Honors Biology 1A
Lab: Identification of Organic Polymers, Dimers and Monomers
Introduction:
As with the relevant ions that determine pH, the constituent chemical units that give
organic polymers (i.e carbohydrates, proteins, lipids, etc.) their unique physical and
chemical properties cannot be assessed visually. However, some substances have
intensive chemical properties that allow them to undergo observable qualitative changes
when they react with the functional groups or linkages of the organic compounds. This
class of specific reactive substances is known as organic indicators. As with acid/base
indicators like phenolthalien (PTH) and Brom-Thymol Blue (BTB), organic indicators
are highly specific for the unique functional groups/linkages of proteins v. carbohydrates
v. lipids. Therefore, they may be used to identify the general class to which an unknown
organic compound belongs.
MATERIALS PER LAB STATION:
Bunsen burner
Four test tubes
test tube holder
eyedropper
10 ml graduated cylinder
400 ml beaker
Benedict's solution
Lugol's solution
Biuret's solution
gelatin
1% glucose solution
1% starch solution
test tube rack
test tube brushes
GENERAL STOCKING PROCEDURE:
1. Into four test tubes, place 3 ml of appropriate solutions, as indicated in the VISUAL
SUMMARY below. Place 1% glucose in the first, 1% starch in the second, 1%
gelatin (protein) in the third, and distilled water in the last test tube. Place all of the
test tubes in a rack.
PART 1: BENEDICT’S TEST
2. Select the Benedict's bottle and record the color of this solution in the Data Table 1
under Initial Color.
3. Place five drops of Benedict's solution in each of the four test tubes. Gently agitate
all of the tubes. Place the four test tubes in a boiling water bath together. After two
minutes have elapsed, observe the colors of each test tube and record those colors in Data
Table 1 under Final Color.
4. Remove the test tubes from the water bath. Clean thoroughly and dry them for use in
the next step.
5. Once again, set up the four test tubes as indicated in the VISUAL SUMMARY.
PART 2: LUGOL’S TEST
6. Select the Lugol's bottle, put a drop in a spot plate, and record the color of the Lugol’s
solution in Data Table 2 under Initial Color.
7. Place five drops of Lugol's solution into each of the four test tubes. Gently agitate all
of the tubes. Record the colors of the solutions after the addition of Lugol's solution and
record the colors in Data Table 2 under Final Color.
NOTE: YOU DO NOT NEED TO HEAT THE LUGOL’S TEST
8. Clean thoroughly and dry the test tubes. Once again, set up the four test tubes as
indicated in the VISUAL SUMMARY.
PART 3: BIURET TEST
9. Select the Biuret's bottle and record the color of the solution in Data Table 3 under
Initial Color.
10. Place five drops of Biuret's solution into each of the four test tubes. Gently agitate
the tubes. Record the colors of the solutions after the addition of Biuret's solution and
record the colors in Data Table 3 under Final Color.
NOTE: YOU DO NOT NEED TO HEAT THE BIURET TEST . However, you may
need to hold the test tubes against a white background to assess changes.
PART 4: SUDAN IV TEST
9. Select the Sudan IV bottle and record the color of the solution in Data Table 3 under
Initial Color.
10. Place five drops of Sudan IV solution into each of the four test tubes. Gently agitate
the tubes. Record the colors of the solutions after the addition of Sudan IV solution and
record the colors in Data Table 34 under Final Color.
NOTE: YOU DO NOT NEED TO HEAT THE SUDAN IV TEST . However, you
may need to hold the test tubes against a white background to assess changes.
PART 5: IDENTIFICATION OF UNKNOWNS
There are three unknowns that your group will be asked to assess. Your task is to
identify which organic compounds are present in each of the three unknown
mixtures by using a logical sequence of test employing the use of the organic
indicators
11. Obtain an unknown solution from your instructor. Into each of the four clean test
tubes, place 3 ml of the unknown solution.
12. In the Initial Color column of Data Table 4, record the initial colors of all the
"indicators" you will be using (e.g. Benedict's, etc.)
13. Add five drops of Benedict's solution to the first, five drops of Lugol's solution to
the second, five drops of Biuret's solution to the third, and five drops of Sudan IV to the
fourth test tube. Gently agitate all of the tubes.
14. Place the first test tube (with unknown and Benedict's solution) into the boiling
water bath for two minutes. Observe the contents of the tube, and record the results in
Data Table 4.
15. Observe the reactions that may have occurred in the three other test tubes (water
control, Benedict’s and Biuret reagents) and record those color results as well.
16. Clean and dry all of the test tubes thoroughly. Repeat steps 11 through 15 for
unknowns B and C. Clean and return all materials to their proper places.
DATA TABLES: Record the initial and final colors of each of the tested indicators in a
proper data table in your lab notebook.
TABLE 1 Benedict's Solution Test
Tube
Content
1
Initial
Indicator
Color
Final Color
TABLE 2 Lugol's Solution Test
Tube
Content
glucose
1
glucose
2
starch
2
starch
3
protein
3
protein
4
water
4
water
TABLE 3 Biuret Solutions Test
Tube
Content
Initial
Indicator
Color
Final Color
Initial
Indicator
Color
Final Color
Table 4 Sudan IV Test
Tube
Content
Initial
Final Color
Indicator
Color
1
glucose
1
glucose
2
starch
2
starch
3
protein
3
protein
4
water
4
water
TABLE 5 Identification of Unknowns
Unknown Code
Contains starch
Contains Glucose
Contains Proteins
Contains Lipids
ANALYSIS QUESTIONS: Answer the following questions in complete sentences in your
lab notebook.
1. Why would water serve as a control group in this experiment?
2. Which reagent - Benedict's, Lugol's, Biuret's or Sudan IV- is used to test for the
presence of monosaccharides? What qualitative change indicates a positive test for these
monomers?
3. Which reagent - Benedict's, Lugol's, Biuret's or Sudan IV - is used to test for the
presence of the polysaccharide starch? What qualitative change indicates a positive?
4. Which reagent - Benedict's, Lugol's, Biuret's or Sudan IV - is used to test for the
presence of protein? What qualitative change indicates a positive test for protein?
5. Which reagent - Benedict's, Lugol's, Biuret's or Sudan IV - is used to test for the
presence of lipids? What qualitative change indicates a positive test for protein?
5. Lugol’s solution will not indicate for the polysaccharides cellulose or glycogen.
Explain why Lugol’s will not indicate for these polymers but will respond to another
carbohydrate polymer from your experiment. Be explicit re: the difference in terms of
functional groups or linkages.
6. Which indicator would you use to assess the following reactions? Explain why you
would use each of the organic indicators you chose:
6a) Hydrolysis of a polypeptide into amino acids
6b) Synthesis of a polysaccharide from monosaccharide monomers
6c) Hydrolysis of table sugar into monosaccharides
6d) Synthesis of a lipid from fatty acids and glycerol
7. Why would leaving the %1 starch + benedict’s solution in the hot water for 3+
minutes ultimately produce the same results as the %1 glucose + Benedict’s test
8. By comparison, if a protein solution were heated up it would shift from positive to
negative. Based on this information, to what must the indicator for protein be sensitive?
9. An unknown compound is determined to be a polymer containing carbon. Why are
the tests we used in this lab essentially “incomplete” as a means for testing the true
identify of this unknown organic compound?
10. Are the changes you observed in this lab more indicative of intensive or extensive
properties? Explain your answers.