Refractometer

Different measuring principles of a refractometer

Hand refractometer

Vintner measuring degrees Oechsle with the refractometer

The refractometer is a measuring device for determining the refractive index of - liquid or solid - transparent substances by refractometry . It uses the behavior of light at the transition between a prism with known properties and the substance to be tested.

If the general composition of a liquid is known, a refractometer can be used to measure the concentration of dissolved substances in it. In connection with the harvest of wine, sugar beet and also apples, the sugar content of the plants is determined in this way.

In ophthalmology , a refractometer is used to determine the refractive ratios of the eyes .

Measurement principles

Three measurement principles can be distinguished:

Transmitted light
grazing idea
Total reflection

The refraction or total reflection of the light is used. What all three principles have in common is a measuring _prism with a known refractive index (n _prism ). The light spreads at different angles at the transition between the measuring prism and the sample medium (n _fluid ). The unknown refractive index of the sample medium is measured via the deflection of light.

With the transmitted light principle, a parallel bundle of rays is refracted at the interface between the two media.
In the case of grazing incidence and total reflection, the critical angle of a beam with different angles of incidence on the interface is measured.

Designs

View through the eyepiece of a handheld refractometer. Reading: 47 ° Oechsle.

Traditional refractometers often use sunlight or an incandescent lamp as a light source, sometimes with a color filter. A scale serves as a detector, which can be read off with the eye using an optical system.

Examples are:

The first investigations with measuring prisms were already carried out in 1761 and 1802, but usable refractometers were only described by Ernst Abbe in 1874 and Pulfrich (1889) and Jelly (1934).

Differential refractometers compare the refractive index of a reference sample with the sample to be tested in that the sample interface forms a prism (which deflects in the event of refractive index differences).

Today's refractometers use an LED as a light source . A CCD sensor is used as the detector . A built-in temperature measurement or thermostat offers the possibility of compensating the temperature-dependent refractive index.

examples are

Hand and table-top devices for small sample quantities
Process refractometer for direct installation in the process, e.g. B. in pipe or tank

In addition, refractometric measuring methods are used in sensors of more complex machines, such as B. as a rain sensor in vehicles or a detector in equipment for high-performance liquid chromatography (HPLC). Continuously operating refractive index detectors are often used here.

Influence of the wavelength

The refractive index of a sample varies for almost all materials at different wavelengths. This so-called dispersion is characteristic of every material.

In the visible wavelength range, a decrease in the refractive index with increasing wavelength of the light and almost no absorption can be observed. In the infrared wavelength range, several absorption maxima and fluctuations in the refractive index often occur. 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 precisely. Therefore, in modern refractometers, the wavelength is set to a bandwidth of ± 0.2 nm in order to ensure correct results for samples with different dispersions.

Influence of temperature

The temperature has a very large influence on the refractive index. Therefore, the temperature of the prism and the sample must also be controlled. For precise measurements of the refractive index, temperature sensors and Peltier elements are used to thermostate the sample and the measuring prism .

In the past, external water baths were used for temperature control. Today's Peltier element temperature controls work faster and, in contrast to a water bath, do not require any maintenance.

Flow cuvette with pouring funnel for an automatic refractometer enables a quick exchange of the sample, e.g. B. in quality control

Automatic refractometer

Schematic structure of an automatic refractometer: An LED light source illuminates a prism with a sample on its surface at different angles. Depending on the refractive index difference between the prism material and the sample and the angle of incidence of the light beam, the light is partially refracted into the sample and reflected or completely reflected. The critical angle of total internal reflection is determined by measuring the reflected light intensity as a function of the angle of incidence

Automatic refractometers carry out some activities independently, can sometimes communicate with other measuring devices and save data.

The measuring principle is based on the determination of the critical angle of total reflection: A light source, usually a light-emitting diode, is focused on a prism surface via a lens system. An interference filter guarantees the specified wavelength. By focusing the light on a point on the prism surface, a large number of different angles of incidence are covered. As shown in the schematic illustration of the structure of an automatic refractometer, the sample to be measured is in direct contact with the measuring prism. Depending on the refractive index of the sample, the incident light partially penetrates into the sample (refracted) at angles of incidence below the critical angle of total reflection, while the light is totally reflected at the sample / prism interface for higher angles of incidence. This dependence of the reflected light intensity on the angle of incidence is measured with a high-resolution CCD sensor. The refractive index of the sample can be precisely calculated from the video signal recorded with the CCD sensor. This method of measuring the angle of total internal reflection is independent of the sample properties. It is even possible to measure the refractive index of highly absorbent samples or samples that contain air bubbles or solid particles. Only a few microliters of the sample are required and the sample can be recovered. The determination is independent of vibrations and other environmental influences.

