User:Khyra A. Neal/Notebook/Chem 571/2014/09/03

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September 3, 2014 Titration of Amino Acids Today we continued with our amino acid titrations. We started with the second trial of Isoleucine and Histidine,two trials of Tyrosine, and one trial of Alanine. For the first trail the pH was measured every 2 mL just to get a rough curve. We found that our initial pH for some of the amino acids were close to the first pKa values so a very small amount of NaOH would cause a huge spike in pH. On the second trials we decreased the amount of NaOH added per pH measurement to have a clear point of inflection. Titration continued until the pH of the solution was between 10-11. NOTE Only one trial was done for Alanine because there was no time for the second titration. Titration Curves The figures below show the titration curves of amino acids. NOTE Tyrosine was dissolved in a 1:10 1.0 M HCL and H2O solution because tyrosine is aromatic and hydrophobic so a stronger acid was need to dissolve it before titration. The titration curves for Tyrosine are inaccurate because as the titrant was added to the solution a white precipitate formed. It may have formed because the concentration of Tyrosine had reached its maximum solubility for the solution at hand. The second trial for tyrosine was stopped prior to the pH reaching 10 because a large amount of precipitate was formed. 200 μM Amino Acid Solutions We made 100 mL of 200 μM amino acid solutions for Histidine, Tyrosine, Alanine, Aspartic Acid, and Isoleucine. Because of how small the desired concentration is, no less than 100 mL of solution can be made. UV-Vis spectra will be obtained next class.