Brix
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For other uses, see Brix (disambiguation).
Degrees Brix (symbol °Bx) is unit representative of the sucrose content of an aqueous solution by weight. One degree Brix corresponds to 1 gram of sucrose in 100 grams of solution and thus represents the strength of the solution as a percentage by weight (% w/w). It has traditionally been used in the wine, sugar, fruit juice, honey and other industries to express the sugar content of an aqueous solution. It is intended to represent exactly the same thing as the degree Plato (°P), widely used by the brewing industry, and the degree Balling which, while it is the oldest of the three, is still in use in some parts of the world and found in textbooks which are considered current today.[1]. While all three are intended to represent the same thing (the number of grams of sucrose in 100 grams of solution) in fact they do not though the differences are small. For example a particular sucrose solution known to have an apparent specific gravity (20°/20°C) of 1.040 would have its Brix value reported as 9.99325 °Bx and its Plato value as 9.99359 °P while the sugar industry, whose representative body, the International Commission for Uniform Methods of Sugar Analysis (ICUMSA), has obsoleted °Bx [2] in favor of "mass fraction", would report the strength of this solution as 9.99249 %. The differences between these three systems are clearly of little practical significance as their magnitudes are less than the precision of even relatively sophisticated instruments. Because of this and because of the wide historical use of the Brix unit modern instruments may calculate mass fraction using ICUMSA official formulas but report the result as °Bx.
Contents[hide] · 1 Background · 2 Measurement · 3 The Tables: Relationship between °Bx , Specific Gravity and Refractive Index · 5 Brix and Actual Dissolved Solids Content · 7 References |
[edit] Background
First Karl Balling, then Adolf Brix and finally the Normal Eichungskommission under Fritz Plato prepared pure sucrose solutions of known strength, measured their specific gravities and prepared tables of percent sucrose by weight vs. measured specific gravity. Balling measured specific gravity to 3 decimal places, Brix to 5 and the Normal Eichungskommission to 6 with the goal of the Kommission being to correct errors in the 5th and 6th decimal place in the Brix table.
Equipped with one of these tables, a brewer wishing to know how much sugar was in his wort could measure its specific gravity and enter that specific gravity into the Plato table to obtain °Plato which is the same as % sucrose w/w. A vintner could measure the specific gravity of his must and enter the Brix table to find the must °Bx value i.e. its % sucrose w/w. It is important to point out that neither wort nor must is a solution of pure sucrose in pure water. Many other compounds are dissolved as well but these are either sugars, which behave very similarly to sucrose with respect to specific gravity as a function of concentration, or compounds which are present in small amounts (minerals, hop acids in wort, tannins, acids in must). In any case even if °Bx are not representative of the exact amount of sugar in a must or fruit juice they can be used for comparison of relative sugar content.
[edit] Measurement
As specific gravity was the basis for the Balling, Brix and Plato tables dissolved sugar content was originally estimated by measurement of specific gravity using a hydrometer or pycnometer. In modern times hydrometers are still widely used but where greater accuracy is required an electronic Oscillating U-tube meter will be employed. Whichever means are used, the analyst enters the tables with specific gravity and takes out (using interpolation if necessary) the sugar content in percent by weight. If he uses the Plato tables (maintained by the American Society of Brewing Chemists[3] he reports in °P. If using the Brix table (the current version of which is found maintained by NIST and can be found on their website)[4] he reports in °Bx. If using the ICUMSA tables, [5] he would report in mass fraction (m.f.). It is not, typically, actually necessary to consult tables as the tabulated °Bx or °P value can be printed directly on the hydrometer scale next to the tabulated value of specific gravity or stored in the memory of the electronic U-tube meter or calculated from polynomial fits to the tabulated data. Both ICUMSA and ASBC have published suitable polynomials, in fact the ICUMSA tables are calculated from the polynomials. The opposite it true with the ASBC polunomial. Also note that the tables in use today are not those published by Brix or Plato. Those workers measured true specific gravity reference to water at 4°C using, respectively, 17.5 and 20°C, as the temperature at which density of the sucrose solutions was measured. Both NBS and ASBC converted to apparent specific gravity at 20°C/20°C. The ICUMSA tables are based on more recent measurements on sucrose, fructose, glucose and invert sugar and tabulate true density and weight in air at 20 °C against mass fraction.
