Refractometer

The refractometer is a measuring device for determining the refractive index of - liquid or solid - transparent materials by refractometry. It uses for the behavior of light at the interface between a prism having known properties and the test substance.

When the overall composition of a liquid is known, a refractometer can be used to measure the concentration of solutes therein. In connection with the harvesting of sugar beets or wine is determined in this way, the sugar content of plants.

In ophthalmology are determined using a refractometer, the refractive conditions of the eye.

• 4.1 Further applications in chemistry

Measurement principles

Three measuring principles can be distinguished:

• Transmitted light
• Grazing incidence
• Total reflection

The refraction (refraction ) and the total reflection of light is utilized. As common use all three principles a measuring prism of known refractive index ( nPrisma ). The light propagates in the transition between the measuring prism and the sample medium ( nFluid ) at different speeds from. The unknown refractive index of the sample medium is measured by the deflection of light.

• The transmitted-light principle, a parallel beam at the interface of the two media is broken.
• At grazing incidence and at the critical angle of total reflection of a beam with various angles of incidence is measured at the interface.

Designs

One way to distinguish between different refractometer, the division into analog and digital instruments.

Use traditional analog refractometer as the light source often sunlight or an incandescent lamp partly with color filters. The detector is a scale which is read via an optical system with the eye.

Examples are:

• Hand refractometer
• Abbe refractometer
• Pulfrich refractometer
• Wollaston's refractometer (1802 )
• Jelley refractometer
• Automatic and semi-automatic refractometer

Initial studies with measuring prisms already existed in the year 1761 or 1802, but usable by Ernst Abbe refractometer only in 1874., And Pulfrich (1888 ) and Jelly (1934 ) have been described. Use digital refractometer as the light source is an LED. As a detector, a CCD sensor is employed. An accurate temperature measurement is integrated, and thus offers the possibility of compensating the temperature-dependent refractive index. Portable, digital refractometers facilitate the measurement, since they can be used directly on site and offer a high reading accuracy.

Examples are

• Hand and tableware for small sample volumes
• Operating process for the direct installation into the process, such as in pipe or tank

Also refractometric measurement sensors are used in process of more complex machines, such as a rain sensor in vehicles or detector apparatus for high performance liquid chromatography (HPLC). In this case, continuously operating refractive index detectors are often used.

Automatic refractometer

Automatic Refractometers measure the refractive index of a sample automatically. The measurement of refractive index based on the determination of the critical angle of total reflection: a light source, typically a long-life LED is focused on a prism surface of a lens system. An interference filter guarantees the specified wavelength. By focusing the light onto a point on the prism surface, a plurality of different incident angles is covered. As shown in the schematic diagram for the construction of an automatic refractometer, the sample to be measured is in direct contact with the measurement prism. In dependence on the refractive index of the sample, the incident light enters at angles of incidence below the critical angle of total reflection in part in the sample (it is broken ), while for higher angles of incidence, the light at the interface sample / prism is totally internally reflected. This dependence of the reflected light intensity of the incident angle is measured with a high resolution CCD sensor. From the video signal recorded with the CCD sensor, the refractive index of the sample can be calculated precisely. This method for measuring the angle of the total reflection is independent of the sample properties. It is even possible to measure the refractive index of strongly absorbing samples or samples containing air bubbles or solid particles. There are only a few microliters of sample is required and the sample can be recovered. The determination is independent of vibrations and other environmental influences.

Influence of the wavelength

The refractive index of a sample varies for almost all materials at different wavelengths. This so-called dispersion characteristic of each material. In the visible wavelength range, a decrease in the refractive index with increasing wavelength of the light and almost no absorption is observed. In the infrared wavelength range often occur a plurality of absorption maxima and fluctuations of the refractive index. For a high-quality measurement of the refractive index with an accuracy of the refractive index of up to 0.00002, the measurement wavelength must be determined very accurately. Therefore, in modern refractometers, the wavelength is set to a range of / -0.2 nm in order to ensure correct results for samples with different dispersions.

Influence of temperature

Temperature has a great influence on the refractive index measurement. Therefore, the temperature of the prism and the sample temperature must be controlled with high precision. For very precise measurements of the refractive index temperature sensor with high accuracy and rapid and precise control of the temperature of the sample and the measuring prism Peltier elements are required. The control accuracy must be designed so that small changes in the sample temperature does not lead to a detectable change in the refractive index.

In the past, external water baths were used for temperature control, today powerful Peltier temperature control are used, which work faster and more accurate and unlike a water require no further maintenance.

Advanced methods of automatic refractometer

Automatic refractometers are microprocessor-controlled electronic devices. That is, they may have a high degree of automation and also be combined with other instruments.

Flow Cells

There are different types of load cells available from a micro - flow cell for a few microliters up to sample cells with a hopper for rapid sample exchange a cleaning of the measuring prism between the measurements is required without that. The sample cell can also be used for the measurement of toxic samples with minimal contact to the sample. Micro - cells require only a few microliters volume, provide a good recoverability of expensive samples safely and prevent evaporation of volatile samples or solvents. They can also be used in automated systems for the automatic filling of the sample. For easy filling of the sample through a funnel flow cells are available with a hopper available. These are used for rapid sample changing in quality control.

