SOP for Calibration of Refractometer in Pharma : Pharmaceutical Guidelines

SOP for Calibration of Refractometer in Pharma

Standard operating procedure to calibrate the refractometer used to determine the reflective index of liquids.


To lay down the procedure for operation and calibration of Refractometer.


This SOP shall be applicable to Quality Control Dept.


Supervisor Quality Control


Sr. Manager Quality Assurance Manager


5.1 Operating the instrument

5.1.1 Clean the refractometer properly.
5.1.2 Expose the illuminating window and deflect the casing with the upper prism home.
5.1.3 Clean the prism surface by means of soft rag wetted with neutral spirits, ether, or other solvent.
5.1.4 Ensure thorough cleaning of the prism surfaces.
5.1.5 Carry few drops of the liquid to be tested on the measuring surface by means of the rounded glass rod in such a way that after the prisms being closed the entire measuring plane to be covered with said liquid.
5.1.6 Displace the upper prism down and press it against the measuring plane.
5.1.7 The illuminating window of the upper prism directs towards the most intensive light source
5.1.8 In course of measuring in transmitted light, the mirror should cover the opening of the refractometeric prism.
5.1.9 While revolving the knobs and obtain a sharp, distinct colorless demarcation line between the bright and the dark background in the field of view of the eyepiece.
5.1.10 Turn the knob and bring the demarcation line exactly on the central point of the cross hair in the upper window of the eye piece.
5.1.11 The vertical line in the lower window of the eyepiece will then indicate the result of the measurement of the refractive index.
5.1.12 The checking and adjustment should be carried out at a temperature of 20° ± 0.5°C, by using the reference plate attached to the refractometer.
5.1.13 The polished side surface of the plate should be directed towards light directed parallel to the measuring plane.

5.2 Calibration

5.2.1 To achieve accuracy, the apparatus should be calibrated against distilled water which has a refractive index of 1.3325 at 25°C or against the reference liquids given in Table- 1.
Table -1
Reference Liquid
Refractive index
Temp coefficient
Carbon tetrachloride
1.4603 ±  0.003
- 0.00057
1.4969 ±  0.003
- 0.00056
5.2.2 The instrument shall be calibrated every month and record.
5.2.3 Re-calibration shall be done if any unusual readings are obtained. If the error persists instrument shall be given for maintenance/ servicing.


6.1 Standard Operating Procedure

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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