Dissolution of sucrose and other sugars in water changes not only its specific gravity but its optical properties in particular its Refractive Index and the extent to which it rotates the plane of linearly polarized light (see Polarization_(waves)). The refractive index, nD, for sucrose solutions of various strengths by weight has been measured and tables of nD vs. °Bx published. As with the hydrometer, it is possible to use these tables to calibrate a refractometer so that it reads directly in °Bx. The hand held instrument illustrated in the accompanying photograph is typical of an electronic instrument calibrated in this way. Calibration is usually based on the ICUMSA tables [6] but the user of an electronic refractometer should verify this.
Sugars also have known infrared absorption spectra and this has made it possible to develop instruments for measuring sugar concentration using NIR (Near Infra Red) and FT-IR (Fourier Transform Infrared Spectrometry) techniques. In the former case, in line instruments are available which allow constant monitoring of sugar content in sugar refineries, beverage plants, wineries etc. As with any of the other instruments NIR and FT-IR instruments can be calibrated against pure sucrose solutions and thus report in °Bx but there are other possibilities with these technologies as they have the potential to distinguish between sugars and interfering substances.
[edit] The Tables: Relationship between °Bx , Specific Gravity and Refractive Index
Approximate values of °Bx can be computed from 261.3 × (1 − 1/S), where S is the apparent specific gravity of the solution 20°C/20°C. More accurate values are available from °Bx = (((182.4601*S -775.6821)*S +-775.6821)*S -669.5622), derived from the NBS table with S as above. This should not be used above S = 1.17874 (40 °Bx). RMS disagreement between the polynomial and the NBS table is 0.0009 °Bx. The Plato scale can be approximated by the Lincoln Equation °P = (463-205*S)*(S-1) or values obtained with high accuracy with respect to the ASBC table from the ASBC polynomial °P = (((135.997*S - 630.272)*S + 1111.14)*S - 616.868).
The difference between the °Bx and °P as calculated from the respective polynomials is: °P - °Bx = (((-2.81615*S +8.79724)*S - 9.1626)*S +3.18213). The difference is generally less than ±0.0005 °Bx or °P with the exception being for weak solutions. As 0 °Bx is approached °P tend towards as much as 0.002 °P higher than the °Bx calculated for the same specific gravity. Disagreements of this order of magnitude can be expected as the NBS and the ASBC used slightly different values for the density of air and pure water in their calculations converting to apparent specific gravity. It should be clear from these comments that Plato and Brix are, for all but the most exacting applications, the same. Note: all polynomials in this article are in a format that can be pasted directly into a spreadsheet.
When a refractometer is used the Brix value can be obtained from the polynomial fit to the ICUMSA table: Bx = (((((11758.74*nD -88885.21)*nD + 270177.93)*nD - 413145.80)*nD + 318417.95)*nD -99127.4536) where nD is the refractive index, measured at the wavelength of the sodium D line (589.3 nm) at 20 °C. Temperature is very important as refractive index changes dramatically with temperature. Many refractometers have built in "Automatic Temperature Compensation" (ATC) which is based on knowledge of the way the refractive index of sucrose changes. For example, the refractive index of a sucrose solution of strength less than 10 °Bx is such that a 1 °C change in temperature would cause the Brix reading to shift by about 0.06 °Bx. Beer, conversely, exhibits a change with temperature about three times this much. It is important, therefore, that users of refractometers either make sure the sample and prism of the instrument are both at very close to 20 °C or, if that is difficult to insure, readings should be taken at 2 temperatures separated by a few degrees, the change per degree noted and the final recorded value referenced to 20°C using the Bx vs. Temp slope information.
[edit] Usage
The three scales are often used interchangeably since the differences are minor.
· Brix is primarily used in fruit juice, wine making, starch and the sugar industry.
