Which test method involves adding a reagent solution of known concentration into a sample of known volume to analyze the concentration?

The method that involves adding a reagent solution of known concentration into a sample of known volume to analyze the concentration is titration. This analytical technique is widely used in chemistry to determine the concentration of an unknown solution by reacting it with a solution of known concentration (the titrant). During the process, the titrant is added gradually until the reaction reaches its endpoint, which is often indicated by a color change or a specific measurement.

In volumetric analysis, which titration is a part of, the focus is on measuring the volume of the titrant needed to achieve this endpoint. Thus, the methodology and principles behind titration are integral to volumetric analysis, but it's important to note that volumetric analysis encompasses a broader range of techniques beyond titration alone.

The other methods mentioned, such as gravimetric, colorimetric, and volumetric, are employed in different contexts. Gravimetric analysis involves measuring the mass of a substance to analyze its concentration, while colorimetric analysis employs color intensity to determine concentration based on beer's law.