Materials

Burette funnel beaker

Erlenmeyer flask volumetric flask plastic bottle

Assigned salts of acids 0.1 M glycine (ph 1.0) 0.1M HCl

Blue tips and rack 0.1 M aspartic acid (ph 1.0) 0.1 M NaOH

pH meter analytical balance

IV. Experimental

Buffer preparation

The group was assigned to prepare a buffer solution of 250 mL 0.1 M phosphate buffer with pH of 7.4. A weak acid whose pKa is closest to the desired pH was chosen from different reagents shown in the table below. In this case, phosphoric acid (pK2 7.0) was closest to the desired pH of 7.4.

Table 1. pKa values of weak acids

The ratio of the salt and weak acid needed to acquire the desired pH was calculated using the Henderson-Hasselbalch equation. The amounts needed were then weighed using an analytical balance and mixed. The resulting mixture was then transferred to 250 ml volumetric flask and diluted to the mark. To verify the pH of the solution, a pH meter was used. The solution was adjusted until the desired pH was reached. Finally, the solution was transferred to a plastic bottle for storage.

Acid pK1 pK2 pK3

Acetic Acid 4.74

Carbonic Acid 6.1 10.4

Citric Acid 3.1 4.1 6.4

Phosphoric Acid 1.97 7.0 12.5

Aspartic Acid 2.1 3.9 9.8

Glycine 2.3 9.6

Titration Curve of an Acid

10 mL 0.1 M glycine was titrated with 0.1 M NaOH in 0.5 ml increments until pH of 12.0 was reached. A pH meter was used to determine the pH. The pH values were plot against the volume of 0.1 NaOH. The resulting titration curve was compared with that of aspartic acid.

VII Conclusion and Recommendation

Buffers are essential in the body of organisms and in laboratory experiments. Buffer solutions resist dramatic changes in pH despite adding an acid or a base. In preparing a buffer solution the Henderson-Hasselbach equation is useful in determining the ratio of an acid and its conjugate base after successfully choosing the weak acid whose pKa is closest to the desired pH. The pH of the resulting solution can then be adjusted to make it more acidic or basic before diluting.

The titration curves of polyprotic acids differ from monoprotic acids in the greater number of equivalence points, formation of more than one buffer regions and overlapping of equivalence points.

Amino acids are ampholytes that can act as an acid or a base. They are essentially good buffers in the body of an organism. The experiment basically demonstrated how amino acids or buffers could resist changes in pH which may be detrimental in the body of the organism.

In order to fully achieve the objectives of the experiment, it is recommended to make sure that the titrant is well mixed with the analyte for the titration curves to be more accurate. Accuracy of measurements made using different equipments is also needed. Additionally, glasswares used should be cleaned properly to avoid contamination before performing the experiment.