Name _________________________
Testing Protein Concentration of Solutions
(Part IV of lab 3f)
1.
Add 2 mL of the 0.625% gelatin solution to a clean 13 X 100 mm glass test tube. Add 2 mL of
distilled water and mix thoroughly. This is a 1:2 dilution. What is the % concentration of this
tube now? Label it. Take 2 mL of this new sample and dilute again the same way and label. You
should now have six total glass tubes containing decreasing % gelatin solutions.
2.
You will need six 12 X 75 mm polystyrene test tubes. Pipette 1 mL of each gelatin solution into
a clean 12 X 75 mm polystyrene test tube. To each plastic test tube, add 200 uL of 10% NaOH
and then add 100 uL of 5% CuSO4. Mix each tube thoroughly using a tap dancer or vortex mixer.
Create a data table in your lab notebook to record observations.
3. Blank the spectrophotometer using 200 uL distilled water in a clean cuvette. Measure the
absorbance of each clear gelatin solution (in the glass tubes) by pipetting 200 uL of each sample
into the cuvette and placing in the spectrophotometer. Read the absorbance of each of the six
samples at 280 nanometers. (280 nm is a wavelength of UV light that is absorbed by proteins.)
Create a data table in your lab notebook to record the absorbance values. Plot the absorbance
vs. % gelatin on the graph paper provided by Mr. Ward.
4. Get an unknown sample from Mr. Ward. Perform both a Biuret test and read the absorbance at
280 nm using the spectrophotometer. Do both the Biuret test and the spectrophotometer data
agree? Identify the % concentration of gelatin in your unknown sample.