The refractometry is a method of optical analysis of substances that measures the refractive index of a substance to determine its main characteristics. It is based on the fact that light, when passing from one medium to another, undergoes a change of direction that depends on the nature of these media..
The speed of light in vacuum is c = 300,000 km / s, but in water, for example, it decreases to v = 225,000 km / s. Refractive index n is defined precisely as the quotient c / v.
Suppose that light of a certain wavelength falls at a predetermined angle on the surface that limits two different materials. Then the direction of the ray will change, because each medium has a different refractive index.
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Snell's law relates the index of refraction between two media 1 and 2 as:
n1 sen θ1 = ntwo sen θtwo
Here n1 is the index of refraction in the medium 1, θ1 is the angle of incidence of the ray on the boundary surface, ntwo is the index of refraction in the medium 2 andtwo is the angle of refraction, in which direction the transmitted ray continues.
The refractive index of materials is constant and is known under certain physical conditions. With this you can calculate the refractive index of another medium.
For example, if light passes through a glass prism whose index is n1 and then by the substance whose index we want to know, carefully measuring the angle of incidence and the angle of refraction, we obtain:
ntwo = (sin θ1 / sen θtwo). n1
The refractometer is an instrument that measures the refractive index of a liquid or a solid with flat and smooth faces. There are two types of refractometers:
-Optical-manual type such as Abbe's refractometer.
-Digital refractometers.
The Abbe refractometer was invented in the 19th century by Ernst Abbe (1840-1905), a German physicist who contributed significantly to the development of Optics and Thermodynamics. This type of refractometer is widely used in the food industry and teaching laboratories and basically consists of:
-A lamp as a light source, generally sodium vapor, whose wavelength is known. There are models that use normal white light, which contains all the visible wavelengths, but they have built-in prisms called Amici prisms, that eliminate unwanted wavelengths.
-A lighting prism and other refractive prism, between which the sample whose index is to be measured is placed.
-Thermometer, since the refractive index depends on the temperature.
-Adjustment mechanisms for the image.
-The eyepiece, through which the observer carries out the measurement.
The arrangement of these basic parts may vary depending on the design (see figure 3 left). Then we will see the principles of operation.
The procedure is as follows: the sample is placed between the refraction prism - which is fixed - and the illumination prism - tiltable-.
The refractive prism is highly polished and its refractive index is high, while the illumination prism is matte and rough on the contact surface. In this way, when the lamp is turned on, light is emitted in all directions on the sample..
Ray AB in figure 3 is the one with the largest possible deviation, so to the right of point C an observer will see a shaded field, while the sector to the left will be illuminated. The adjustment mechanism comes into action now, since what is sought is to make the two fields have the same size.
For this there is an aid mark on the eyepiece, which varies according to the design, but it can be a cross or another type of signal, which serves to center the fields.
By making the two fields the same size, the critical angle or limit angle can be measured, which is the angle at which the transmitted ray would pass grazing the surface that separates the media (see figure 4).
Knowing this angle allows to directly calculate the refractive index of the sample, taking that of the prism. Let's look at this in more detail below..
In the following figure we see that the critical angle θc is the one in which the ray travels just over the boundary surface.
If the angle is further increased then the beam does not reach the middle 2, but is reflected and continues in the middle 1. Snell's law applied to this case would be: sin θtwo = sin 90º = 1, which leads directly to the refractive index in medium 2:
ntwo = n1 sen θc
Well, the critical angle is obtained precisely by equating the size of the fields of light and shadow that are seen through the eyepiece, through which a graduated scale is also observed..
The scale is usually calibrated for direct reading of the refractive index, so depending on the refractometer model, the operator will see something similar to what is observed in the following image:
The upper scale, with the help of the vertical line, indicates the main measurement: 1.460, while the lower scale shows 0.00068. When adding, we have the refractive index 1.46068.
The light that falls on the illumination prism will change its direction. But since it is an electromagnetic wave, the change will depend on λ, the length of the incident wave.
Since white light contains all wavelengths, each one is refracted to a different degree. To avoid this mixing that results in a fuzzy image, the light used in a high resolution refractometer must have a unique and known wavelength. The most used is the so-called sodium D line, whose wavelength is 589.6 nm.
In cases where too much precision is not required, natural light is sufficient, even if it contains a mixture of wavelengths. However, to avoid blurring the edge between light and dark in the image, some models add Amici's compensating prisms..
Refractometry is a fast, inexpensive and reliable technique to know the purity of a substance, which is why it is widely used in chemistry, bioanalysis and food technology..
But since there are different substances with the same refractive index, it is necessary to know which one is being analyzed. For example, cyclohexane and some sugary solutions are known to have the same refractive index at a temperature of 20 ° C..
On the other hand, the refractive index is highly dependent on temperature, as stated above, in addition to the pressure and concentration of the refractive solution. All these parameters must be carefully monitored when high precision measurements are required..
As for the type of refractometer to use, it depends a lot on the application for which it is intended. Here are some characteristics of the main types:
-It is a reliable and low-maintenance instrument.
-They are usually cheap.
-Very appropriate to become familiar with the fundamental principles of refractometry.
-Care must be taken not to scratch the surface of the prism in contact with the sample..
-Must be cleaned after each use, but cannot be done with paper or rough materials.
-The refractometer operator must have training.
-Each measurement must be recorded by hand.
-They usually come with scales calibrated specifically for a certain range of substances..
-They require to be calibrated.
-Water bath temperature control system can be cumbersome to use.
-They are easy to read, since the measurement appears directly on a screen.
-Employ optical sensors for high precision readings.
-They have the ability to store and export the data obtained and to be able to consult them at any time.
-They are extremely accurate, even for substances whose refractive index is difficult to measure.
-Different scales can be programmed.
-They do not require temperature adjustment with water.
-Some models incorporate density measurements, for example, or can be connected to density meters, pH meters and others, to save time and obtain simultaneous measurements.
-They do not need to be re-calibrated, but check their proper operation from time to time by measuring the refractive index of well-known substances, such as distilled water.
-They are more expensive than manual refractometers.
Knowing the refractive index of a sample indicates its degree of purity, which is why the technique is widely used in the food industry:
-In the quality control of oils, to determine their purity. For example, through refractometry it is possible to know if a sunflower oil was lowered by adding other lower quality oils.
-It is used in the food industry to know the sugar content in sugary drinks, jams, milk and its derivatives and various sauces.
-They are also necessary in the quality control of wines and beers, to determine the sugar content and alcohol content..
-In the chemical and pharmaceutical industry for quality control of syrups, perfumes, detergents and all kinds of emulsions.
-They can measure the concentration of urea - a waste product of protein metabolism - in the blood.
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