· Plato is primarily used in brewing.
· Balling appears on older saccharimeters and is still used in the South African wine industry and in some breweries.
See also: Ripeness in viticulture
A Brix-measuring instrument for use in the vineyard
Brix is used in the food industry for measuring the approximate amount of sugars in fruits, vegetables, juices, wine, soft drinks and in the starch and sugar manufacturing industry. Different countries use the scales in different industries; in the UK brewing is measured with specific gravity X 1000, European brewers use Plato degrees, and US industries use a mix of specific gravity, Brix, degrees Baumé and Plato degrees. For fruit juices, one degree Brix is about 1-2% sugar by weight. This usually correlates well with perceived sweetness.
Modern optical Brix meters are divided into 2 categories. In the first are the Abbe based instruments in which a drop of the sample solution is placed on a prism and image of which is observed through an eyepiece. The critical angle (the angle beyond which light is totally reflected back into the sample) is a function of the refractive index and the operator detects this critical angle by noting where a dark-bright boundary falls on an engraved scale. The scale can be calibrated in Brix or refractive index. Often the prism mount contains a thermometer which can be used to correct to 20°C in situations where measurement cannot be made at exactly that temperature. These instruments are available in bench and hand held versions.
Digital refractometers also find the critical angle but the light path is entirely internal to the prism. A drop of sample is placed on its surface (at the center of the circular well in the accompanying photograph) and so the critical light beam never penetrates the sample. This makes it easier to read turbid samples. The light/dark boundary, whose position is proportional to the critical angle, is sensed by a CCD array. These meters are also available in bench top (laboratory) and portable (pocket) version. The photograph shows an example of the latter. These are the easiest to use of all the methods for estimating Brix and can be used on location with minimal training. A drop of distilled water is placed on the prism and the calibrate button pressed. The distilled water is now replaced by a drop of juice from the fruit being measured. The read button is presses and the display indicates °Bx directly. This ability to easily measure Brix in the field makes it possible to determine ideal harvesting times of fruit and vegetables so that products arrive at the consumers in a perfect state or are ideal for subsequent processing steps such as vinification.
[edit] Brix and Actual Dissolved Solids Content
When a sugar solution is measured by refractometer or densitometer the °Bx or °P value obtained by entry into the appropriate table only represents the amount of dry solids dissolved in the sample if the dry solids are exclusively sucrose. This is seldom the case. Grape juice (must), for example, contains little sucrose but does contain glucose, fructose, acids and other substances. In such cases the °Bx value clearly cannot be equated with the sucrose content but it may represent a good approximation to the total sugar content. For example, an 11.0 %w/w D-Glucose ("grape sugar") solution measured 10.9 °Bx using a hand held instrument similar to the one in the photograph. For these reasons the reported value is often stated as the "refractometric dried solids" (RDS) which could be thought of as an equivalent sucrose content. Where it is desirable to know the actual dry solids content empirical correction formulas can be developed based on calibrations with solutions similar to those to be tested. For example in sugar refining dissolved solids can be accurately estimated from refractive index measurement corrected by an optical rotation (polarization) measurement.
Alcohol has a higher refractive index at 1.361 than water at 1.333. As a consequence a refractometer measurement made on a sugar solution once fermentation has begun will result in a reading substantially higher than the actual solids content. Thus an operator must be certain that the sample he is testing has not begun to ferment if the results are to be relied upon. Specific gravity based Brix or Plato measurements are also effected by fermentation but in the opposite direction. As ethanol is lighter than water an ethanol/sugar/water solution gives a Brix or Plato reading which is low compared to the actual dissolved sugar content.
How To Use A Refractometer in Winemaking
http://www.grapestompers.com/refractometer_use.asp
It's important for winemakers to know how to use a refractometer, because it is used to measure the amount of sugar (actually, the percentage Brix) in the juice of grapes or other fresh fruit. Winemakers know there is a direct correlation between the amount of sugar present and the ability to make wine. This portable instrument (it'll fit in your pocket) allows the winemaker to assess the ripeness of fruit by measuring Brix in the field or vineyard so he or she can decide the proper harvest time depending upon the readings taken.
