A refractometer is an optical instrument designed to measure the concentration of solutions, which is based on the phenomenon of light refraction.
Refractometry is a method for studying substances based on determining the index (coefficient) of refraction (refraction) and some of its functions. Refractometry (refractometric method) is used to identify chemical compounds, quantitative and structural analysis, determination of physical and chemical parameters of substances.
Classification :
1. Industrial
2. Laboratory
3. Portable digital
4. Portable handheld
Industrial and laboratory refractometers are designed for the study of substances in scientific laboratories and the control of technological processes in production. They have high precision measurements but also relatively large sizes.
Portable refractometersdesigned for the operational control of substances in the laboratory, in production or in the field. In turn, portable refractometers can be classified into digital and manual.
Portable digital refractometers usually have a liquid crystal display that shows the measured results. Most often, they also have additional options, such as simultaneous measurement of the density and refractive index of the solution, converting the results into different units of measurement, maintaining the temperature of the sample, and so on.
Hand-held (not digital) refractometers are usually more compact in size and do not have any electronic circuits and batteries (with the exception of some backlit models), which makes them easy to use for measurements not only in production, but also at home. Today, such refractometers are very popular due to their accuracy, ease of use, portability and low affordable price.
What is the principle of operation of a refractometer based on?
The principle of operation of the refractometeris based on the use of the phenomenon of refraction (refraction) of the light flux. When a beam of light passes from one substance to another, it deviates from the rectilinear direction by a certain angle. The ratio of the angle of entry of a light beam into a substance and its angle of refraction at the interface between two media is called the coefficient (index) of refraction.
The structure of a typical refractometer is shown schematically in the figure below. The main optical element of a refractometer is a prism onto which the test substance is applied. A prism is made of a material with a high refractive index.
Due to this, the incident light, passing through the substance and the prism, is refracted at a sufficiently large angle. Further, through a system of optical lenses, the light enters the refractometer scale (graded circle). Depending on the angle of refraction, the light beam is higher or lower on the scale of the instrument. The illuminated part of the scale will then be light; the part on which the light beam does not fall will be dark. The value of the angle of refraction of light depends on the composition of the solution and its concentration. Thus, by the position of the interface between light and shadow, one can unambiguously determine the refractive index or optical density of the solution under study.
However, it must be borne in mind that the refractive index of a substance also depends on temperature. Some models of hand-held refractometers take into account the influence of temperature using the ATC (Automatic Temperature Compensation System) function. Inside their body is a bimetallic plate. It shrinks or stretches depending on temperature changes. The bimetallic plate is connected to the optical system of the refractometer, smoothly moving it with temperature changes. The magnitude of the shifts is calculated so that the effect of temperature on the refractive index of the substance is completely compensated. When buying a refractometer, be sure to pay attention to the presence of the ATC function in it. In case of its absence, it is necessary to use special tables to recalculate the obtained values depending on the ambient temperature.
Taking measurements
Before taking measurements, the hand-held refractometer must be calibrated. Most refractometers are calibrated with distilled water. A few drops of water are applied to the main prism with a pipette, then the protective glass is closed. At the same time, care must be taken to ensure that the water protective glass evenly covered the surface of the prism, leaving no air bubbles. Next, using the calibration screw (or in the case of more simple models calibration screw) on the instrument scale is set to 0.0. After calibration, the prism must be carefully wiped with a soft cloth (it is advisable to use a cloth for glasses lenses, its material will not damage the refractometer lens). The refractometer is now ready for measurements. If the refractometer scale does not start from zero (0 is distilled water), then the refractometer is calibrated using a special oil.
To carry out measurements, the same actions are performed as during calibration, but instead of distilled water, the test solution is applied to the prism of the device. The calibration screw remains in its original position. After applying the solution, wait 30 seconds for the temperature of the solution to equalize with the temperature of the instrument. The refractometer is then pointed at a light source ( daylight or incandescent lamp) and take readings.
After taking measurements, the prism should be wiped again with a soft cloth. Hand refractometer can not be lowered into the water; this may cause water to enter the instrument and fog the scale. Do not measure hard or corrosive substances with the refractometer as they may damage the prism coating. Also, do not measure very hot solutions, as the main lens may come off. For most refractometers, the temperature limit is 50C.
