2.4
Question
What nutrients are present in the unknown sample?
Materials
goggles
test tube racks
thermometer
distilled water
medicine droppers
test tube brush
utility stand
lab apron
test tube brushes
400-mL beaker
wax pencil
depression spot plate
detergent solution
8–16 test tubes
hot plate
10-mL graduated cylinder
test-tube holder
rubber stoppers
ring clamp
For monosaccharide test:
Benedict’s reagent
5% sucrose solution
5% glucose (dextrose)
solution
5% starch suspension
5% fructose solution
5% glucose solution
unknown solution
5% sucrose solution
unglazed brown paper
(2 letter-sized sheets)
whipping cream
unknown solution
gelatin
unknown solution
egg albumin
unknown solution
For starch test:
Lugol’s solution
5% starch suspension
For lipid tests:
Sudan IV indicator
vegetable oil
skim milk
Benedict’s reagent, Lugol’s
solution, Sudan IV indicator,
and Biuret reagent are toxic
and can cause an itchy rash.
Avoid skin and eye contact.
Wash all splashes off your skin
and clothing thoroughly. If you
get any chemical in your eyes,
rinse for at least 15 min and
inform your teacher.
Sudan IV indicator is flammable.
Keep away from the hot plate.
For protein test:
Biuret reagent
skim milk
Procedure
Before you begin
• make sure that all the glassware is clean and well rinsed;
• note the location of the eyewash station;
• put on your apron and goggles.
Part 1: Monosaccharide Test
Benedict’s reagent is an indicator for monosaccharides and some disaccharides,
such as maltose. Table 1 summarizes the quantitative results obtained when a
simple sugar reacts with Benedict’s reagent.
1. Prepare a water bath by heating 300 mL of tap water in a 400-mL beaker
on a hot plate. Heat the water until it reaches approximately 80°C. (Use the
thermometer to monitor the temperature.) See Figure 5.
utility stand
thermometer
Table 1
Colour of Benedict’s reagent
Approximate sugar concentration
blue
nil
light green
0.5%–1.0%
green to yellow
1.0%–1.5%
orange
1.5%–2.0%
red to red brown
>2.0%
hot plate
Figure 5
A water bath
Chemistry of Life 43
2. Using a 10-mL graduated cylinder, measure 3 mL each of distilled water,
fructose, glucose, sucrose, starch, and the unknown solution. Pour each
solution into a separate test tube. Clean and rinse the graduated cylinder
after pouring each solution. Label each test tube using the wax pencil.
Record the code for your unknown sample. Add 1 mL of Benedict’s
reagent to each of the test tubes.
3. Using a test tube holder, place each of the test tubes in the hot water bath.
Observe for 6 min. Record any colour changes in a chart.
Part 2: Starch Test
Lugol’s solution contains iodine and is an indicator for starch. Iodine turns blueblack in the presence of starch.
4. Using a medicine dropper, place a drop of water on a depression spot plate
and add a drop of Lugol’s solution. Record the colour of the solution.
5. Repeat the procedure, this time using drops of starch, glucose, sucrose, and
your unknown solution. Record the colour of the solutions in a chart.
Which solutions indicate a positive test?
To mix the contents of a stoppered test tube thoroughly,
make sure the stopper is on
tight. Place your index finger
on top of the stopper, gripping
the test tube firmly with your
thumb and other fingers. Then
shake away from others.
Part 3: Sudan IV Lipid Test
Sudan IV solution is an indicator of lipids, which are soluble in certain solvents.
Lipids turn from a pink to a red colour. Polar compounds will not assume the
pink colour of the Sudan IV indicator.
6. Using a 10-mL graduated cylinder, measure 3 mL each of distilled water,
vegetable oil, skim milk, whipping cream, and the unknown solution.
Pour each solution into a separate labelled test tube. Clean and rinse the
graduated cylinder after each solution. Add 6 drops of Sudan IV indicator
to each test tube. Place stoppers on the test tubes and shake them vigorously for 2 min. Record the colour of the mixtures in a chart.
Part 4: Translucence Lipid Test
Lipids can be detected using unglazed brown paper. Because lipids allow the
transmission of light through the brown paper, the test is often called the
translucence test.
7. Draw one circle (10-cm diameter) on a piece of unglazed brown paper.
Place 1 drop of water in the circle and label the circle accordingly. Using
more sheets, draw a total of 7 more circles (10-cm diameter). Place 1 drop
of vegetable oil, skim milk, whipping cream, and unknown solution, each
inside its own circle, labelling the circles as you do. When the water has
evaporated, hold both papers to the light and observe. In a chart, record
whether or not the papers appear translucent.
Part 5: Protein Test
Proteins can be detected by means of the Biuret reagent test. Biuret reagent reacts
with the peptide bonds that join amino acids together, producing colour changes
from blue, indicating no protein, to pink (+), violet (++), and purple (+++). The
+ sign indicates the relative amounts of peptide bonds.
8. Measure 2 mL of water, gelatin, albumin, skim milk, and the unknown
solution into separate labelled test tubes. Add 2 mL of Biuret reagent to
each of the test tubes, then tap the test tubes with your fingers to mix the
contents. Record any colour changes in a chart.
Analysis
(a) What laboratory evidence suggests that not all sugars are monosaccharides?
(b) Explain the advantage of using two separate tests for lipids.
44
Chapter 2