Name ____________________________________________ Block _____ Date __________________
Testing for Proteins
A protein is a chain of amino acids, held together by peptide bonds. Biuret reagent identifies the presence
of peptide bonds, causing a change in color.
PROCEDURE FOR PROTEINS
1. Label four test tubes: water, albumin, starch, and potato.
2. Add 3 mL of the appropriate solution to each.
3. Add 3 mL of Biuret reagent to each tube.
4. Record the color of the solutions in each test tube. If the color is purplish-violet, then proteins are
present. If the color stays blue or becomes a slight green, then proteins are not present. If you
aren’t sure, hold a white piece of paper behind your test tube while checking the color.
Data & Questions
Solution
Color of Solution
Proteins Present? (Yes or
No)
Albumin
Water
Starch
Potato
a) Which test tube represented the negative control situation when testing for proteins?
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b) Which test tube represented the positive control situation when testing for proteins?
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c) Does the starch or potato solution contain a protein?
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Name ____________________________________________ Block _____ Date __________________
Post-Lab Questions
a) What test would you use to determine if starch were present?
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b) If a biuret solution turned dark purple in a casein solution, what kind of substance was present?
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c) What color would a Benedict’s solution appear if no simple sugar was present?
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d) If an unknown solution turned biuret from blue to dark purple and Benedict’s solution from blue
to orange, what substance(s) would be present?
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e) If an unknown solution caused iodine to appear light brown in solution and caused paper to
appear translucent, what substance(s) were present?
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f) If an unknown solution caused Benedict’s to appear blue and iodine to turn black, what
substance(s) were present?
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g) If an unknown solution caused biuret to appear blue and Benedict’s solution to appear orange,
what substance(s) would be present?
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