Analytical methods for sucrose and other optically active sugars: the concentration of a sugar solution is most directly determined using which optical technique that measures the angle of rotation of plane-polarized light?

Introduction / Context:
Sugar quality control in food, fermentation, and biochemical labs often requires determining concentration quickly and accurately. Many instrumental techniques exist, but for optically active sugars (such as sucrose), the classical and still-relevant method is polarimetry, which exploits their ability to rotate plane-polarized light.

Given Data / Assumptions:

• Analyte is an optically active sugar (e.g., sucrose, glucose).
• Sample is a clear solution in a suitable cell length.
• Standard temperature, wavelength (often sodium D-line), and path length are controlled or known.

Concept / Approach:
According to the polarimetric principle, the observed rotation (alpha) is proportional to concentration (c) and path length (l): alpha = [alpha]_specific * l * c. By measuring the rotation with a polarimeter and knowing the specific rotation (from tables) and cell length, one calculates concentration directly. This method is rapid, non-destructive, and widely standardized for sugar solutions (e.g., International Sugar Scale).

Step-by-Step Solution:
Prepare a clear, bubble-free sugar solution of known path length.Place it in the polarimeter tube and align the instrument.Measure optical rotation at the specified wavelength and temperature.Compute concentration using the known specific rotation and path length.

Verification / Alternative check:
Results can be checked by refractometry (Brix) when applicable; however, polarimetry directly targets optical activity and is less confounded by non-optically-active solutes if calibrated correctly.

Why Other Options Are Wrong:
Flame photometry/emission spectroscopy: Elemental analysis of metals; not for organic sugar concentration.Oscillometry: Measures properties like blood pressure or impedance, not optical rotation.Potentiometric titration: Useful for acids/bases or redox systems, not the standard for sugar concentration.

Common Pitfalls:
Ignoring temperature control (specific rotation is temperature-dependent), using contaminated cells, or misreading the end-point alignment can introduce bias. Calibrate with certified sucrose solutions for accuracy.

Final Answer:
Polarimetry