Question

How would you use the titration curve to figure out how many ionizable protons are on an amino acid? Hint: is the titration curve a straight line or did the slope vary? If the slope varied, what was the significance of the slope changes? Is the completely uncharged form of any amino acid ever seen; in other words, can the uncharged amine group and the uncharged carboxylic acid group ever exist at the same pH? Explain your answer

Answer 1

1. A titration curve of neutralization reactions appear as a sigmoid curve on a plot of pH vs. volume of base added. In it, the volume of base at which a sharp change in pH is found is taken as the equivalence point for that neutralization reaction. Such sharp changes occur for every ionizable proton in the acid sample and so for n no.of ionizable protons, there be n sigmoids merged into one another and every point of sharp pH change indicates the equivalence point of an ionizable proton getting completely consumed.

2. An amino acid is composed of a basic amine group and an acidic carboxylate group. At low pH, the amine group attains a charge while at high pH the acid group attains a negative charge because at low pH there is an abundance of protons which protonate the amine group and at high pH there is an abundance of hydroxide ions that deprotonate the carboxylic acid group. So, at any given pH either the amine group or the acid group will possess a charge due to their drastically different pKa values also (acids have low pKa and amines have high pKa or low pKb). However, at one particular pH, known as the isoelectric point, both the amine and the acid group will be charged, rendering the amino acid itself with zero charge.

Therefore, the amine and acid groups of an amino acid will never be neutral simultaneously at any particular pH.