Flow cells

Various types of measuring cells are common, from micro flow cells for a few microlitres to sample cells with a filling funnel. Micro cells ensure good recoverability of expensive samples and prevent evaporation of volatile samples or solvents. Many flow cells have a filling funnel for filling.

Automatic sample feed

Automatic refractometer with sample changer for measuring multiple samples

If an automatic refractometer is equipped with a flow cell, the sample can be added either with a syringe or by using a pump. Some refractometers also offer the option of controlling a peristaltic pump built into the device. A hose pump makes it possible to carry out several measurements on a sample without user interaction. This eliminates human errors and increases sample throughput.

If an automated measurement of a large number of samples is required, some refractometers can be combined with a sample changer. The sample changer is controlled by the refractometer. However, the samples must first be filled into the sample changer container.

Applications

Traditional uses

Many applications serve to determine concentrations in a carrier medium: Traditionally, refractometers are used to determine the sugar content in aqueous solutions, e.g. B. Determination of ripeness during the grape harvest , measurement of the original wort when brewing beer and as a beekeeping device to determine the water content of honey . Separate scales have been established for these applications in the food sector ( degrees Oechsle , degrees Brix , degrees Plato ).

Refractometers are also used to determine the acid concentration in batteries. The oil-water concentration in cooling emulsion mixtures is measured in metalworking machines with the handheld refractometer, as is the measurement of the glycol content in coolants or the salinity of seawater. In the medical field, a refractometer is used to determine the protein content in urine . Alternatively, for the applications mentioned, the density is often determined with the aid of a hydrometer , or a drop scale. The most famous one is probably the cider scale .

Further applications in chemistry

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

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

{\ displaystyle {\ text {Specific refraction}} = {\ frac {n ^ {2} -1} {n ^ {2} +2}} \ cdot {\ frac {1} {d}}}

Here, the refractive index, the specific gravity of the substance. ${\ displaystyle n}$ ${\ displaystyle d}$

Multiplying the specific refraction by the molar mass of the substance gives the molecular refraction : ${\ displaystyle M}$

{\ displaystyle {\ text {Molecular refraction}} = {\ frac {n ^ {2} -1} {n ^ {2} +2}} \ cdot {\ frac {M} {d}}}

.

If the refractive index of a substance is determined at different wavelengths (e.g. the yellow sodium D line or the red hydrogen line), the molecular dispersion of substances is obtained.

The refractive power of a substance depends on the functional groups in each individual molecule. The molecular refraction is the sum of the individual functional groups, atoms in a molecule. By adding the increments for each functional group (e.g. C (monovalent): 2.41, C = C: 1.69, C≡C: 2.38, C = O: 2.19, CH: 1 , 09, -O-: 1.64) the molecular refraction can be calculated for each molecule and compared with the measured value.

Refractometry was one of the earliest physical methods for checking the structure and functional groups in molecules.

Refractometers are also used to determine the optical properties of solid media, e.g. B. 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 using a Max Le Blanc method.

Vehicle technology

Refractometry is used to determine the frost resistance of the coolant . These refractometers have a temperature scale for the pour point of the coolant. Other methods are swimmers who measure the glycol content by buoyancy .

Ophthalmology

In ophthalmology and optometry , manual or automatic refractometers are used to determine and measure the objective refraction of the eyes , the basis for adjusting corrective lenses such as glasses or contact lenses . When Autorefractometer can Foucault cutting methods are used. The automation accelerates the examination process, makes it usable for the layperson, but does not always produce accurate measurement results. An alternative to the use of a refractometer, especially for small children, is retinoscopy .

Gem lore

The ER604 optical gem refractometer

Gemstones are transparent minerals and can therefore be examined using optical methods. Since the refractive index is a material constant that depends on the chemical composition of a substance, it provides information about the type and quality of a gemstone. The determination with a special gemstone refractometer is an easy-to-use method with which the authenticity and quality of a stone can be assessed. The gemstone refractometer is therefore part of the basic equipment of a gemological (gemstone) laboratory. Due to the dependence of the refractive index on the wavelength of the light used ( dispersion ), the measurement is usually carried out at the wavelength of the sodium D line (nD) of 589 nm. This is either filtered out of the daylight or generated by a monochromatic light emitting diode ( LED ) . Certain stones such as ruby, sapphire, tourmaline or topaz are optically anisotropic . They have a birefringence that depends on the polarization plane of the light . The two different refractive indices are determined by using a polarization filter. Gemstone refractometers are offered both as classic optical instruments and as electronic measuring devices with a digital direct display.

Web links

Commons : Refractometer - collection of images, videos and audio files

Wiktionary: Refractometer - explanations of meanings, word origins, synonyms, translations

Automatic refractometer in comparison to monocular subjective refraction (PDF; 1.1 MB) Archived from the original on April 30, 2006. Retrieved on December 29, 2009.
Modern automatic refractometers in comparison (PDF; 135 kB) Archived from the original on April 26, 2005. Retrieved on December 29, 2009.
Refractometer calculator for determining the final degree of fermentation and alcohol content of beer in the hobby brewing sector

literature

aprentas (Ed.): Laborpraxis . 6th edition. tape 2 : Measurement methods. Springer International Publishing Switzerland, Cham 2017, ISBN 978-3-0348-0967-2 , Chapter 10: Determining the refraction , p. 83-92 , doi : 10.1007 / 978-3-0348-0968-9_10 .
Standard DIN EN ISO 10342: 2010-11: Ophthalmic instruments - eye refractometers ( beuth.de ).

Individual evidence

^ Herbert Feltkamp, Peter Fuchs, Heinz Sucker (eds.): Pharmaceutical quality control. Georg Thieme Verlag, 1983, ISBN 3-13-611501-5 , pp. 248-249.
^ Clairaut, Mem. Acad. R. 388 (1761).
^ William Hyde Wollaston: XII. A method of examining refractive and dispersive powers, by prismatic reflection . In: Philosophical Transactions of the Royal Society of London . tape 92 , 1802, pp. 365-380 , doi : 10.1098 / rstl.1802.0014 (free full text).
↑ Ernst Abbe: New apparatus for determining the refractive and dispersive power of solid and liquid bodies . Mauke's Verlag (Hermann Dufft), Jena 1874, OCLC 9297565 ( digital copy - new edition: Forgotten Books, [sl] 2016, ISBN 978-1-334-01028-6 ).

↑ C. Pulfrich: A new refractometer . In: Journal for analytical chemistry . tape 28 , no. 1 , 1889, p. 81-82 , doi : 10.1007 / BF01375871 .

↑ EE Jelly: XVI. - A Microrefractometer and Its Use in Chemical Microscopy . In: Journal of the Royal Microscopical Society . tape 54 , no. 4 , 1934, pp. 234–245 , doi : 10.1111 / j.1365-2818.1934.tb02319.x .

↑ differential refractometer RIDK-102 from the year 1989th

↑ JW Brühl: About the influence of the single and the so-called multiple bonding of atoms on the light refractive power of the body . In: Journal of Physical Chemistry . 1U, no. 1 , 1887, p. 307 , doi : 10.1515 / zpch-1887-0136 .

↑ James D. Forbes: On the color of steam under certain circumstances . In: Annals of Physics . tape 123 , no. 8 , 1839, pp. 593-599 , doi : 10.1002 / andp.18391230805 ( digitized on Gallica ).

↑ JW Brühl: Investigations on the molecular refraction of organic liquid bodies of great color dispersibility . In: Reports of the German Chemical Society . tape 19 , no. 2 , 1886, p. 2746–2762 , doi : 10.1002 / cber.188601902246 ( digitized on Gallica - here: p. 2760).

↑ E. Conrady: Calculation of the atomic refraction for sodium light . In: Reports of the German Chemical Society . tape 22 , 1889, pp. Ref. 224 ( digitized on Gallica ).

↑ JW Brühl: About the influence of the single and the so-called multiple bond of atoms on the light refractive power of the body. A contribution to the investigation of the constitution of benzene and naphthalene compounds . In: Reports of the German Chemical Society . tape 20 , no. 2 , 1887, p. 2288–2311 , doi : 10.1002 / cber.18870200239 / full ( digitized on Gallica ).

↑ JW Brühl: on the molecular refraction of organic liquid bodies with a great ability to disperse colors . In: Justus Liebig's Annals of Chemistry . tape 235 , no. 1-2 , 1886, pp. 1–106 , doi : 10.1002 / jlac.18862350102 (here: p. 35).

↑ M. Le Blanc: A simple method for the determination of refraction exponents of optically isotropic bodies . In: Journal of Physical Chemistry . 10U, no. 1 , 1892, doi : 10.1515 / zpch-1892-1027 .

↑ Stefan Sobotta: Heat pump practice: technology, planning, installation . 2nd Edition. Beuth Verlag, Berlin 2015, ISBN 978-3-410-23362-6 , pp. 216 ( limited preview in Google Book search).

↑ Bernhard Lachenmayr, Annemarie Buser: Eye-glasses-refraction: Schober-course: understand-learn-apply . 4th edition. Thieme, Stuttgart 2006, ISBN 978-3-13-139554-2 , p. 38 ( limited preview in Google Book search).

[Feltkamp-1] Herbert Feltkamp, Peter Fuchs, Heinz Sucker (eds.): Pharmaceutical quality control. Georg Thieme Verlag, 1983, ISBN 3-13-611501-5 , pp. 248-249.

[2] Clairaut, Mem. Acad. R. 388 (1761).

[3] William Hyde Wollaston: XII. A method of examining refractive and dispersive powers, by prismatic reflection . In: Philosophical Transactions of the Royal Society of London . tape 92 , 1802, pp. 365-380 , doi : 10.1098 / rstl.1802.0014 (free full text).

[4] Ernst Abbe: New apparatus for determining the refractive and dispersive power of solid and liquid bodies . Mauke's Verlag (Hermann Dufft), Jena 1874, OCLC 9297565 ( digital copy - new edition: Forgotten Books, [sl] 2016, ISBN 978-1-334-01028-6 ).

[5] C. Pulfrich: A new refractometer . In: Journal for analytical chemistry . tape 28 , no. 1 , 1889, p. 81-82 , doi : 10.1007 / BF01375871 .

[6] EE Jelly: XVI. - A Microrefractometer and Its Use in Chemical Microscopy . In: Journal of the Royal Microscopical Society . tape 54 , no. 4 , 1934, pp. 234–245 , doi : 10.1111 / j.1365-2818.1934.tb02319.x .

[7] rential refractometer RIDK-102 from the year 1989th

[8] JW Brühl: About the influence of the single and the so-called multiple bonding of atoms on the light refractive power of the body . In: Journal of Physical Chemistry . 1U, no. 1 , 1887, p. 307 , doi : 10.1515 / zpch-1887-0136 .

[9] James D. Forbes: On the color of steam under certain circumstances . In: Annals of Physics . tape 123 , no. 8 , 1839, pp. 593-599 , doi : 10.1002 / andp.18391230805 ( digitized on Gallica ).

[10] JW Brühl: Investigations on the molecular refraction of organic liquid bodies of great color dispersibility . In: Reports of the German Chemical Society . tape 19 , no. 2 , 1886, p. 2746–2762 , doi : 10.1002 / cber.188601902246 ( digitized on Gallica - here: p. 2760).

[11] E. Conrady: Calculation of the atomic refraction for sodium light . In: Reports of the German Chemical Society . tape 22 , 1889, pp. Ref. 224 ( digitized on Gallica ).

[12] JW Brühl: About the influence of the single and the so-called multiple bond of atoms on the light refractive power of the body. A contribution to the investigation of the constitution of benzene and naphthalene compounds . In: Reports of the German Chemical Society . tape 20 , no. 2 , 1887, p. 2288–2311 , doi : 10.1002 / cber.18870200239 / full ( digitized on Gallica ).

[13] JW Brühl: on the molecular refraction of organic liquid bodies with a great ability to disperse colors . In: Justus Liebig's Annals of Chemistry . tape 235 , no. 1-2 , 1886, pp. 1–106 , doi : 10.1002 / jlac.18862350102 (here: p. 35).

[14] M. Le Blanc: A simple method for the determination of refraction exponents of optically isotropic bodies . In: Journal of Physical Chemistry . 10U, no. 1 , 1892, doi : 10.1515 / zpch-1892-1027 .

[15] Stefan Sobotta: Heat pump practice: technology, planning, installation . 2nd Edition. Beuth Verlag, Berlin 2015, ISBN 978-3-410-23362-6 , pp. 216 ( limited preview in Google Book search).

[16] Bernhard Lachenmayr, Annemarie Buser: Eye-glasses-refraction: Schober-course: understand-learn-apply . 4th edition. Thieme, Stuttgart 2006, ISBN 978-3-13-139554-2 , p. 38 ( limited preview in Google Book search).