An Abbe refractometer is a tool that can measure the refractive index (nD) and average dispersion (nF-nC) of liquids or solids that are clear or translucent (mainly for measuring transparent liquids). nD has a refractive index of between °C and 70°C. The two important optical constants for dispersed substances are the refractive index and the average dispersion.

These constants can be used to learn about the important optical properties, purity, concentration, and dispersion size of substances. The device can also measure how much sugar is in a solution of sucrose (0–95%, which is the same as a refractive index of 1.333–1.531). So, this instrument can be used for a lot of different things and is one of the most important pieces of equipment for study and research units in factories like the petroleum, oil, pharmaceutical, paint, food, daily chemical, sugar, and geological exploration industries.

Principle

The law of refraction is the main idea behind the refractometer. n1 and n1n2 are the refractive indices of the two media on either side of the interface.

If light comes from a medium that is optically dense and goes into a medium that is optically sparse, the angle of entry is smaller than the angle of refraction. By changing the angle at which the light comes in, the refraction can reach 90°. At this point, the incident angle is called the critical angle. The critical angle measurement method is used by these instruments to figure out how much refractive index there is in a substance.

When light comes into the AB plane from different directions, it bends more than I. If you use a telescope to look at the light going out, you can see that the telescope’s field of view is split into two parts: light and dark. There is a clear line between the two. The critical angle is where the line between light and dark is.

Instrument structure

Optical part

The optical part of the instrument consists of two parts of the telescope system.

  1. Incoming prism
  2. Refraction prism
  3. Swing mirror
  4. Achromatic prism group
  5. Telescopic objective lens group
  6. Parallel prism
  7. Reticle
  8. Eyepiece
  9. Reading goal
  10. Reflector
  11. Scale plate
  12. Condenser

There is a small uniform gap between the light-inlet prism and the refraction prism of the liquid to be measured is put in this gap in the Abbe refractometer. When natural light or incandescent light enters the light-inlet prism, a diffuse reflection happens on its frosted surface. The liquid layer being tested gets light from different angles, and the refracting prism creates a beam of light with a refraction angle greater than the critical angle.

The light beam is sent into the group of prisms that don’t change color. The achromatic prism group is made up of two Amish prisms with the same amount of dispersion. Its purpose is to get a variable dispersion to balance out the different measurements of the object’s dispersion. Then, look at this line of light and dark on the reticle through the telescope. The reticle has a cross on it, and you can look through the eyepiece to see it.

The light goes through the condenser lens and lights up the scale plate. The scale plate and the swinging reflector are part of the same body and move around the center of the scale. Through the reflecting mirror, the reading objective lens, and the parallel prism, the refractive index indication values of different parts of the scale plate are shown on the reticle.

Structural part

The instrument is held up by the base, which is where the case is attached. Except for prisms and eyepieces, all of the optical parts and main structures are inside the housing. The prism group is attached to the casing. It is made up of a prism that lets light in, a prism that bends the light, a prism seat, and other parts. The two prisms are stuck in the prism with a special glue.

Instrument Features

It uses visual aiming and dial reading, which makes it easy to use and operate. Find the n-average color of a liquid or solid (n-n) and the mass fraction of dry solids in an aqueous sugar solution (Brix). It can be used to find sugar and analyze it, as well as drugs, drinks, oil, food, the production of chemicals, scientific research, and teaching departments.

How to operate

Ready to work

Before the measurement can begin, the readings need to be checked against a standard sample. Add 12 drops of bromonaphthalene to the polished surface of the refracting prism, and then stick it to the polished surface of the standard sample. When the reading field of view shows the value on the standard sample, check to see if the line in the telescope that divides light from dark is in the middle of the cross line.

If there is any deviation, use a screwdriver to slightly turn the screw in the small hole while the objective lens is moving back and forth. This will move the image of the dividing line to the center of the cross line, and repeated observation and correction will reduce the error in the initial reading. At a bare minimum (this includes operator aiming error), Once the calibration is done, this part can’t be moved around at will during the next measurement process.

If there is any doubt about the measured refractive index indication value in daily work, it can be checked with the standard sample using the above method to see if there was an error in the beginning and to fix it.

Before each measurement and calibration of the indication value, anhydrous alcohol and ether must be mixed with the rough surface of the light-entering prism, the polished surface of the refracting prism, and the polished surface of the standard sample (1:4). Wipe it clean with absorbent cotton so that you don’t leave anything else on it that could affect how clear the image is and how well it measures.

Determining work 

(1) How to tell if a liquid is clear and see-through:

Using a clean dropper, put the liquid to be tested on the refraction prism’s surface. Then, cover the light entrance prism and lock it with the hand wheel. Open the shading plate, close the reflector, adjust the viewing angle of the mirror so that the image of the cross line is clear, turn the hand wheel to find the position of the line between light and dark in the eyepiece’s field of view, and then turn the hand wheel to make the line without any color. Fine-tune the hand wheel so that the dividing line is in the middle of the cross line, and then turn the condenser lens the right way. At this time, the liquid to be measured is the number that is shown below the eyepiece field of view.

