml of a sample solution is placed in a test tube. Two ml of Bial's reagent (a solution of orcinol, HCl
and ferric chloride) is added. The solution is then heated gently in a Bunsen Burner or hot water bath. If
the color is not obvious, more water can be added to the tube.
A positive test is indicated by:
The formation of a bluish product. All other colors indicate a negative result for pentoses. Note that
hexoses generally react to form green, red, or brown products.
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/bial/bials.htm 1/2

10. 9/24/13 Carbohydrates - Bial's Test
two negative tests (left, middle) and a positive test (right)
www.harpercollege.edu/tm-ps/chm/100/dgodambe/thedisk/carbo/bial/bials.htm 2/2

11. 9/24/13 Seliwanoff's test - Wikipedia, the free encyclopedia
Positive Seliwanoff’s test
Seliwanoff's test
From Wikipedia, the free encyclopedia
Seliwanoff’s test is a chemical test which distinguishes between aldose
and ketose sugars. Ketoses are distinguished from aldoses via their
ketone/aldehyde functionality. If the sugar contains a ketone group, it is a
ketose and if it contains an aldehyde group, it is an aldose. This test is
based on the fact that, when heated, ketoses are more rapidly
dehydrated than aldoses. It is named after Theodor Seliwanoff, the
chemist that first devised the test.
Seliwanoff-Reaction
The reagents consist of resorcinol and concentrated hydrochloric acid:
The acid hydrolysis of polysaccharides and oligosaccharides yields simpler sugars followed by furfural.[1]
The dehydrated ketose then reacts with the resorcinol to produce a deep cherry red color. Aldoses may
react slightly to produce a faint pink color.
Fructose and sucrose are two common sugars which give a positive test. Sucrose gives a positive test as it is a
disaccharide consisting of fructose and glucose.
References
1. ^ Abramoff, Peter; Thomson, Robert (1966). An experimental approach to biology. WH Freeman & Company,
San Francisco. p. 47.
Seliwanoff, Theodor (1887). "Notiz über eine Fruchtzuckerreaction". Berichte der deutschen chemischen
Gesellschaft 20: 181. doi:10.1002/cber.18870200144
(http://dx.doi.org/10.1002%2Fcber.18870200144).
en.wikipedia.org/wiki/Seliwanoff's_test 1/2

12. 9/24/13 Seliwanoff's test - Wikipedia, the free encyclopedia
Katoch, Rajan (2011-06-30). Analytical Techniques in Biochemistry and Molecular Biology
(http://books.google.de/books?id=rCU2-gvkjo0C&pg=PA71). p. 71. ISBN 978-1-4419-9784-5.
Chawla (2003-01-01). Practical Clinical Biochemistry: Methods and Interpretations
(http://books.google.de/books?id=gP2whA97gPgC&pg=PA35). p. 35. ISBN 978-81-8061-108-7.
Retrieved from "http://en.wikipedia.org/w/index.php?title=Seliwanoff%27s_test&oldid=540745469"
Categories: Reagents for organic chemistry Chemical tests Analytical chemistry stubs
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