7/17/2019 Expt. 1 PH and Buffer Systems

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EXERCISE 1: pH and Buffer Systems

Procedures:

A. Factors affecting Buffer capacity

1. 

Effect of concentration of the buffer

Using 0.1 acetate buffer, pH 4.7, prepare 25 mL each of the buffer solutions with the following

concentrations at pH 4.7:

i. 

0.005 M

ii. 

0.05 M

iii. 

0.10 M

Record the pH of the buffer solutions. Add 2 mL of 0.1 M NaOH to each 25 mL buffer

samples. Record the pH of each buffer solution after the addition of alkali. Account for the

magnitude of the change in pH.

2. 

Effect on the ration of the Conjugate Base to the Weak acid

From the Henderson-Hasselbalch equation, calculate the ratio of acetic acid and acetate

required to produce buffer solution with:

i. 

pH 3.7

ii. 

pH 4.7

iii. 

pH 5.7

Using 0.1 M stock solutions of the weak acid and conjugate base, make up 25 mL of the buffer

solution. Measure and record the pH of the buffer solution. Divide into two equal parts. Add

2 mL of 0.1 N NaOH to one part and 2 mL 0.1 N HCl on the other. Record the pH of each buffer

solution after addition of alkali or acid. Account for the magnitude of the pH shift in each withreference to the direction of pH shift.

B. Titration of an Amino Acid

In this part of the experiment you will titrate an amino acid sample which will be assigned to you

by your instructor.

Pipette 10 mL of the amino acid sample into 25 mL or 50 mL beaker. Adjust pH to 1.5 using 1 N

HCl. Use a burette to add 0.1 M KOH in approximately 1.0 mL increments until about pH 12 is reached

(record the accurate volume of each increment

Stir well and measure the pH after each addition. Construct a titration curve by plotting pH against

the volume of KOH added. Label your graph properly. From your graph, determine the pKa’s of your

sample. Draw the structure of your amino acid at each pKa value.