Application of refractometers
Refractometers widely used in various fields human activity. The following are some of the most common applications for refractometers:
In the food industry:
1. quality control ;
2. determination of the mass fraction of soluble solids in processed fruits and vegetables;
In medicine:
1. determination of protein in blood serum;
2. determination of the density of urine, subretinal fluid of the eye;
Concentration measurement process various substances method of measuring refraction and determining the refractive index got its name - refractometry. Instruments that use the principle of refractometry in their work are called refractometers. Refractometers are widely used in various industries: for the identification of chemical compounds, the determination of physico-chemical parameters, for quantitative and structural analysis. In the food industry - to measure the alcohol content in alcoholic products, control the sugar content in sugar production- in general, to establish the quality food products. In pharmacology, refractometers are used to determine the amount of glucose in biological fluids and drugs in solutions. Advantages of refractometric methods of chemical quantitative analysis- speed of measurements, small consumption of substance and high accuracy.
Objective of the work: Refractometric method as a method for the analysis of medicinal substances. The relevance of using ATAGO refractometers and polarimeters in pharmaceuticals and, as a private matter, in pharmacies.
The refractive index (refractive index) is the ratio of the speed of light in vacuum to the speed of light in the test substance (absolute refractive index). The refractive index depends on the temperature and wavelength of the light at which the determination is made. In solutions, the refractive index also depends on the concentration of the substance and the nature of the solvent. In this case, in practice, the so-called relative refractive index (n) is determined, which is calculated as the ratio of the sine of the angle of incidence of the beam (α) to the sine of the angle of refraction (β) for two contiguous media.
The refractive index is also equal to the ratio of the speeds of light propagation in these media:
In laboratory conditions, the so-called relative refractive index (RI) of a substance with respect to air is usually determined. PP is measured on refractometers various systems. Previously, the measurement of RI was most often carried out using Abbe refractometers, which operate on the principle of total internal reflection when light passes through the interface between two media with different refractive indices. Nowadays, automatic refractometers are increasingly common in the laboratory. ATAGO series RX.
The range of measured RI when measured in transmitted light using Abbe refractometers is 1.3000 - 1.7000. If it is necessary to push the boundaries of the ranges, special models with low or high ranges are used, as well as multi-wavelength Abbe refractometers.
The range of measured RI when measured on automatic refractometers of the RX series is 1.32500 - 1.70000. The measurement accuracy of the refractive index should not be lower than ±2·10-4. The value of the refractive index depends on the nature of the substance, the wavelength of the light, the temperature at which the measurement is made, and the concentration of the substance in the solution. Typically, the measurement of the refractive index is carried out at a light wavelength of 589.3 nm (line D of the sodium spectrum). But in some cases, different wavelengths are used in the range from 450nm to 1550nm. Very important condition definition of PP is compliance temperature regime. As a rule, the determination is carried out at 20 degrees Celsius. At temperatures above 20 degrees - the value of the PP decreases, at temperatures below 20 degrees - the value of the PP increases.
The temperature correction is calculated using the formula: n1=n20+(20-T)*0.0002
The refractive index measured at 20°C and a light wavelength of 589.3 nm is denoted by the index n20. The refractive index can be used as a constant to establish the authenticity and purity of those medicines, which are liquids in nature. The refractometric method is widely used in pharmaceutical analysis to quantify the concentration of substances in a solution, which is found from a graph of the refractive index of a solution versus concentration. On the graph, select the concentration interval in which there is linear dependence between refractive index and concentration. This method can be used in the practice of intra-pharmacy control.
The dependence of the refractive index on the concentration of a substance in percent is expressed by the formula:
where n and n0 are the refractive indices of the solution and solvent; C is the concentration of the substance in the solution; F is the refractive index factor.
The refractive index of a solution is the sum of the refractive index of the solvent and the refractive indices of the solutes.
The values of refractive indices and factors for different concentrations of solutions of medicinal substances are given in the refractometric tables, which are available in the manual for intra-pharmacy control. The use of tables greatly simplifies calculations.
The dependence of the refractive index of aqueous solutions of certain substances on the concentration: In refractometry, two methods are used to calculate the concentration of a substance in a solution from the measured refractive index.
