Linda Phengsavath
Sec: 013L
The major goals of this lab were to correctly identify an unknown amino acid through
different methods such as thin layer chromatography and a titration. To be able to use those
different methods of identification a knowledge of pH, concentration and buffers had to be
utilized.
The first method used in identifying the unknown amino acid was thin layer
chromatography. The silica gel coated plastic sheet known as the stationary phase was really
polar so polar amino acids would not travel as far from the line of origin when compared to a
nonpolar amino acid. Rf values are used to measure the rate of travel of the amino acids with the
polar values closer to zero and the nonpolar values closer to 1. Knowing the chemical properties
of the different amino acids it was not surprising to see that Arginine which is polar molecule
with a positive charge have the lowest Rf value. What was surprising was that Glycine which is
a nonpolar molecule have a lower Rf value than Cysteine, a polar molecule with no charge.
Based on the Rf values of the different amino acids when compared to the unknown, the
unknown was determined to be Cysteine (known Rf value = 0.67 cm, unknown Rf value = 0.68).
A buffer solution was prepared based on the measurements in problem 1 (15 mL of 0.2 M
acetic acid with 25 mL of 0.1 M sodium acetate) in Set A buffer problems. A pH meter was
standardized and used to measure the pH of the buffer solution which was 4.47. When
comparing to the expected pH value of 4.67 the measured pH was low but with a percent error or
4.48 the buffer prepared was relatively accurate. Buffering capacities were observed by first
taking pH values of distilled water, the prepared buffer solution and a 0.5 % albumin solution
before and after an addition of a drop of 0.1 N HCl to each solution. When comparing the
buffering capacities, the distilled water was the worse with a pH difference of -1.44, albumin
came in second with a difference of -0.49 and the buffer prepared did the best with only a
difference of 0.05. The results of the study were not surprising as distilled water would not have
many too many hydroxide ions to interact with HCl. It was expected for the albumin to have a
moderate buffering capacity because it is found in the human body which can only function
within a limited pH range.
acid/base mixture can be used to treat the acidic water.
Reference: https://www.ncbi.nlm.nih.gov/pubmed/16910118