This page explains what a refractometer does, how it works, how to calibrate and use it, tips for buying the best model, and how to take care of it.
What Is A Refractometer?
A refractometer is a relatively inexpensive yet essential piece of test equipment used by vineyard managers and winemakers. The rugged exterior of metal, rubber and plastic protects the highly polished optical glass, mirrors and prisms that are contained within. Once the sample is in place underneath the daylight plate, the winemaker can see the percentage Brix reading by looking through the monocular / eyepiece and reading the scale that is seen when he or she holds the refractometer in natural light.
What Does A Refractometer Do, and How Does It Work?
As previously stated, a refractometer allows the winemaker to figure the percentage Brix (the relative "sugar weight" of a sample compared to distilled water) of the juice of grapes or other fresh fruit. Brix is sometimes referred to as Balling - don't worry, the terms are interchangeable. Depending upon the readings observed, a winemaker can monitor the progress of ripening and adjust his/her plans for harvest, if necessary.
In simplest terms, the refractometer works much like a prism. Remember how, as a child, you could use a prism to separate out the different wavelengths of light (red, orange, yellow, green, blue, indigo, violet) when a source of light was shone on the prism at the correct angle? Well, the modern refractometer works on the same principle - it reacts differently to light (by giving a reading on a scale) depending upon the amount of sugar that is available in the liquid sample held between the daylight plate and the main prism assembly.
How to Calibrate and Use Your Refractometer
Before you start taking readings, it's very important to calibrate the refractometer. Some refractometers require the use of a special calibration liquid to perform this task, while others (like the ones sold at grapestompers.com) are calibrated with distilled water.
Let's get to it!
| Begin the calibration of your refractometer by lifting up the daylight plate and placing 2-3 drops of distilled water on top of the prism assembly. Close the daylight plate so the water spreads across the entire surface of the prism without any air bubbles or dry spots. | |
Allow the test sample to sit on the prism for approximately 30 seconds before you attempt calibration in the next step. This allows the sample to adjust to the ambient temperature of the refractometer. | ||
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Hold the refractometer in the direction of a natural light source and look into the eyepiece. You will see a circular field with graduations down the center. You may have to focus the eyepiece to clearly see the graduations. Figure 1 (below) shows what you would see if you looked through the refractometer without any sample present. |
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| Turn the calibration screw (see photo at left) until the boundary between the upper blue field and the lower white field meet exactly at ZERO on the scale. See example (Figure 2, shown below) of the interior view you'll see when you look through the eyepiece of the refractometer. | |
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Once the refractometer has been properly calibrated, you are ready to take readings of grape juice or whatever else you want to sample. Put away the calibration screwdriver. Clean the instrument (both the daylight plate and the top of the main prism assembly) using a soft, damp cloth, then place 2-3 drops of the desired sample on top of the prism. Close the daylight plate and take your reading as before. Figure 3 (see below) illustrates what you might see at this point. | ||
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| FIGURE 1 The image to the left illustrates what the winemaker would see if he looked through the refractometer without any sample at all. Notice how the entire scale is colored blue; no white at all. When looking through the monocular, be sure you are using natural light to view the readings; you should not read a refractometer in the presence of fluorescent light. | |
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| FIGURE 2 This is what the winemaker sees once he has properly calibrated the refractometer. Notice that the reading is taken where the blue and the white meet. Calibrate to ZERO using distilled water as the sample. If your refractometer does not automatically compensate for the temperature of the sample, you must take this into account or your readings will be off. | |
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| FIGURE 3 Finally, we get to sample some real grapes! Don't fall into the trap of sampling only one or two grapes - select a group of grapes at random from across your vineyard and combine their juice to get a good cross section sample of your crop. As you can see, this sample is reading 23% Brix. Looks like it's time to make wine! Be sure to cleanse and dry the refractometer before putting it away in storage. |
Warnings and Maintenance of Your Refractometer
Accurate measurement depends on careful calibration. Follow the instructions above closely. A reminder: Differences between the ambient room temperature of the prism and the temperature of the sample will throw off the accuracy of your reading. Remember to allow the sample to rest on the prism assembly for 30 seconds before taking a reading.