The value of the refractive index factor is taken from the refractometric tables.
By measuring the refractive index, the corresponding concentration value is found in the table. If the measured refractive index is not given in the table, interpolation is carried out. The refractometric method is used to quantify concentrated solutions. Concentrated solutions are working solutions of medicinal substances (PM) of a certain, higher concentration than these solutions are prescribed in pharmacies.
When preparing concentrated solutions, concentrations close to saturated should be avoided, because. when the temperature of the solution decreases, crystallization of the dissolved substance is possible.
Deviations allowed in concentrates:
1) The concentration of the solution was higher than required.
The volume of water required to dilute the resulting solution is calculated by the formula:
where X is the amount of water required to dilute the prepared solution (ml); A is the volume of the prepared solution (ml); B - required solution concentration (%); C is the actual concentration of the solution (%).
2) The concentration of the solution was lower than required.
The mass of the drug to strengthen the resulting solution is calculated by the formula:
where X is the mass of the substance to be added to the solution (g); A is the volume of the prepared solution (ml); B - required solution concentration (%); C is the actual concentration of the solution (%); ρ20 - density of the solution at 20°C (g/ml, g/cm3)
NAR-1T Liquid 
DR-A1 Refractometers of the NAR or DR-A1 series are designed to measure the refractive index and average dispersion of non-corrosive liquids. These are very high quality devices. Easy to maintain. Minimum content. Actually consumable for these refractometers - a light bulb (light source). Refractometers ATAGO series NAR or DR-A1 are used:
To determine the proportion of solids in various musts (GOST 5900-73), sugar-agar syrup, syrup for marmalade, marshmallows, creams and gingerbread.
To determine the mass fraction of soluble solids by sucrose (% Brix) in processed fruits and vegetables, to determine the percentage of fat in solid foods (gingerbread, wafers or bakery products) salt concentration.
4. When servicing equipment, ATAGO refractometers are used to determine with greater accuracy the volumetric concentration of the anti-crystallization liquid "IM", which is added to aviation fuel in an amount of 0.1 to 0.3%. Further processing of the results is carried out according to " methodological recommendations on the analysis of the quality of fuels and lubricants in civil aviation "Part II p.
Automatic refractometers Auto laboratory refractometers The microprocessor-controlled RX series are designed to test the concentration of a wide range of liquid media, both low and high viscosity, regardless of transparency and color. The RX series provide high measurement accuracy, accurate temperature control. The entire measurement process (heating/cooling) takes place automatically. Just press the Start button. The device automatically measures the refractive index of the solution sample, calculates its concentration and displays the result on a digital LCD screen. The RX-I series is equipped with a screen, which is made using the "Touch Screen" technology - a touch screen, all control of the device is carried out from the screen. Automatic refractometers of the RX series can both heat/cool the sample due to the built-in Peltier elements, and use the function of automatic temperature compensation during the measurement. The ideal device for pharmacists in this series is the RX-9000-i and RX-5000-i Plus automatic refractometer.
Ethyl alcohol (ethanol, Spiritus aethylicus) is one of the most widely used organic solvents in medical and pharmaceutical practice. Ethanol has bacteriostatic and bactericidal properties. It is widely used to obtain tinctures, extracts, dosage forms for external use. The quality of alcohol solutions depends on the concentration of alcohol in which the drug is dissolved. In each case, an optimal concentration is required at which the drug substance will not precipitate. Therefore, water-alcohol solutions are prepared with different concentrations of alcohol. The quantitative content of ethyl alcohol is easily determined refractometric method. There is a clear relationship between the concentration of ethanol and the refractive index. It is known that the refractive index depends on temperature, wavelength of light, the nature of the substance and solvent, and the concentration of the substance. So, the refractive index of aqueous solutions of alcohol from 1% to 70% has a linear characteristic, which means that you can easily measure the concentration with a refractometer. ATAGO manufactures special refractometers for measuring alcohol concentration. In general, these are ordinary refractometers, but a special correction factor is "sewn" into the processor, which allows you to immediately display the concentration of a water-alcohol solution on the display, bypassing the refractive index. At a concentration of 70% to 96% - the dependence is non-linear. Thus, the refractometric method can determine the strength of alcohol in the range from 1% to 70%.
