Amino acids are amphotheric compounds, which means they simultaneously display acidic and basic properties. Here you can see the generic amino acid on its three ionisation forms: protonated, double ion (“zwitterion”) and deprotonated:
The simplest way to study these ionisation equilibria is to perform an acid-base titration.
A way to do it is by dissolving the amino acid at a very low pH (e.g., pH=1) and then adding small volumes of a strong base, while the pH is being measured. In this way, a titration curve like this is obtained: