Lab Food Science and Human Nutrition Department University Glycine0

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					                                                      Laboratory 2

                            Water solute interactions - ionization of weak acids

Introduction

    The behavior of many food components is a function of the pH of their environment. The pH of most
food systems is a result of the combined action of weak acids, weak bases, ionizable groups on proteins
and the buffering capacity of these compounds. The hydrogen ion concentration (more specifically
hydronium ion activity) is a major factor influencing the rate of many chemical reactions in foods. Physical
properties of many macromolecules are strongly influenced by pH. In addition, the function of many food
chemicals (i.e., leavening agents, antimicrobials, sanitizers) are influenced by pH. Food flavor (sourness)
is related to pH, among other factors.

    This laboratory exercise will demonstrate the buffering capacity of common weak acids and the amino
acid glycine. Data obtained will be used to evaluate the buffering capacity as a function of pH and the
pKa values of each compound will be determined. The ionic behavior of glycine will serve as an example
of the ionization of the weakly acidic and basic functional groups of proteins.


Important Equations and Definitions
                 +
A. pH = -log [H ]     pKa = - log Ka

B. Constants for weak acids:
                            +    -
  ionization constant = [H ][A ]/[HA][H2O]
                                     +   -
  dissociation constant = Ka = [H ][A ]/[HA]

C. Henderson-Hasselbach equation:

  pH = pKa - log [HA]/[A]

D. Buffering capacity = [added meq of base]  change in pH at each addition.

  Total mL 0.3       Total meq KOH           Added meq               pH    Change in pH        Buffer
    N KOH                                      KOH                                            capacity
        2                 0.6                   0.6                  2.1         -                -
        4                 1.2                   0.6                  3.2        1.1            0.545
        6                 1.8                   0.6                  4.6        1.4             1.14
        8                 2.4                   0.6                  4.8        0.2             3.00

Buffering capacity is highest at pH 4.8 in this example.


Procedures

You may wish to set-up your lab notebook page with the following headings for data entry.

                                                                 pH
  mL of KOH             formic                  HCl             Glycine       Lysine          aspartic

Buffering action of hydrochloric acid, formic acid, glycine, lysine and aspartic acid (may vary between
years)
                                                           1
1. Into separate 150 mL beakers, add 25 mL of 0.1 M formic acid and 0.1 M hydrochloric acid. Turn on
         the pH meter and standardize using the pH 7 and 10 buffers. Place the beaker on a stir plate,
         gently stir the solution and add the pH electrode. Be careful because you can break the electrode
         if it hits the stir bar while stirring to vigorously. Set up a buret and load carefully with 0.3 N KOH
         as shown by the instructor [load the buret so that the top of the buret is below eye level]. Slowly
         titrate each of the acids with 0.3 N KOH to a final pH of 12. Record the pH of the well mixed
         solution and the volume of KOH added after each addition of 1 mL in your lab notebook.
2. Repeat step 1 above with 25 mL 0.3 M aqueous solution of glycine and with 25 mL 0.3 M lysine (be
         sure the pH of these solutions are adjusted to pH 2 with HCl before beginning). Use 2 mL
         aliquots.
3. Repeat step 1 above with 25 mL of aspartic acid and the other samples provided (be sure the pH of
         the solution is adjusted to pH 2 before beginning). Use 2 mL aliquots.

You may wish to set-up your lab notebook page with the following headings for data entry.

                                    pH                                                  pH
  mL KOH        CW        OJ          SM         AJ        mL HCL        CM        OJ        SM        AJ

CM - cow’s milk; OJ - orange juice; SM - soy milk; AJ - apple juice

Buffering action of various foods
(may vary between years)

1. You will be assigned either milk, orange juice, apple juice or soybean milk. Take two separate 25 mL
       samples from the product assigned to you. Measure the pH of both samples. Take one of the
       two samples and titrate with 0.3 N KOH. Record the pH of the well mixed solution and the
       volume of KOH added after each addition of 1 mL in your lab notebook until you reach pH 11.
       Take the other sample and titrate with 0.3 N HCl to pH 2. Record the pH of the well mixed
       solution and the volume of HCl added after each addition of 1 mL. Note any changes in the food
       products affected by the pH changes and record in your laboratory notebook.

Calculations and discussion

        1] You will need to place the data in a spreadsheet program (Excel or QuatroPro) for
        incorporation into your report. These data should be as a tables.

        2] You will need to graph the data from your spreadsheet. You must use a computer program to
        generate the graphs.

        3] Calculate total meq KOH added and graph these vs pH for all samples. What
        differences were observed between the plots for formic and HCl? Between glycine,
        lysine and aspartic? Why ? Think about the structures of these compounds.

                 Remember:          1 N = 1 equivalent/L = 1 meq/mL

        4] Plot buffering capacity vs pH for all samples. Discuss the buffering capacity of the
        various compounds analyzed. Determine the apparent Ka and pKa values for the amino
        and carboxyl groups. These are areas of high buffering capacity.




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