Consider the use of refractometers in the manufacture and analysis of a 10% glycerol solution for injection: Glycerol solution 10% Glycerol (in terms of anhydrous) 100 g
1. Sodium chloride 9.0 g. Water up to 1 liter.
Manufacturing. Glycerin is purchased from suppliers ( top grade, dynamite) with a quantitative content of 86-90%, 94-98% or more. Therefore, in order to calculate the amount of initial glycerol, it is necessary to know exactly what is the mass fraction of anhydrous substance in it. A refractometer is used to accurately measure the concentration of glycerol. The refractive index of the original glycerol n=1.4569 corresponds to the mass fraction of anhydrous substance 89%. The initial amount of glycerin that is required to make the solution, according to prescription 68:
2. Weight of glycerin = 100 g / 0.89 = 112.36 g.
Quantitative determination of glycerol in solution. We calculate the concentration of glycerol: Spline = / Fglitz
where n is the refractive index of the solution; n0 is the refractive index of purified water, measured at the same temperature; СNaCl is the concentration of sodium chloride in the solution, determined by the method of argentometry; FNaCl is the refractive index factor of the sodium chloride solution for the found concentration; Fglycine is the refractive factor of a 10% solution of glycerin (0.001156).
IN pharmaceutical industry ATAGO refractometers can be used to study aqueous solutions of various drugs: calcium chloride (0% and 20%); novocaine (0.5%, 1%, 2%, 10%, 20%, 40%); ephedrine (5%); glucose (5%, 25%, 40%); magnesium sulfate (25%); sodium chloride (10%); cordiamine, etc.
If for one of the substances included in the solution, the refractive index factor is unknown or its insignificant concentration does not allow obtaining accurate data, then a control solution is prepared containing this substance in the concentration that was determined by the titrimetric method.
Refractometers and polarimeters of the Japanese company ATAGO have found application in all industries of the Russian Federation. ATAGO refractometers and polarimeters are included in the state register of measuring instruments of the Russian Federation. This allows the use of ATAGO devices in the most tightly controlled areas of production - such as, for example, pharmaceuticals.
It is also used during vision correction with glasses or contact lenses.
The device is a complete system for converting and recording infrared radiation. Based on such indicators of reflected radiation as intensity and wavelength, a special program of the device performs calculations. The results of the decoding will indicate the refraction (refractive power) of the optical media of the eye.
To understand why the study is used in ophthalmology, you should know the device and the basic principles of the refractometer.
They are based on these key points:
There are several medical indications for conducting research using a special automatic analyzer:
Also, a study using a refractometer is necessary when conservative methods vision correction. Without fail, an examination on the device is prescribed when worn and in order to monitor the course of treatment.
Refractometry is an outpatient procedure that is performed in a specially equipped room. The examination is carried out in a non-contact way, therefore it does not require medical staff preliminary preparation in compliance with the rules of asepsis and antisepsis.
To obtain a reliable result, as well as to eliminate errors, the pupil is first dilated using medicinal product atropine. It belongs to the pharmacological group of M-anticholinergics and leads to temporary paralysis of the eye muscles that constrict the pupil.
Atropine in the form of eye drops is prescribed 3 days before the proposed study. Eyes are instilled 2 times a day at approximately the same time intervals (morning and evening).
During the procedure, the patient sits on a chair in front of the device, and rests his head against a special limiter. The doctor asks to focus on the sensors and not move.
After the start of operation of the device, the sensors emit infrared radiation, which is reflected from the retina, passes back and is recorded. The duration of the study using an automatic refractometer does not exceed two minutes for each eye.
After conducting an examination using an automatic device, it issues a printout that displays the main indicators in the form of alphabetic values and numbers.
They have the following decoding:
The number of indicators depends on the specific model of the automatic ophthalmic refractometer that was used for the examination.
The examination, which is carried out using a refractometric device, helps to avoid medical errors and accurately establish the diagnosis. This saves time for the doctor and the patient. And due to non-contact refractometry also eliminates physical discomfort for the patient.
