REFRACTOMETRY

Refractometry is the method of calculating the refractive index (a fundamental physical property of any substance) of a sample to, for example, know its composition or purity. Refractometers are the instruments used to determine this refractive index. Although refractometers are most effective for measuring liquids, they are also used for measuring solids and gases, such as glass or gems.

Fig.1 Refractometer

WHAT IS REFRACTION

When a pencil is placed in water, the tip of the pencil appears tilted. Then, if you do the same thing but put the pencil in a sugar water solution, the tip of the pencil will appear more inclined. This is the phenomenon of refraction of light.

Refractometers are measuring instruments, in which this phenomenon of light refraction is put into practice. They are based on the principle that when the density of a substance increases (for example: when sugar dissolves in water), the refractive index increases proportionally.

Refractometers were invented by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th century.

Fig.2. Refraction

There are two types of refractometers based on refractive index detection; transparent systems and reflection systems. Portable refractometers and Abbe refractometers use transparent systems, while digital refractometers use reflection systems.

Fig.3 Abbe refractometer

DESCRIPTION OF THE TRANSPARENT SYSTEM.

In the figure below the detection is made using the refractive phenomenon produced at the limit of the prism. The refractive index of the prism is much larger than that of the example.
if the sample is of low concentration, the angle of refraction is large (see "A") due to the large difference in refractive index between the prism and the sample
If the sample is concentrated, the angle of refraction is small (see "B") due to the small difference in refractive index between the prism and the sample.

Fig.4. Transparent System

DESCRIPTION OF THE REFLECTIVE SYSTEM.

The refraction system for the digital refractometer (reflection system) will be detailed below. In the figure below, light beam A, incident from the bottom left of the prism, is not reflected by the boundary, but passes through the sample. Light beam B is reflected from the right face, directly along the boundary of the prism. Light beam C is incident at an angle too great to pass through the sample, but is totally reflected to the lower right side of the prism. As a result, the boundary line is produced by dividing the light and shadow on the other side of the dotted line B in the figure. The reflection angle of this line is proportional to the refractive index, the position of the boundary line between light and dark fields is captured by a sensor and converted into refractive indices.

Fig.5. Reflection System

Fig. 6 Digital refractometer

UNIT OF MEASUREMENT (BRIX)

The Measurement Scale (%) shows the percentage concentration of the soluble solids contained in a sample (water solution). The content of soluble solids is the total of all the solids dissolved in the water, including sugar, salts, proteins, acids, etc., and the measurement read is the total of the sum of these. Basically, the Brix percentage (%) is calibrated to the number of grams of sugar contained in 100g of sugar solution. Thus, when measuring a sugar solution, Brix (%) must be perfectly equivalent to the actual concentration. With solutions containing other components, especially when one wants to know the exact concentration, a conversion table is necessary.

Fig.7. Brix Scale