SOP for Operation and Calibration of Refractometer

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STANDARD OPERATING PROCEDURE
Operation and Calibration of Refractometer

1.0 Purpose

To describe the procedure for operation, calibration, and verification of the refractometer to ensure accurate determination of refractive index in accordance with IP, BP, USP <831>, and EP 2.2.6.

2.0 Scope

This SOP applies to all refractometers used in the Quality Control Laboratory for testing of raw materials, intermediates, and finished pharmaceutical products.

3.0 Responsibility

DesignationResponsibility
QC AnalystOperation, calibration, testing, and documentation
QC SupervisorReview and verification of results
QC ManagerOverall compliance and approval
EngineeringMaintenance and servicing

4.0 Accountability

The QC Manager shall be accountable for implementation and compliance of this SOP.

5.0 References

  • Indian Pharmacopoeia
  • British Pharmacopoeia
  • USP <831> Refractive Index
  • European Pharmacopoeia 2.2.6
  • Refractometer Manufacturer’s Instruction Manual

6.0 Definitions and Abbreviations

TermDescription
Refractive IndexRatio of the velocity of light in vacuum to that in a substance
nDRefractive index measured at Sodium D line (589 nm)
ATCAutomatic Temperature Compensation

7.0 Safety and Precautions

  • Handle certified reference liquids with care.
  • Ensure prism surface is clean, dry, and scratch-free.
  • Avoid direct contact with prism surface.
  • Maintain measurement temperature at 20°C.

8.0 Instrument Description

The refractometer is an optical instrument used to measure refractive index of liquids and transparent solids based on refraction of light.

9.0 Operating Procedure

  1. Switch ON the refractometer and allow warm-up for 15–20 minutes.
  2. Verify temperature control or ATC function.
  3. Clean prism using purified water and dry with lint-free tissue.
  4. Apply 2–3 drops of sample on the prism.
  5. Close prism cover and allow reading to stabilize.
  6. Record refractive index value.

10.0 Calibration and Verification

Certified Standard Refractive Index (nD, 20°C) Acceptance Criteria
Purified Water 1.3330 ± 0.0002
n-Hexane (CRS) 1.3750 ± 0.0002
Toluene (CRS) 1.4969 ± 0.0002

11.0 Calibration Frequency

  • Daily: Verification using purified water
  • Monthly: Calibration using certified reference standards
  • After maintenance, repair, or relocation

12.0 Acceptance Criteria

The refractometer shall be considered suitable if observed values of all certified standards are within the specified acceptance criteria.

13.0 Deviations and Corrective Actions

  • Do not use the instrument if calibration fails.
  • Inform QC Supervisor immediately.
  • Initiate deviation and perform corrective actions.

14.0 Cleaning and Maintenance

  • Clean prism after each use.
  • Use appropriate solvent for oily samples.
  • Do not use abrasive materials.
  • Perform preventive maintenance as per schedule.

15.0 Documentation and Records

  • Record operation and calibration details in Refractometer Log Book.
  • Maintain certificates of reference standards.
  • Document deviations and corrective actions.

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