List of sources:
Refractometry, performed using refractometers, is one of the most common methods for identifying chemical compounds, quantitative and structural analysis, and determining the physicochemical parameters of substances.
In this work, an Abbe refractometer is used, the operation of which is based on measuring the limiting angle of refraction. The optical scheme of the refractometer is shown in fig. 4. The test solution is placed between the planes of two prisms - lighting 3 and measuring 4 made of glass with a high refractive index ( n = 1.9 ). The large refractive index of the measuring prism keeps the condition n p < n st for a wide range of densities of measured liquids. The scale of the instrument is calibrated to the value n p =1.7 .From source 1 a beam of light is guided by a condenser 2 on the input face of the illuminating prism. Passing the illuminating prism 3, light falls on a frosted hypotenuse face AB given prism , adjacent to a thin layer of the investigated liquid. The matte surface has irregularities, the dimensions of which are several wavelengths. The light is scattered by these irregularities over the entire surface and, having passed through a thin layer of the solution, falls on the “solution-glass” interface at various angles of incidence, i.e. the angle of incidence varies from 0 0 before 90 0 .
On the mirror hypotenuse face CD measuring prism 4 light is refracted (the size of the irregularities on this face is less than the wavelength). Owing to the fact that n p < n st , the angle of refraction varies from zero to γ etc . Angled γ > γ etc radiation is not observed. Thus, at an angle of refraction equal to γ etc , there is a border between light and shadow. Value n p is determined from the relation sinγ etc = n p / n st , where the value n st known.
The course of the light rays when it leaves the measuring prism is easily taken into account when calibrating the device, since the refraction of light occurs at the “glass-air” boundary , where the refractive indices of both media are known. The angle of light refraction at this boundary does not affect the measurement accuracy n p .
Due to the illumination of the entire layer of the solution, the boundary between light and shadow is observed quite sharply. Therefore, when setting up the device for operation, the light from the illuminator must be directed to the prism so that it evenly illuminates the entire surface of the face. AB scattering prism. To determine the angle at which the rays exit the measuring prism, a telescope formed by the lens is used. 6 and eyepiece 9, light enters through a system of direct vision prisms 5 . In this case, the property of the telescope is used that the rays going to it parallel to its axis are collected in the back focus, where a transparent plate is placed 7 with a grid crosshair applied to it. The crosshair exactly matches the focus.
Rice. 4. Path of rays in a refractometer when measuring the refractive index by the grazing beam method.
Optical layout of the device: 1-light source, 2-condenser, 3-illuminating prism, 4-measuring prism, 5-direct vision prism, 6-objective telescope lens, 7-grid with crosshair, 8-scale, 9-eyepiece of the telescope , 10-eyepiece field of view.
Direct vision prisms and spotting scope are rigidly connected and can be rotated relative to the measuring prism. The angle of rotation is measured on a fixed scale 8, located in the common focal plane of the objective and the eyepiece. The scale is graduated in terms of the refractive index of the test solution based on formula (6). By turning the telescope, you can set its axis parallel to the rays refracted on the edge CD at the limit angle γ etc. In this case, in the field of view of the eyepiece, light and dark areas will be observed, the border between which will coincide with the crosshair. The bright area is formed by rays refracted at the edge CD at angles less than the limit, and the dark area arises due to the absence of rays going at angles greater than the limit. The position of the border of light and shadow formed by rays refracted at a limiting angle will indicate on the scale 8 the desired value of the refractive index of the solution.
Light source 1 is not monochromatic. Therefore, due to the dispersion of both the substance under study and the material of the measuring prism (the dependence of their refractive indices on the wavelength of light), the border of light and shadow, observed through the telescope, is blurred and colored. Direct vision prisms are used to eliminate this effect. 5 , forming dispersion compensator. The prisms are designed so that rays with a wavelength λ D= 589.3 nm (the average value of the wavelength of sodium) did not deviate when passing through them. When one prism is rotated relative to the other, their total dispersion changes, which makes it possible to compensate for the difference in the exit angles of rays with different wavelengths from the measuring prism and direct them into the telescope parallel to the rays with a wavelength λ D. In this case, the border of light and shadow turns out to be sharp, uncolored and gives the value of the refractive index of the test solution n D at a wavelength λ D .
