Refractometry

Refractometry is a method for measuring the refractive index of solid, liquid and gaseous substances, which uses the limiting angle of refraction, or the total reflection of light. Refractive index is a characteristic quantity of substances, it is a sign of their purity and it can also be used to determine concentration. Instruments designed to measure the index of refraction are called refractometers and can work in both transmitted and reflected light. They can be divided according to several points of view, according to the construction, the purpose it serves, or according to the radiation used (monochromatic or polychromatic).

According to their construction, we divide them into two groups:

a)	refractometers in which the refracting prism and the telescope are movable relative to each other (Pulfrich's and Abbe's)

b)	refractometers in which the refracting prism and the telescope occupy a mutually invariable position (submersible and handheld)

They are used to quickly determine the concentration, purity of substances, quality of substances, etc.

The refractometer consists of an illuminating and measuring prism, between which a thin layer of the examined sample is placed, a reading microscope and an adjusting telescope. The diaphragm wall of the illumination prism is roughened so that scattered light enters the layer of the measured substance. If we observe a reflected or refracted light beam through a telescope, whose optical axis is parallel to the emerging beam, we see bright and dark fields, the boundary line of which corresponds to the limiting angle of total reflection. The boundary line is set by rotating the prism body to the center of the reticle. In the reading microscope, the index of refraction is read. Since the refractive index also depends on the temperature, it is necessary to connect to the thermostat and perform the measurement at a constant temperature.

Abbe refractometer
'Abbe's refractometer belongs to the first group (the distance of the telescope from the refracting prism can be changed). Due to its versatile applicability, it is referred to as a universal refractometer. It is most commonly used to measure the refractive index of liquids as a double-prism refractometer, the core of which is formed by an illuminating and measuring prism. These prisms are made from Icelandic limestone. It measures a refractive index ranging from 1.3 to 1.7. It is also possible to work with polychromatic light, as it has an optical dispersion compensator. The designer is the German physicist Ernst Karl Abbe.

The principle is to spread the investigated liquid between the walls of the prisms, which are folded together. During illumination, light enters through the illumination prism. Thanks to its roughened wall, the light scatters and begins to pass in all directions in the measured liquid, its refraction occurs. Thanks to this, we can see in the telescope, with a suitable rotation, the interface that is given by the boundary rays. These are based on the measuring prism. We can directly read the refractive index value for a given wavelength on the telescope scale. A typical value is 589.3 nm, which is the average wavelength of the spectral lines of sodium discharge light. It is the most common source of light.

Pulfrich refractometer
The Pulfrich refractometer belongs to the first group (the distance of the telescope from the refracting prism can be changed). Its basic feature is that it does not have a dispersion compensating device, and therefore the light source must be monochromatic. It can be used to measure the index of refraction as well as dispersion. And that of liquid and solid substances. The core is a glass prism with a high refractive index. The range of refractive index measurement depends on the choice of prism, but normally it is a range from 1.3 to 1.8.

For measuring liquids, a hollow tube is glued to the measuring prism. It is also favorable for the accuracy of the measurement that the temperature is constant.

For the measurement of powder substances, cuvettes attached to the measuring prism are used. First, about 1 to 2 ml of a liquid with a lower refractive index is dripped into the cuvette, then the powder is added (it must not dissolve). Next, we drop in a liquid with a higher refractive index, we add it until the refractive index of the powder and liquids are equal. We can tell by seeing a sharp interface in our field of vision.

The scale is in degrees, so we find the values ​​by calculations or from tables.

Immersion refractometer
It belongs to the second group (the telescope and the refracting prism do not move relative to each other). The core is a prism with a refracting angle of 60°, which is mounted directly on the telescope. It is mainly used to measure the refractive index of liquids.

The measuring prism is immersed in the beaker with the measured liquid. The beaker must have a transparent window in the bottom so that white light can enter, which illuminates the given liquid. The rays are rotated by a mirror to reach the Amici prism, eliminating dispersion.

Hand-held refractometer
Due to special purposes, it is similar in construction to an immersion refractometer. It is not as variable as the previous types, it has one fixed measuring prism, which is chosen according to requirements and specialization. There is no need to correct the temperature, but it is best to use it at a temperature of 20 °C.

Distilled water is dripped onto the measuring prism of the refractometer and covered with a plate, the value on the given scale should be zero. Then the plate is removed, the prism is wiped dry and the measured sample is applied. Calibration with distilled water is required after each measurement.

The scales are different, for example Oeschle, Brix and Baumé.

Use of refractometers
This method can be used to determine the purity of substances in the solid and liquid phase. In general, the determination of impurities is more sensitive and accurate, the more their refractive index differs from the refractive index of the pure substance. It is also used in the detection of binary mixtures of substances, especially organic solvents.

Hand-held refractometers are currently used in the healthcare sector, in laboratories, but also in the automotive and food industries.

In healthcare, they are used in the analysis of body and tissue fluids (determining the specific gravity of blood serum, urine, plasma). The refractive index of these substances can be measured in the range of 1.333 to 1.357. 0.005 ml of sample is enough to perform the measurement.

Digital refractometers are used in laboratories, mainly for drug analysis. A positive feature is that they can be connected to a computer and the results can be recorded automatically. Automatic temperature compensation is a matter of course for these devices.

In the automotive industry, they are used to check brake and cooling fluid, cutting oils, lubricants or to determine the state of electrolytes in car batteries. These instruments have temperature compensation, so there is no need to keep the calibration, measured sample and the prism at the same temperature.

In the food industry, refractometers measure the sugar content of, for example, wine and honey. Also salinity in food brine and seawater salinity. Refractometers are also widely used in the construction and cosmetic industries. They are also used in determining the refractive index of precious stones.

Refractive index
When a ray passes from one environment to another, its speed and direction of propagation change, due to the different densities of the environments. The ratio of the beam penetration speed in the first medium c1 to the speed in the second medium c2 is called the refractive index n and is therefore characterized by the relation:

n=c1/c2

Since the index of refraction is a relative quantity, it was necessary to choose a standard one, precisely defined environment. The vacuum environment was chosen as the basic (standard) environment, in which the speed of the light beam is the largest and is equal to the speed of light c = 2.997925.108 m.s-1. During the transition of rays from a vacuum to a given medium, we find the so-called absolute index of refraction, which is defined by the relation:

n=c0/c2

Refractive index we can also express using the change in the direction of the medium beam ('Snell's law):

n=sin α/sin β

When a beam passes through another medium, the beam is refracted due to the difference in the speed of light in the two mediums. The angle of refraction β is smaller than the angle of incidence α when the ray passes into a medium in which the speed of light is lower compared to the original medium. So a fracture to the perpendicular occurs. Otherwise, the fracture occurs from the perpendicular. The angles of incidence, reflection and refraction are measured between the beam and the perpendicular lowered to the phase interface. During refractometric measurements, an incidence angle of 90 °C is chosen, the so-called sliding beam, and the maximum limiting angle is monitored.

related articles
Refractive index of light