Calibration of Refractometer
Calibration of Refractometer
1. Introduction
A refractometer is used in pharmaceutical, food, and chemical industries to measure the refractive index (RI) of liquids, which is related to concentration or purity. Calibration ensures measurement accuracy and compliance with GMP and pharmacopeial requirements.
2. Principle
Calibration is performed by measuring the refractive index of a reference standard (usually distilled water or certified calibration liquids) at a controlled temperature (commonly 20 °C) and comparing the reading with the known standard value.
3. Materials & Equipment
-
Distilled or deionized water (reference)
-
Certified reference liquids (optional)
-
Clean, lint-free tissue
-
Thermometer (to monitor temperature)
-
Calibrated refractometer
4. Procedure
-
Clean the Prism Surface – Use a soft, lint-free tissue to remove any residues.
-
Set Temperature – Ensure refractometer temperature is stabilized at calibration temperature (e.g., 20 °C).
-
Apply Calibration Standard – Place 2–3 drops of distilled water (RI = 1.3330 at 20 °C) on the prism.
-
Close Cover Plate – To spread the sample evenly and avoid air bubbles.
-
Take Reading – Look through the eyepiece or read the digital display.
-
Adjust Calibration – If the value differs from the standard RI, adjust the calibration screw or press the calibration button until it matches the known RI.
-
Verify with Secondary Standard – Optional: Check with a certified calibration liquid for higher accuracy.
-
Clean After Calibration – Wipe the prism surface to prevent contamination.
5. Calibration Frequency
-
Before first use
-
Daily or before each critical measurement
-
After repairs or suspected damage
6. Documentation
Record date, refractometer ID, reference liquid used, temperature, measured RI, standard RI, and pass/fail status in the calibration log.
7. References
-
USP <831> Refractive Index
-
ISO 3657: Animal and Vegetable Fats and Oils — Determination of Refractive Index
-
Good Laboratory Practice (GLP) guidelines