What tool is used to determine the concentration of a particular substance within a given solution?

The refractometer is specifically designed to measure the refractive index of a solution, which can directly relate to the concentration of a solute within that solution. When light passes through a solution, its speed and direction change depending on the concentration of dissolved substances. By knowing the refractive index of pure solvent and comparing it to that of the solution, one can determine how much solute is present.

A barometer measures atmospheric pressure and is not used for determining concentrations in solutions. A hydrometer measures the specific gravity of liquids, which indirectly relates to concentration but is not as precise for specific solute levels as a refractometer. A calorimeter is used to measure heat changes in physical and chemical processes rather than concentration. Thus, the refractometer is the most suitable tool for determining solute concentration in a solution.