TECHNIQUES USED IN THE FOOD AND DRUGS DIVISIONREFRACTOMETRYRefractometry is the method of measuring substances' refractive index in order to, for example, to assess their composition or purity. A refractometer is the instrument used to measure refractive index. Refractometers can also be used to measure sugar content of a sample in Brix and Salinity.

DGAL uses refractometry to determine Brix (sugar content) and refractive index (RI) of a sample.

PRINCIPLES OF REFRACTOMETRY The refractometer works using the principle of light refraction through liquids. As light passes from air into a liquid it slows down. The more dissolved solids water contains, the slower light travels through it, and the more pronounced the "bending" effect on light.

The ratio of the angle of refraction in a vacuum versus that in a material of interest is the material's index of refraction. For practical purposes, air is normally used as a reference instead of a vacuum, since it's index of refraction is very close to (but not precisely) 1.0.

A refractometer measures the extent to which light is bent (refracted) when it moves from air into a sample and is typically used to determine the index of refraction (refractive index or n) of a liquid sample.

Refractometers use this principle to determine the amount of dissolved solids in liquids by passing light through a sample and showing the refracted angle on a scale. The scale most commonly used is referred to as the Brix scale.

How to Calibrate and Use a Refractometer (see appendix __ for diagram)Before starting to take readings, the refractometer has to be calibrated using distilled water or a special calibration liquid depending on the type of refractometer.

a) The calibration is initiated by lifting up the plate and placing 2-3 drops of distilled water on top of the prism assembly. The plate is closed so the water spreads across the entire surface of the prism without any air bubbles or dry spots.

The test sample is left to sit on the prism for approximately 30 seconds before attempting the next step. This allows the sample to adjust to the ambient temperature of the refractometer.

b) The refractometer is then shifted towards the direction of a natural light source and the eyepiece in looked into. A circular field with graduations down the center will be seen. The eyepiece is focused to clearly see the graduations.

c) The calibration screw is then turned until the boundary between the upper blue field and the lower white field meet exactly at ZERO on the scale (see figure on right).

d) After calibration, the refractometer is ready to take readings of samples.

Before adding a sample, the instrument is cleaned (both the plate and the top of the main prism assembly) using a soft, damp cloth, then place 2-3 drops of the desired sample is added on top of the prism. The daylight plate is closed and the reading is taken..

e) The image on the right illustrates what would be seen if one looked through the refractometer without any sample at all. The entire scale is colored.

   

f) After putting the sample, the scale will appear as shown on the left. This sample is reading 23% Brix.

     

Brix Degrees Brix (°Bx) is unit representative of the sucrose content of an aqueous solution by weight. One degree Brix corresponds to 1 gram of sucrose in 100 grams of solution and thus represents the strength of the solution as a percentage by weight (% w/w).

Dissolution of sucrose and other sugars in water changes its optical properties in particular its Refractive Index and the extent to which it rotates the plane of linearly polarized light. Therefore, a refractometer can be used to measure Brix since it proportional to the refractive index of the liquid.

Brix is used in the food industry for measuring the approximate amount of sugars in fruits, vegetables, juices, wine, soft drinks and in the starch and sugar manufacturing industry.

The scale of a refractometer can be calibrated in Brix or refractive index.

REFERENCEShttp://www2.ups.edu/faculty/hanson/labtechniques/refractometry/intro.htm http://en.wikipedia.org/wiki/Brix http://www.chemistry.oregonstate.edu/courses/ch361-464/ch362/refract02.htm

APPENDIXAnatomy of a Refractometer