Do not expose the refractometer to damp working conditions. Do not immerse the instrument in water. If the instrument becomes foggy, water has entered the body. Call a qualified service technician or contact your dealer to purchase a new refractometer.
Do not measure abrasive or corrosive chemicals with this instrument, because they can damage the prism's coating.
Clean the instrument between each measurement using a soft, damp cloth. Failure to clean the prism on a regular basis will lead to inaccurate results and damage to the prism's coating.
The refractometer is an optical instrument. It requires careful handling and storage. Failure to do so can result in damage to the optical components and its basic structure. With care, this instrument will provide years of reliable service.
Buying Tips
When you purchase a refractometer, you'll need to know:
· The range of readings (highest to lowest), to make sure it will suit your purpose. A standard range for home brewers is 0 to 32% Brix. For example, in order to achieve a 13% wine, you'll want to start your wine at a Brix of 23.
· The ease with which the refractometer can be read and understood. Some less expensive refractometers are difficult to read, either due to a lack of a focus adjustment, inferior optics, or the eyepiece lacks a rubber seal and will not fit snugly over your eye.
· The calibration temperature of the refractometer. The most common calibration temp is 20° C or 68° F. If your sample is not exactly 68° F, you will need to make mathematical corrections to compensate for the temperature difference. Luckily, many modern models of refractometers (like the ones stocked by grapestompers) are sold with ATC (automatic temperature compensation), so you never have to worry about the temperature of your sample.
· How easy it is to calibrate. Must you purchase a calibration liquid, or can you calibrate with distilled water? Does it calibrate with a set screw or a dial or knob?
· How easy it is to clean.
· If it comes with a protective case (they're pretty fragile) and instruction manual.
Conclusion
There are many reasons why a winemaker might want to use a refractometer:
· To measure the percentage Brix of grapes or other fresh fruit
· To determine progress of crop ripening
· To measure progress of fermentation
· To measure the amount of sugar present in grapes or other fruit
· To allow the winemaker to determine when fruit is at its peak of ripeness and should be harvested
Here's some other refractometer pages we recommend:
· Refraction of Light - the first part of this page explains the principles of refractometry.
· How a refractometer works - the bottom half of this page has a great explanation of how the refractometer works, as well as a diagram of its internal construction.
PRINCIPLES OF REFRACTOMETERY
Refractometers are instruments used to measure substances dissolved in water and certain oils. The refractometer works using the principle of light refraction through liquids. As light passes from air into a liquid it slows down. This phenomenon is what gives a "bent" look to objects that are partially submerged in water. To put it simply, the more dissolved solids water contains, the slower light travels through it, and the more pronounced the "bending" effect on light. Refractometers use this principle to determine the amount of dissolved solids in liquids by passing light through a sample and showing the refracted angle on a scale. The scale most commonly used is referred to as the Brix scale. The Brix scale is defined as: the number of grams of pure cane sugar dissolved in 100 grams of pure water (grams sugar/100 grams H20). Other scales have been developed to measure salt, serum proteins (albumen) and urine specific gravity.
Refractometry
http://www.chemistry.oregonstate.edu/courses/ch361-464/ch362/refract02.htm
The refractive index is a physical property that is characteristic of a pure compound. Like a melting point, it can be used to confirm the identity of a compound, or to assess its purity, by comparison with a known (literature) value.
Principles of Refractometry
You should be familiar with the general principle of refraction: any time a light ray moves from one material to another at an angle, it changes direction. This is the principle by which a rainbow is formed. The ratio of the angle of refraction in a vacuum versus that in a material of interest is the material's index of refraction.
For practical purposes, air is normally used as a reference instead of a vacuum, since it's index of refraction is very close to (but not precisely) 1.0.
The Abb� refractometer passes light through a thin film of your liquid sample. This illuminates a reference mark, and movement of an adjustment knob to adjust the visual alignment of this mark with a reference is correlated with a calibration scale to make the measurement.
