Working Principles and Uses of the Abbe Refractometer
The Abbe Refractometer is designed to measure the refractive index of liquids, glass, plastics and film. It requires a sample, an index liquid (that matches the refractive index of the sample) and a power source.
The sample is placed between a double prism – the measuring prism and the illuminating one. Light enters the sample through the illuminating prism, gets refracted at a critical angle and is measured by the telescope.
Working Principles of the Abbe Refractometer
The refractometer works on the measuring principle of total refraction. Light absorbed by the liquid sample is measured by an illuminating prism and, through a second refracting prism, transmitted to a refractive index scale in the eyepiece of the instrument. Since the refractive index of liquids depends on their temperature, a thermostat is often fitted into the instrument to maintain constant measurement temperatures. The refractometer can also be supplied with a thermometer for the measurement of solids.
The measurement values determined by the Abbe refractometer are displayed on the touch screen and can be saved in one of 100 memory locations. The measurement data can be retrieved again using different criteria such as user ID or sample number, and the results can also be exported to other devices.
Refractometers are used for a wide range of applications in the laboratory and process industry. For example, the concentration of raw and auxiliary materials as well as operating and maintenance fluids are checked. Similarly, sugar concentration and Brix value are frequently determined in beverage production. The refractometer is also used for the analysis of liquid mixtures.
With the Abbe refractometer, the concentration of solids in liquids can be determined precisely and quickly. Unlike many modern refractometers, the Abbe-type refractometer does not need to be calibrated before use. Nevertheless, a calibration is recommended to ensure error-free measurements.
To calibrate the Abbe refractometer, you simply need to place a known solution of the same concentration in the measuring chamber. Then, you move the rotatable focusing mirror to find the shadow boundary of the sample in relation to the light / dark line. The refractometer automatically calculates the refractive index from this information.
The measurement results are displayed on the refractometer’s display and can be interpreted with the aid of an appropriate scale. The scale can be a standard scale (degrees Oechsle, degrees Brix) or a user-specific scale. For a more precise determination of the refractive index, you can optionally connect the Abbe refractometer to a spectrophotometer.
This option provides a more accurate measurement of the Brix value, which is a function of the sucrose content in the sample. In this way, a high level of accuracy can be achieved when measuring liquids with a low concentration of sugar.
Uses of the Abbe Refractometer
The Abbe refractometer is ideal for testing the refractive index of liquids. This measurement is independent of the turbidity, viscosity or transparency of the sample. This is made possible by measuring the total reflection of a beam of rays with different angles of incidence at the interface between the illuminating prism and the sample. In this way the critical angle is measured and the refractive index can be determined precisely. The Abbe refractometer is used in many laboratories and is particularly suitable for examining pharmaceutical, food, cosmetics and chemical products.
The basic design of the Abbe refractometer has remained unchanged since Ernst Abbe’s booklet of 1874 (Neue Apparate zur Bestimmung des Refraktiven Indes). The sample is placed between two prisms – the illuminating and the measuring prism. Light enters the illuminating prism and is refracted at the critical angle on the bottom surface of the measuring prism. The reflected light is detected with a telescope and the position of the bright / dark boundary is determined. The result is then displayed on a scale. Traditionally the scale is calibrated in degrees Brix, but modern instruments also have a numerical display that allows the user to read the result directly.
Due to the constant temperature control of the illuminating and measuring prisms, refractometers are characterized by their extremely high level of precision. The measurement is most accurate at room temperature (20 degC). Therefore, it is important to keep the refractometer at this temperature, even between measurements. It is also advisable to carry out a calibration test each time before using the instrument. This ensures that the correct temperature is selected.
Most digital laboratory refractometers have a solid-state Peltier effect device that heats and cools the instrument and the sample, thus eliminating the need for a circulating water bath. This makes the instrument very convenient to use. The refractometer also has a history function that recalls the last several measurements.
Measurement data can be transferred to a PC via the RS232 interface or stored on one of 100 memory locations in the instrument itself. The measured values can be retrieved again by entering a user ID or the date of the measurement.
Components of the Abbe Refractometer
Refractometers are essentially optical instruments that measure the refractive index of liquid samples or transparent solids. A small sample of the material to be tested is inserted into a cube-shaped part of the instrument and sandwiched between a refracting prism and an illuminating prism. When light enters the sample via the illuminating prism it is refracted at a critical angle at the bottom surface of the measuring prism. The measured value of the angle gives the sample’s refractive index.
To ensure accurate measurement results the refractometer is equipped with a thermometer that is mounted on the enclosure of the measuring prism. This is because the refractive index of a substance changes with temperature, and for an error-free measurement the ambient temperature must be constant.
In addition, the Abbe refractometer has a special feature that eliminates dispersion, which blurs the boundary between the light and dark parts of the sample. It has a special compensator, consisting of two equal Amici prisms within the eyepiece cylinder that rotate uniformly in opposite directions and focus the light on the sample. This enables an almost sharply defined boundary between the light and dark areas to be observed, which guarantees accurate measurement values.
The refractometer’s storage case is fitted with a small bottle of 1-bromonaphthalene, a fluid that has a high index and is used as an embedding agent for solids. It is used to improve the quality of the refracted signal when the index of the sample is unknown. It also serves to normalise the result for temperature fluctuations.
Most modern digital refractometers can use solid-state Peltier effect devices to heat and cool the instrument and the sample, eliminating the need for an external water bath. These units are also equipped with software that allows programmable user-defined scales and offers a history function to recall the most recent measurements. Some can even transfer the readings to a connected computer for processing or reporting purposes.
Besides measuring the refractive index, digital Abbe refractometers can also be used to determine the Brix value of sugar solutions. This is the most commonly used unit for describing sugar concentration worldwide, and it corresponds to the amount of sucrose in a solution. There is a direct correlation between the Brix value and the sugar concentration, which makes the refractometer an indispensable tool for measuring the level of sucrose in food.
Servicing
Refractometers are used for measuring the refractive index of liquids and solids. In order for a correct measurement of the refractive index, the temperature of the sample and instrument needs to be constant. This is why most laboratory models of Abbe Refractometer have a thermostated tank where the sample is stored. The tank also contains a small Peltier device to heat and cool the instrument and the contact liquid, so that they stay at the same temperature, for repeatable and accurate measurements.
The principle of this instrument is that a sample is placed between two prisms, the illuminating and the measuring prism. The light enters the sample from the illuminating prism, and gets refracted at a critical angle on the bottom surface of the measuring prism. The position of the boundary line between the bright and dark areas is measured by the telescope, and using a scale that can be calibrated to show the index value of the liquid (or the Brix value), a measurement can be made.
This measurement method can be used for a variety of materials, liquids or even thick slurries. Nevertheless, it is important that the sample is not corrosive. It is also recommended that the index-matching liquid is added to the sample, in order to get a more precise result. This is especially important in cases where the index of a liquid changes with temperature.
Abbe Refractometers are widely used in educational establishments and laboratories, as well as in some industrial processes. For example, refractometers are used to measure the concentration of pharmaceutical products, or the density of petroleum products. They are also used to determine the salinity of seawater, and in geology to identify minerals that look similar but have different refractive indices.
ATAGO manufactures several different models of Abbe Refractometers. These include the DR-A1 which is a very simple refractometer for everyday use, and the DR-A2 which has a much more wide range of application possibilities, such as the ability to measure slurries or thick samples. Both models of refractometers are easy to operate, and provide a direct indication of the measured index or Brix value on the digital display.
