REFRACTOMETRY: RATIONALE, TYPES OF REFRACTOMETERS, APPLICATIONS - CHEMISTRY - 2025

The refractometer is a method of optical analysis of substances which measures the refractive index of a substance to determine its main features. 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. The refractive index n is precisely defined as the c / v ratio.

Figure 1. Refractometer used to measure the sugar content in fruits. Source: Wikimedia Commons.

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 index of refraction.

How to calculate the index of refraction

Snell's law relates the index of refraction between two media 1 and 2 as:

Here n ₁ is the index of refraction in medium 1, θ ₁ is the angle of incidence of the ray on the boundary surface, n ₂ is the index of refraction in medium 2 and θ ₂ is the angle of refraction, in which direction the transmitted beam continues.

Figure 2. Ray of light striking two different media. Source: Wikimedia Commons.

The refractive index of materials is constant and is known under certain physical conditions. With this the refractive index of another medium can be calculated.

For example, if light passes through a glass prism whose index is n ₁ and then through the substance whose index we want to know, carefully measuring the angle of incidence and the angle of refraction, we obtain:

Types of refractometers

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 like the Abbe refractometer.

-Digital refractometers.

- Optical-manual type like Abbe's refractometer

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 source of light, generally sodium vapor, whose wavelength is known. There are models that use normal white light, which contains all visible wavelengths, but they have built-in prisms called Amici prisms, which eliminate unwanted wavelengths.

-An illumination prism and another 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). Next we will see the principles of operation.

Figure 3. On the left an Abbe refractometer and on the right a basic operating diagram. Source: Wikimedia Commons. 丰泽 一号

How Abbe Refractometer Works

The procedure is as follows: the sample is placed between the refraction prism –which is fixed-, and the illumination prism –fattable-.

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 greatest 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 you want is to make the two fields have the same size.

For this there is an aid mark on the eyepiece, which varies depending on 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.

The critical angle

In the following figure we see that the critical angle θ _c is the one at which the ray travels just over the boundary surface.

If the angle is increased further 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 θ ₂ = sin 90º = 1, which leads directly to the refractive index in the medium 2:

Figure 4. Critical angle. Source: F. Zapata.

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:

Figure 5. The scale of a refractometer is calibrated to give the refractive index directly. Source: Refractometry. Oregon State University.

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, the refractive index is 1.46068.

Importance of wavelength

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.

Advantages and disadvantages

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 mentioned 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:

Manual Abbe Refractometer

-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 registered by hand.

-They usually come with scales calibrated specifically for a certain range of substances.

-They require to be calibrated.

-The water bath temperature control system can be cumbersome to use.

Digital refractometers

-They are easy to read, since the measurement appears directly on a screen.

-They use 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.

-It is possible to program different scales.

-Does 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.

-It is not necessary to recalibrate them, but check from time to time that they are working properly by measuring the refractive index of well-known substances, such as distilled water for example.

-They are more expensive than manual refractometers.

Applications

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 the 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.

Figure 6. Food technology laboratory. Source: Piqsels.

-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 the quality control of syrups, perfumes, detergents and all kinds of emulsions.

-They can measure the concentration of urea - a waste from protein metabolism - in the blood.

References

1. Chemistry Lab Techniques. Refractometry. Recovered from: 2.ups.edu.
2. Gavira, J. Refractometry. Recovered from: triplenlace.com
3. Mettler-Toledo. Comparison of different techniques for measuring density and refractometry. Recovered from: mt.com.
4. Net InterLab. What is a refractometer and what is it for? Recovered from: net-interlab.es.
5. Oregon State University. Principles of refractometry. Recovered from: sites.science.oregonstate.edu.