We have two types of refractometer. The older, Bausch and Lomb instrument is described first; the newer Mark II is described second.
Bausch and Lomb Refractometer
Use distilled water as a test sample to learn how to use the refractometer; its index of refraction is 1.3329 at 25�C.
Place several drops on the sample prism. (If you do not use enough sample, it will be difficult to see the mark; if you use too much it will splash out of the sample area and potentially contaminate the area.) Close the sample prism firmly, and swing the lamp up against it. Press the switch button at the back left of the instrument to illuminate the scale in the eyepiece; turn the large knob on the right of the instrument until 1.333 appears in the crosshairs:
If the scale is out of focus, readjust the eyepiece.
Releasing the switch causes the view to change to the alignment mark. Center the mark in the crosshairs:
Re-illuminate the scale, and read to 5 significant figures using the top scale. The image above reads "1.4606."
To run multiple measurements, move the adjustment knob off your reading, recenter the mark, and reread the scale. Record each measurement in your notebook, along with the temperature.
Clean the instrument for the next person.
Mark II Refractometer
Use distilled water as a test sample to learn how to use the refractometer; its index of refraction is 1.3329 at 25�C.
The optics of this model are similar to the Bausch and Lomb instrument; application of the sample is identical:
Apply the sample: use enogh to cover the prism, but not too much. Adjust the focus until you can see the horizontal mark (Hint: turn counterclockwise if the field is dark, clockwise if it's light.) Now, turn the control knob to center the horizontal mark in the crosshairs:
A photo showing what you will see. |
Press the READ button on the front of the instrument; the refractive index will appear in the LED display.
To run multiple measurements, move the adjustment knob off your reading, recenter the mark, and reread the scale. Record each measurement in your notebook, along with the temperature.
Clean the instrument for the next person.
Cleaning the Instrument
Careful cleaning is critical to our continued use of these instruments. The major enemy is dust and grit. Also Kim-wipes are very abrasive and can easily scratch the prism glass. Always use cotton balls to carefully wipe away sample; then rinse with methanol and a clean cotton ball.
· Never use paper (including Kim-wipes) to clean the refractometer prisms!
· Never "scrub" the prism!
Temperature correction
Refractive index is a function of temperature, not least because of density changes in condensed materials with changes in T. A simple correction can be applied in most circumstances to allow you to report a value in the standard way: at a temperature of 25�C.
hD25 = hDy - (25.0 - y)(0.00045)
The actual temperature of the measurement is y (in �C). hD25 is the index of refraction, using the sodium D line, at 25�C; hDy is the index of refraction you measure at y�C. You want to report your measurement, with its 95% confidence limit, corrected to 25�C.
Refractometry:
http://www2.ups.edu/faculty/hanson/labtechniques/refractometry/intro.htm
Introduction
A refractometer measures the extent to which light is bent (i.e. refracted) when it moves from air into a sample and is typically used to determine the index of refraction (aka refractive index or n) of a liquid sample.
The refractive index is a unitless number, between 1.3000 and 1.7000 for most compounds, and is normally determined to five digit precision. Since the index of refraction depends on both the temperature of the sample and the wavelength of light used these are both indicated when reporting the refractive index:
The italicized n denotes refractive index, the superscript indicates the temperature in degrees Celsius, and the subscript denotes the wavelength of light (in this case the D indicates the sodium D line at 589 nm).
The refractive index is commonly determined as part of the characterization of liquid samples, in much the same way that melting points are routinely obtained to characterize solid compounds. It is also commonly used to:
· Help identify or confirm the identity of a sample by comparing its refractive index to known values.
· Assess the purity of a sample by comparing its refractive index to the value for the pure substance.
· Determine the concentration of a solute in a solution by comparing the solution's refractive index to a standard curve.
Theory
Theory
The speed of light in a vacuum is always the same, but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material. The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction (aka refractive index or n) for the substance.