(2) Determination of transparent solids by Abbe refractometer:

To use an Abbe refractometer to find out about transparent solids, the thing to be tested needs to have a smooth, flat surface. Open the light entrance prism and add 12 drops of brominated naphthalene to the polished surface of the refraction prism. Wipe the polished surface of the object to be tested and place it so that it touches the good. Now it can be seen through the eyepiece.

Find the line that separates the two parts of the field of view by aiming and reading as described above.

(3) Determining the type of solid:

The solid being tested must also have a flat, polished surface. During the measurement, put some brominated naphthalene on the polished side of the solid in the refractive prism, turn on the mirror, and change the angle so you can measure with the reflected beam. The exact method of operation is the same as what was said above.

(4) Determine the amount of sugar in the sucrose:

The process is the same as when you find out how a liquid bends light. At this point, the reading can be taken directly from the upper half of the display value in the field of view. This is the percentage of sugar concentration in the sucrose solution.

(5) An Abbe refractometer for determining the average dispersion value

The basic way to use it is the same as when measuring the refractive index, except that when you turn the dispersion adjustment hand wheel in two different directions, the line between light and dark in the field of view is colorless. Take the average value of the scale value Z and put it on it to figure out the refractive index nD. Find A and B in the same row of the Abbe refractometer dispersion table based on the nD value of the refractive index (if nD is in the middle of the two values in the table, it is obtained by interpolation).

Then, based on the Z value, find the value in the table that goes with it. When Z is greater than 30, take a negative value; when Z is less than 30, take a positive value. Plug the values you got for A, B, and dispersion into the dispersion formula to get the average dispersion value.

(6) Measure the refractive index at different temperatures:

Screw the thermometer into the thermometer base, connect the thermostat water pipe, set the thermostat temperature to the desired measurement temperature, connect the circulating water, and wait ten minutes. After the temperature has stabilized, you can take the measurement.

Instrument calibration

The instrument can also be calibrated from time to time or when the data from the measurements isn’t clear. Distilled water or glass are used as standards. If the measurement data doesn’t match the standard, put a clock screwdriver through the small hole in the dispersion correction hand wheel, carefully turn the screw inside, move the cross line on the reticle up and down, and then measure again until the measurement meets the requirements. When the sample is a block, it should be the same. The data that was used to calibrate the standard block should match up with the numbers.

In order to keep the instrument accurate and avoid damage, please pay attention to the maintenance of the user, and the following points are given as a guide:

(1) The instrument should be placed in a dry, air-ventilated room to avoid mold growth after the optical parts are damp.

(2) When testing corrosive liquids, cleaning should be done in time (including optical parts, metal parts, and paint surfaces) to prevent erosion and damage. After the instrument is used, it is necessary to do a good job of cleaning, and there should be a desiccant (discolored silica gel) in the wooden box to absorb moisture.

(3) Before and after the instrument is used and when changing samples, the working surface of the refractive prism system must be cleaned first.

(4) There should be no hard impurities in the sample to be tested. When testing solid samples, the surface of the refractive prism should be prevented from being roughed up or indented.

(5) Always keep the instrument clean, and never touch the optical parts with oily or sweaty hands. If there is dust on the surface of the optical parts, it can be wiped with high-grade deerskin or long-fiber absorbent cotton and then blown off with a leather blower. If the surface of the optical parts is stained with grease, it should be wiped clean with an alcohol-ether mixture in time.

(6) To avoid damage to optical parts and affecting accuracy, the instrument should avoid strong vibration or impact.

Principle and working

When light and matter interact, different optical phenomena can happen (such as light refraction, reflection, scattering, transmission, absorption, optical rotation, and stimulated emission of matter, etc.). Through the study and analysis of these optical phenomena, scientists can learn a lot about the structures of atoms, molecules, and crystals. So, optical measurement is an essential part of figuring out a substance’s composition, structure, and photochemical reaction. This section describes a few optical measuring tools that are often used in physics and chemistry experiments.

The Abbe refractometer

The index of refraction is one of the important constants of matter. Many pure things have a certain index of refraction. If they have impurities, the index of refraction will change, and there will be differences. The deviation will be bigger if there are more impurities. So, by measuring the refractive index, you can figure out how much of the substance is there.

  1. The construction principle of the Abbe refractometer

When a beam of monochromatic light enters medium II from medium I (the density of the two mediums is different), the light changes its direction when passing through the interface. This phenomenon is called light refraction.

