SOP and Calibration of Refractor Meter | Refractometer Callibration Procedure

This procedure is applicable to the determination of the refractive index and Refractometer Calibration procedure a sample at a known temperature.


To lay down the procedure for operating REFRACTOR METER 


This procedure applies to Quality Control Department. 


  • QC chemists to prepare and follow the SOP. 
  • Head – QA, and QC for implementation of SOP.
Operation Procedure 

Ensure that the Instrument is clean and ready for use Set the desired temperature at which the refractive index of the sample is to be calculated.

Open the rectangular prism by unlocking the lever place a few drops of the liquid (Temp. of the prisms, which has a matt and ground surface.   

Focus the eyepiece on the crossed index lines seen against the patch of light.

With the borderline near the center of the view. Focus the borderline sharply by adjusting the eyepiece.

Bring the borderline into the crosshair, as shown below, using the coarse (and Fine) Prism Adjust knob.

Look through the eyepieces and focus the lens till the refractive index scale and the gratitude index line are in clear focus .take the reading of the scale against the index mark.

This gives the refractive index of the specimen correct up to the third decimal place by the graduated division and by estimation to the fourth decimal place.

Repeat the process to take a number of readings whose average is the refractive index of the liquid.


         SOP:           Standard Operating Procedure

            QA:           Quality Assurance

            QC:           Quality Control           


N.B after use put all the LDPE bag .outer and threads to the designated place.

Methods No 02


a). Abbe’s Refractometer

b). Source of light.

c). A dropper or jet tube.


Calibration Procedure

1. Take distilled water as a standard sample for the calibration of Abbe’s Refractometer.

2. Open the prism box and clean the prism surface with acetone soaked clean swab of cotton. Allow the prism to dry.

3. Place 2-3 drops of the standard sample of distilled water with the help of a dropper or a jet tube between the prisms and close the prism box.

4. Allow light from diffused sunlight or from a frosted electric lamp to fall on the mirror and adjust at such an angle so that maximum light is reflected in the prism box when viewed through the eye wil piece.

5. Gently turn the prism box by means of a milled head screw forward and backward until a colored band appears in the field of view. By turning the milled ring as the base of the telescope, a position will be found at which the colored band will be eliminated, the field of view being partly bright and partly dark with a sharp dividing line.

6. Bring the sharp dividing line to coincide with the intersection of the cross wires by turning the milled head screw.

7. Read the refractive index of the distilled water on the circular scale through the reading lens.

8. Record at least three readings and the average value of the readings denotes the refractive index of the liquid.

9. Refractive index of standard distilled water = 1.33 at 25 °C. and refractive index of water =1.333

Refractive Index

The refractive index of a substance is the ratio of the speed of light in a vacuum to the speed of light in a substance. For practical measurements, including this method, the scales of standard instruments indicate refractive indices with respect to air rather than vacuum. 

The index of refraction of oils is characteristic within certain limits for each kind of oil. It is related to the degree of saturation, particularly to the extent of conjugation, but it is affected by other factors such as free fatty acid content, oxidation, and heat treatment.

Scope: Applicable to drying oils and all fatty acids and another solvent.


(a) Refractometer, Abbe, or dipping type. The temperature of the refractometer prisms must be controlled to within ± 0.1 °C and for this purpose, it is preferably provided with a thermostatically controlled water bath and motor-driven pump to circulate water through the instrument. The instrument is standardized, following the manufacturers' instructions, with a liquid of known purity and refractive index prism with a glass prism of known refractive index. Distilled water has a refractive index of  1.33299 at 20 °C and 1.33251 at 25°C.

(b) Light source, if the refractometer is equipped with compensation, a tungsten lamp or daylight bulb is permissible. Otherwise, a monochromatic light such as an electric sodium vapor lamp is required.


Toluene or some other solvent satisfactory for cleaning the prisms. Lens tissue or cotton is recommended for cleaning and drying the prisms in order to avoid injury to them.

Procedure of RI 

Melt the sample, if it is not already liquid, and filter through filter paper to remove any impurities and the last traces of moisture. The sample must be completely dry.

Make certain that the temperature of the refractometer is 25°C for fatty acid liquid at room temp. or 65°C for fatty acid solid at room temp. and that the prisms are clean and completely dry. Place several drops of the sample on the prism of the refractometer. Close the prisms and tighten firmly with a screw head. Allow standing for 3 minutes so that the sample comes to the temp. of the instrument. If the dipping refractometer is used immerse the prism in the sample which must be maintained at the designated temp. and allow 3 min. for temp. adjustment before reading.

Adjust the instrument and light to obtain the most distinct reading possible and then determine the refractive index. Take several readings and calculate the average of all.

Note:- Approximate temperature corrections may be made by the following calculations 

Abbe refractometer

    R = R' +K(T'-T)

    R = the reading reduced to temperature T

    R' = the reading at T' °C

   T = the standard temperature 

   T' = the temp. at which the reading R' is made

   K = 0.000385

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Determination Refractive Index at 20° or 25°C

Principle of Refractive Index

A sample is placed into the Refractometer and the Refractive Index read directly from the scale.

Safety: Observe proper laboratory safety rules, personal protection, and chemical handling procedures at all times. Read MSDS before handling chemicals and products.


Suitable refractometer

Water bath capable of maintaining the desired temperature

Procedure of Refractive Index

1) Check specification to determine whether the refractive index is to be tested at 20°C or 25°C.

2) Make sure that water from the water bath at 20°C or 25°C is circulating through the refractometer.


All Concentrate & Finished Goods Specific Gravity specifications for routine production QC checks must be specified at25°C.

3) Ensure that the instrument is calibrated by checking that the Refractive Index of distilled water is 1.3330 at 20°C or 1.3325 at 25°C.

4) Make sure the prism is clean. The prism may be cleaned with Luke-warm water or alcohol. The prism must be dried using either filter paper or tissue paper, dryness is important as small traces of water can have a significant effect on the results obtained.

5) Place a small drop of sample on the lower prism and lower the top prism onto the sample; making sure that the sample smears thinly between the prisms.

6) Close the prisms and look through the eyepiece. Adjust the instrument as required and read the Refractive Index.


Report the Refractive Index as determined and compare to the specification.


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