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Glycine Amino Acid pH Titration Analysis

This document summarizes the results of an amino acid pH titration experiment. It provides the experimental pKa values for the carboxylic acid and amine groups of glycine, which were 2.15 and 10.55 respectively. The student notes potential sources of error in determining the equivalence points that could account for deviations from accepted pKa values. The student expresses that performing the experiment helped reinforce concepts from class by allowing them to obtain an expected titration curve graph.

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66 views3 pages

Glycine Amino Acid pH Titration Analysis

This document summarizes the results of an amino acid pH titration experiment. It provides the experimental pKa values for the carboxylic acid and amine groups of glycine, which were 2.15 and 10.55 respectively. The student notes potential sources of error in determining the equivalence points that could account for deviations from accepted pKa values. The student expresses that performing the experiment helped reinforce concepts from class by allowing them to obtain an expected titration curve graph.

Uploaded by

Minich
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
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Name

Date

Section Number

Amino Acid pH Titration Post-Lab

Post-Lab Questions

1. Just to the left of the carboxylic acid pKa, most of the glycine molecules are protonated at

both their carboxylic acid and amine groups. At the carboxylic pKa point, exactly half of the

carboxylic groups have been deprotonated while the majority of the amine groups are still

protonated. Just to the right of this point, the majority of the carboxylic acid groups are

deprotonated while the majority of the amine groups are protonated.

2. Just to the left of the amine pKa, all the carboxylic acid groups are deprotonated while the

majority of the amine groups are protonated. At the amine pKa point, exactly half of the

amine groups are deprotonated. Just to the right of this point, most of the amine groups are

deprotonated.

3. The experimental values obtained for the pKa of the carboxylic acid group and the amine

group are 2.15 and 10.55 respectively. The carboxylic pKa obtained is slightly lower than the

accepted value. On the other hand, that of the amine is significantly higher than its accepted

value by almost 1 pH unit. These differences may be due to errors in locating the equivalence

points. For instance, the pH reading is known to be unstable around the equivalence point

(Harris, 2010). Determining the exact equivalence pH is also based on an estimate of the

midpoint of the steep region on the graph. These factors lower the overall accuracy and

precision of the data, resulting in deviations from the actual values.


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Results and Discussion

The pKa of the carboxylic acid group was found to be 2.15 whereas that of the amine

group was found to be 10.55. In this experiment I learned that amino acids have two pKa values.

I was surprised to find out that this results in a potentiometric titration curve with two distinct

end points. My favorite part was performing the experiment and obtaining a graph very similar to

the one expected from theory. I liked this part in particular as it brought theory to life and

allowed me to reinforce what I learned in class.


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References

Harris, D. C. (2010). Quantitative chemical analysis. Macmillan.

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