Hd EXPERIMENT 8: BUFFERS Maintaining the pH of a solution can be very important,

Question

Hd EXPERIMENT 8: BUFFERS Maintaining the pH of a solution can be very important, especially in bio- logical systems. Practically all biological molecules require a particular pH in order to function properly. Thus the pH of the fluids found in living organisms must be maintained at rather specific values or the chemical re- actions necessary to maintain life will not occur as they should. Blood is a very good example of this. Normally human blood has a pH of 7.1. Should the pH rise above 7.3 or fall below 6.9, it will no longer be able to carry sufficient oxygen and other necessary molecules to sustain life. To ensure its pH remains at 7.1, blood contains a very complex system of acid-base buffers. These buffers counteract the acids and bases generated by the vari- ous chemical reactions that occur in the blood, thereby maintaining the correct pH. 51 OBJECTIVES Educational Objectives A student who has successfully completed this experiment will be able to: . explain how an acid/base buffer works and prepare a solution buffered to a specific pH value. Experimental Objectives A student who performs this experiment is asked to: .prepare a buffer solution and investigate its ability to buffer pH change, . prepare a buffer solution having a specific pH value, and demonstrate the ability of the buffer solution to resist pH change.

Explanation / Answer

If you have equal amount of weak acid and its conjugate base in a buffer, the pH is always = pKa where Ka is the dissociation constant of weak acid. Here Ka = 1.8X10^-5.

So pKa = -log(Ka) = 4.745.

We would use the Henderson-Hasselbalch Equation:

pH = pKa + log (base/acid)

for acetic acid, let us assume 1.00 M in acetic acid and 1.00 M in sodium acetate

pH = 4.745 + log (1.00/1.00)

pH = 4.745 + zero

pH = 4.745

The same result would happen no matter what the two concentrations were.

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