Figure 1. Light crossing from any transparent medium into another in which it has a different speed, is refracted, i.e., bent from its original path (except when the direction of travel is perpendicular to the boundary). In the case shown, the speed of light in medium A is greater than the speed of light in medium B. |
Whenever light changes speed as it crosses a boundary from one medium into another its direction of travel also changes, i.e., it is refracted (Figure 1). (In the special case of the light traveling perpendicular to the boundary there is no change in direction upon entering the new medium.) The relationship between light's speed in the two mediums (vA and vB), the angles of incidence (qA) and refraction (qB) and the refractive indexes of the two mediums (nA and nB) is shown below:
Thus, it is not necessary to measure the speed of light in a sample in order to determine its index of refraction. Instead, by measuring the angle of refraction, and knowing the index of refraction of the layer that is in contact with the sample, it is possible to determine the refractive index of the sample quite accurately. Nearly all refractometers utilize this principle, but may differ in their optical design.
Figure 2. Cross section of part of the optical path of an Abbe refractometer. The sample thickness has been exaggerated for clarity. |
In the Abbe' refractometer the liquid sample is sandwiched into a thin layer between an illuminating prism and a refracting prism (Figure 2). The refracting prism is made of a glass with a high refractive index (e.g., 1.75) and the refractometer is designed to be used with samples having a refractive index smaller than that of the refracting prism. A light source is projected through the illuminating prism, the bottom surface of which is ground (i.e., roughened like a ground-glass joint), so each point on this surface can be thought of as generating light rays traveling in all directions. Inspection of Figure 2 shows that light traveling from point A to point B will have the largest angle of incidence (qi) and hence the largest possible angle of refraction (qr) for that sample. All other rays of light entering the refracting prism will have smaller qr and hence lie to the left of point C. Thus, a detector placed on the back side of the refracting prism would show a light region to the left and a dark region to the right.
Samples with different refractive indexes will produce different angles of refraction (see Equation 2 above and recall that the angle of incidence and the refractive index of the prism are fixed) and this will be reflected in a change in the position of the borderline between the light and dark regions. By appropriately calibrating the scale, the position of the borderline can be used to determine the refractive index of any sample. In an actual Abbe' refractometer there is not a detector on the back of the refracting prism, and there are additional optics, but this is the essential principle.
(It is also possible to design a refractometer based on the reflection of light from the boundary between the prism and the sample. These types of refractometers are often used for continuous monitoring of industrial processes.)
In most liquids and solids the speed of light, and hence the index of refraction, varies significantly with wavelength. (This variation is referred to as dispersion, and it is what causes white light moving through a prism to be refracted into a rainbow. Shorter wavelengths are normally refracted more than longer ones.) Thus, for the most accurate measurements it is necessary to use monochromatic light. The most widely used wavelength of light for refractometry is the sodium D line at 589 nm.
If white light were used in the simple Abbe' refractometer optics shown in Figure 2, dispersion would result in the light and dark borderline being in different places for different wavelengths of light. The resulting "fuzziness" of the borderline would make precise work impossible. However, many Abbe' refractometers are able to operate satisfactorily with white light by introducing a set of "compensating prisms" into the optical path after the refracting prism. These compensating prisms are designed so that they can be adjusted to correct (i.e., compensate for) the dispersion of the sample in such a way that they reproduce the refractive index that would be obtained with monochromatic light of 589 nm, the sodium D line.
As mentioned earlier, the speed of light in a substance is slower than in a vacuum since the light is being absorbed and reemitted by the atoms in the sample. Since the density of a liquid usually decreases with temperature, it is not surprising that the speed of light in a liquid will normally increase as the temperature increases. Thus, the index of refraction normally decreases as the temperature increases for a liquid (Table 1). For many organic liquids the index of refraction decreases by approximately 0.0005 for every 1 °C increase in temperature. However for water the variation is only about -0.0001/°C.
Many refractometers are equipped with a thermometer and a means of circulating water through the refractometer to maintain a given temperature. Most of the refractive index measurements reported in the literature are determined at 20 or 25 °C.
Anatomy of a Refractometer
Click on a part of the refractometer to get more information about it.