Automatic sample feeder

If an automatic refractometer is equipped with a flow cell, the sample may be filled either by a syringe or by use of a pump. Modern refractometer also offer the possibility to control a built- in the device peristaltic pump. This is configured and controlled via the device software. A peristaltic pump confers the batch processes or in the laboratory to monitor to perform a plurality of measurements on a sample without any user interaction. This eliminates human error and ensures a high sample throughput.

If an automated measurement of a large number of samples is required, modern automatic refractometer can be combined with an automatic sample changer. The sample changer is controlled by the refractometer and allows fully automatic measurements of the samples were filled into the vials of the sample changer.

Multi-parameter measurements

Today's labs want for efficient quality control not only measure the refractive index of the sample, but also determine additional parameters such as density, optical rotation or viscosity. Due to the microprocessor control and a plurality of interfaces, automatic refractometer are able to communicate with other computers or instruments, for example, with density meters, pH meters, polarimeters or viscometers. In addition to the refractive index of the data are measured in a single pass and stored together in the database.

Pharmaceutical documentation and validation

Refractometers are widely used in pharmaceutical applications for quality control of raw materials and finished products. Pharmaceutical manufacturers have to many international regulations such as FDA 21 CFR Part 11, GMP, GAMP 5 and USP < 1058 > note that require detailed documentation. The manufacturer of automatic refractometers support these users by providing a device software with user -level, electronic signature and audit trail functionality fulfills the requirements of 21 CFR Part 11. In addition, pharmaceutical validation and qualification packages are available. These include:

• Qualification Plan (QP )
• Design Qualification (DQ )
• Risk analysis
• Installation Qualification (IQ)
• Operational Qualification (OQ )
• Check- list 21 CFR Part 11 / SOP
• Performance Qualification (PQ )

Applications

Many applications are used to determine concentrations in a carrier medium: Traditional refractometer to determine the sugar content to be used in aqueous solutions, eg maturity determination in the wine harvest, measuring the wort in beer cooking and as beekeeping equipment for the determination of the water content of honey. For these applications in the food industry to your own scales have been established ( degrees Oechsle degrees Brix, ° Plato). Refractometers are also used in the determination of the acid concentration in the battery. The oil-water concentration in cooling emulsion mixtures is measured at metal working machines with the hand-held refractometer. Similarly, when measuring the glycol content in coolants or the salinity of sea water. In the medical field, a refractometer is used to determine the protein content in urine. Alternatively, for the mentioned applications also often the density using a hydrometer, a hydrometer, determined. The best known is probably the Mostwaage here.

Other applications in the chemical

In chemistry, the refractometry of purity testing of organic materials used. Any organic liquid has a characteristic refractive index. In addition to the purity testing refractometry is also used for quantitative analysis of two- or multi-component mixtures and the identification of substances.

The specific refraction of a substance is obtained from the Lorentz formula Lorenz'sche

Here, the refractive index, the specific gravity of the substance.

Multiplying the specific refraction with the molar mass of the substance, we obtain the molecular refraction:

If one determines the refractive index of a substance at different wavelengths (eg, the yellow sodium D line or the red hydrogen line ), we obtain the molecular dispersion of substances.

The refractive power of a substance is dependent on the functional groups in each molecule. The molecular refraction is the sum of the functional groups, the atoms in a molecule. By means of the summation of the increments for each functional group (eg, C ( single-valued ): 2.41, C = C: 1.69, C ≡ C: 2.38, C = O: 2.19, CH: 1.09, - O: 1.64 ) can be the molecular refraction for each molecule calculated and compared with the measured value.

The refractometry was one of the earliest physical methods to verify structural and functional groups in the molecule.

To determine the optical properties of solid media refractometer also be used, for example, in the manufacture of glasses and in the quality assessment or identification of gemstones. It is also possible to determine the refractive index of organic solids by a method of Max Le Blanc.

Ophthalmology

In ophthalmology and optometry manual or automatic refractometer for the determination and measurement of objective refraction of the eye are used, the basis for the adjustment of corrective lenses such as glasses or contact lenses. When refractometer can be used the Foucault knife edge method. The automation speeds up the examination process, making it applicable also for the layman, however, does not always produce accurate results. A, particularly in young children, used alternative to the use of a refractometer is the retinoscopy.

Gemology

Gemstones are transparent minerals and can therefore be studied by optical methods. Since the refractive index is a depending on the chemical composition of a substance material constant, and focuses on the nature and quality of a gemstone. The determination with a special Gemstone is an easy to use method that can be assessed with the authenticity and quality of a stone. Therefore, the Gemstone belongs to the basic equipment of a gemological ( gemmology ) laboratories. Because of the dependence of the refractive index from the wavelength of light used (dispersion) of the measurement is usually carried out at the wavelength of Natium D-line (nD ) of 589 nm, this is produced either filtered from the daylight or by a monochromatic light-emitting diode (LED). Certain stones such as ruby ​​, sapphire, tourmaline or topaz are optically anisotropic. They have a dependent of the polarization plane of the light birefringence. The two different indices of refraction are determined by using a polarizing filter. Gemstone offered both as a classical optical instruments as well as electronically -working instrument with digital direct display.