Making Glue
Purpose:
To prepare and test a casein glue. To deduce the charge distribution in proteins as determined by
pH.
Background:
Cow’s milk contains 4.4% fat, 3.8% protein, and 4.9% lactose. At the normal pH of milk, 6.3 to
6.6, the protein remains dispersed because it has a net negative charge due to the dissociation of
the carboxylic acid group as shown in Figure A below. As the pH is lowered by the addition of
an acid, the protein acquires a net charge of zero, as shown in Figure B. After the protein loses
its negative charge, it can no longer remain in solution and it coagulates into an insoluble mass.
The precipitated protein is known as casein. The pH at which the net charge on a protein
becomes zero is called the isoelectric pH. For casein, the isoelectric pH is 4.6.
H2N – protein – COO
+
Figure A
H3N – protein – COO
Figure B
In this experiment you will coagulate the protein in milk by adding acetic acid (vinegar). The
casein, a precipitate in this reaction, will then be separated from the remaining solution by
filtration. This process is known as separating the curds from the whey. The excess acid in the
curds can be neutralized by the addition of sodium hydrogen carbonate (baking soda), NaHCO3.
HC2H3O2(aq) + NaHCO3(s)  NaC2H3O2(aq) + H2O(l) + CO2(g)
Acetic acid
Sodium bicarbonate
Sodium acetate
The product of this reaction is casein glue.
Equipment:
Hot plate
250-mL Beaker
100-mL Graduated cylinder
Funnel
250-mL Flask
Glass stirring rod
Beaker tongs
Thermometer
Pieces of paper
100-mL Beaker
Safety goggles (Must be worn at all times during the experiment!)
Chemicals:
Nonfat milk
Acetic acid (vinegar, HC2H3O2)
Sodium bicarbonate (baking sodaNaHCO3) Distilled water
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Procedure:
1. Wear safety goggles throughout the experiment.
2. Pour 125 mL of nonfat milk into a 250 mL beaker. Add 20 mL of 4% acetic acid (vinegar) to
the milk.
3. Place the mixture on a hot plate and heat it while stirring to 60oC. Use a stirring rod to stir the
mixture and not the thermometer.
4. Decant the mixture to remove the majority of excess liquid leaving most of the solid in the
beaker. Always decant into a waste beaker in case you lose product you want to recover.
5. Filter the remaining mixture through a piece of filter paper into a 250 mL flask.
6. Discard the filtrate that contains the whey. Scrape the curds from the filter paper back into
the 250 mL beaker.
7. Weigh 1.2 g of NaHCO3 in a 100-mL beaker. Add the NaHCO3 to the solid curds to
neutralize any acid remaining from the vinegar and stir. Watch for bubbles of gas to appear. Add
a little more sodium bicarbonate, if necessary, until no more bubbles appear.
8. Slowly add drops of water, stirring intermittently, until the consistency of white glue is
obtained. The glue should flow freely, but not be too runny.
9. The substance in the beaker is glue. Obtain two small pieces of paper and use the substance
to glue the two pieces together. Put your names on the top piece of paper and hand in the papers
to be tested and graded.
10. Fasten together two wooden splints. Hand in these samples with your pieces of paper to be
tested and graded.
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