Class: BE-210
Group: W2
Members: David Kellman, Hyung Lee, Jim Oh, Greg Saybolt
Date: Sprin 1997
Full Text

Abstract:
The objective of this project was to observe the outcome of pKa’s when the mixtures of two amino acids with two distinct pKa’s were titrated. Our hypothesis was that there would be no reaction between amino acids and that the pKa would fall between the pKas of the individual amino acids, falling closer to the amino acid of greater concentration. Glycine and Serine were the two selected amino acids for this project. In the process of titrations, the automatic pipette was utilized instead of the burette. It was determined that using a burette to conduct our titrations would give rise to greater inaccuracies and inefficiencies in obtaining our result. Conducting our titrations with an automatic pipette solved the problems that were posed by using burette.

A total of 13 titrations were conducted. Glycine and Serine were individually titrated twice to provide a benchmark for the latter mixtures’ results. Individual pKas of Glycine and Serine were compared with the literature values given in the BE 210 lab manual. The average of Glycine’s pKa was 9.57 with 0.31 % error from actual and the average pKa of 9.02 for Serine with 1.45 % error from actual. By varying the molar ratios of the two constituents, three different types of mixtures were produced and titrated. Three titration trials of each 1 to 1, 2 to 1, and 1 to 2 mole ratio of Glycine to Serine mixtures were conducted. The average pKas of the mixtures were 9.33, 9.37 and 9.19 respectively with uncertainties below 0.02 for all results. From this result, it is concluded that the greater the concentration of one constituent, the closer the pKa of the mixture was to that particular constituent’s pKa. Thus our hypothesis was corroborated.

In the titration of the mixtures a discontinuity point was discovered on the graph of ‘Change of pH vs. pH.’ These values of discontinuity were obtained and further studied. The averages of these points’ occurrences were pH of 10.19, 10.03 and 9.86 in order of mixtures’ mole ratio, 1 to 1, 2 to 1, and 1 to 2 (Glycine:Serine). Uncertainties of these values were all below 0.03. It was concluded that discontinuity points signified complete dissociation of Serine.

Lastly, the pKa of a mixture falls between the pKa of its constituents. This hypothesis arose from the definition of the pK point. The pK is the equilibrium point of the reaction, not the individual species. The pK found in the mixture will be the point in the acid/base reaction, where the mixture would be the most resistant to change of pH.