Which analytical instrument measures composition by using the change of refractive index (angle of refraction) with solution composition?

Introduction / Context:
Many process analyzers exploit a physical property that varies predictably with composition. Refractive index is one such property; it correlates strongly with concentration for many binary liquid mixtures (e.g., sugar solutions, certain organics), enabling robust in-line or laboratory concentration measurements.

Given Data / Assumptions:

• Property used: refractive index and the associated angle of refraction.
• Goal: identify the instrument whose principle is based on refractive index variation with composition.
• Normal operating ranges and optical clarity are assumed sufficient for measurement.

Concept / Approach:
According to Snell’s law, n1 sin θ1 = n2 sin θ2. If the sample’s refractive index n2 changes with composition, the measured critical angle (or refraction angle) shifts. A refractometer measures that shift and converts it to concentration via calibration. Polarimeters measure optical rotation of chiral samples; polarographs involve electrochemical current-voltage curves; spectrometers resolve light by wavelength, not necessarily refraction angle alone for concentration; calorimeters measure heat effects.

Step-by-Step Solution:
Identify the property: refractive index dependence on composition.Match to instrument: refractometer uses critical/refraction angle.Select “Refractometer” as the correct option.

Verification / Alternative check:
Industrial inline refractometers (e.g., in sugar, chemicals, and pulp & paper) continuously output Brix or concentration based on refractive index.

Why Other Options Are Wrong:
Polarimeter: Measures optical rotation, not refraction per se.Polarograph: Electroanalytical, not optical refraction.Spectrometer: Measures intensity vs wavelength; not the refraction angle change directly.Calorimeter: Relates to heat, not optics.

Common Pitfalls:
Confusing refractive index with optical rotation; both are optical but arise from different physical phenomena.

Final Answer:
Refractometer