  1. Dissipating handle;
  2. Constant temperature water inlet;
  3. Thermometer;
  4. Measuring prism;
  5. Hinge;
  6. Auxiliary prism;
  7. Filling tank;
  8. Reflector;
  9. Reading telescope;
  10. Shaft;
  11. Dial cover;
  12. Closing knob;
  13. Base

We don’t use monochromatic light to measure the refractive index. Instead, we use regular white light, which is made up of many different monochromatic lights. Different wavelengths of light have different refractive indices, so they bend in different ways. In the eyepiece, we can see bands of colored light where the line between light and dark is not clear.

In the Abbe refractometer, a set of achromatic prisms, which are also called compensation prisms, are used to do this. By changing how the achromatic prism is set up, the light that spreads out from the measuring prism goes away, and the line between light and dark becomes clear. The refractive index of the liquid at this time is the same as the refractive index measured by the monochromatic sodium light D line (5890!) nD, which is the same thing.

Instructions

(1) Put the Abbe refractometer in a bright place, but not in direct sunlight, so that the liquid sample doesn’t evaporate quickly when it gets hot. Use the super constant temperature bath to pour water at a constant temperature into the prism jacket, and use the thermometer on the prism to see if the reading matches the requirements. (Most of the time, choose either (20.0±0.1)°C or (25.0±0.1)°C)

(2) Add sample: Unscrew the measuring prism and auxiliary prism’s closed knobs so that the frosted slope of the auxiliary prism is horizontal. Do not wipe in both directions. After washing and drying the mirror surface, use a dropper to put a few drops of the sample on the hair mirror surface of the auxiliary prism. Close the auxiliary prism quickly and tighten the closing knob. If the liquid is volatile, move quickly or close the two prisms first. Then, use a dropper to add the sample through the filling hole (be careful not to break the dropper in the hole).

(3) Adjusting the light: Turn the lens barrel so that it is vertical, adjust the reflector so that light from the front enters the prism, and adjust the eyepiece’s focal length so that the cross line is clear and bright. Adjust the achromatic compensator so that the colored light band in the eyepiece goes away. Then, adjust the reading spiral so that the line where light and dark meet is right in the middle of where the crosshairs meet.

(4) Reading: Read the refractive index value from the dial on the reading telescope. The Abbe refractometer, which is used a lot, can be read to the fourth decimal place. For the reading to be accurate, the sample should be measured three times, with no more than 0.0002 difference between each time, and then the average value should be used.

Precautions for use

The Abbe refractometer is a precise optical instrument. The following points should be paid attention to when using it:

(1) When using the prism, take care to protect it and clean it with lens paper rather than filter paper. When adding the sample, the mouth of the dropper should not touch the mirror surface. The Abbe refractometer should not be used for corrosive liquids such as acid and alkali.

(2) During each measurement, the sample should not be added too much. Generally, only 2 to 3 drops are needed.

(3) Take care to keep the instrument clean and the dial protected. each experiment, add a few drops of acetone to the mirror and wipe it dry with lens tissue. Finally, use two layers of lens cleaning paper between the two prism mirror surfaces to avoid damage to the mirror surface.

(4) When reading, sometimes the clear dividing line between light and dark is not observed in the eyepiece, but it is deformed because the prisms are not filled with liquid; if there is an arc-shaped halo, it may be because the light does not pass through the prism and is directly irradiated onto the condenser lens.

(5) If the refractive index of the sample to be tested is not within the range of 1.3 to 1.7, the Abbe refractometer cannot be measured, and the dividing line between light and dark cannot be seen.

Maintenance and calibration

The zero point on the Abbe refractometer‘s dial sometimes moves, so it needs to be fixed. Most of the time, pure water is used as the standard liquid because it has a known refractive index. Use the tool to measure the refractive index of pure water and read the value. If the standard refractive index of pure water under these conditions doesn’t match, change the value on the dial until it does. It can also be set up using the refractory glass that comes with the instrument from the factory. In general, the method is explained in detail in the manual for the instrument.

After you use the Abbe refractometer, you need to take care of it. The tool needs to be cleaned. If there is dust on the optical parts, it can be wiped off with high-quality deerskin or absorbent cotton and then blown off with an ear-washing ball. If there is an oil stain, absorbent cotton soaked in a little gasoline and then wiped with ether can get rid of it. After using the instrument, keep it from getting wet by putting it in a box with desiccant and putting it in a dry, well-ventilated room. Be careful not to shake or hit the instrument too much when you move it. This could hurt the instrument’s optical parts and make it less precise.

FAQ:

What do the upper and lower readings of an Abbe refractometer represent?

The upper and lower readings on an Abbe refractometer show the percentage content and refractive index.

Change the interface so that a line goes through the center of where the diagonal crosses meet. Again, read it to the fourth decimal place. A tool for measuring the refractive index (ND) and average dispersion (NF-NC) of liquids or solids that are clear or translucent. The instrument is connected to a thermostat that can measure the refractive index ND between 0 °C and 70 °C and the concentration of sugar in the sugar solution.