Application Note ANTCREFRACT0311 The Importance of Temperature Control with Abbe Refractometers
Scott Pratt, Thermo Fisher Scientific, Newington, NH, USA
Key Words: Abstract speed of light through the solution Benefits of Tight Temperature Control Refractometry is an important compared to the speed of light The speed of light in a substance is • Refractometry tool used to detect, identify, and in a vacuum (which is a physical slower than in a vacuum since the • Refractive index quantify substances in solution and constant). light is being absorbed and remitted • Liquid density is widely used in food & beverage, In solutions, the density by the atoms in the sample. Since • Temperature agricultural, medical, and industrial will typically decrease as the the density of a liquid usually control applications. Natural temperature temperature increases. As a result, decreases with temperature, it variations in the samples being the speed of light passing through is not surprising that the speed measured could result in grossly will also increase as the solution of light in a liquid will normally inaccurate readings. Accurate becomes less dense, thus altering increase as the temperature refractometric results can only the refractive index. In order to increases. Thus, the index of be achieved by stabilizing the accurately measure the refractive refraction normally decreases as sample temperature of the index, it is imperative that the the temperature increases for a solution during measurement. temperature of the solution be kept liquid. For many organic liquids Stabilizing the solution temperature at a constant, typically 20°C. the index of refraction decreases by typically requires the use of a approximately 0.0005 for every refrigerated bath circulator or How Refractometers Work 1°C increase in temperature. laboratory chiller. Selecting the best A refractometer has an illuminating Most of the refractive index temperature control equipment will prism, a refracting prism, and a measurements are determined at 20 not only improve the accuracy of light source. It works by placing or 25 °C. Variations in temperature the measured refractive index, but a thin layer of solution between between measurements can result offer efficient operation and limited the refractometer’s prisms and in great variation in the index of workspace. reading the angle at which the light refraction reported (Table 1). changes direction. The angle will Many refractometers are Introduction vary depending on the physical equipped with a thermometer to Refractometers are a simple cost properties of the solution. indicate the sample temperature. effective optical tool used to Whenever light changes speed Because they are coarsely measure the amount of dissolved as it crosses a boundary from one graduated, this can cause a different substances in solution or to help medium into another its direction interpretation between users, so identify unknown substances. This of travel also changes, i.e., it is achieving an accurate reading is is accomplished by determining refracted. The relationship between difficult. the refractive index of the solution, lights speed in the two mediums is These same refractometers i.e., measuring the ratio of the the refractive index. are often equipped to circulate It is not necessary to measure temperature controlled water the speed of light in a sample in allowing for control and order to determine its refractive stabilization of the sample Accel 250LC laboratory chiller index. Instead, by measuring the temperature. The result is a refraction (change in direction highly accurate refractive index of the light) of the solution, and reading without the need of an knowing the index of refraction applying a correction factor to the of the layer that is in contact with refractometer reading. This can be (refracting prism), it is possible to achieved by using a temperature accurately determine the refractive control device such as the Thermo index of the solution. Nearly all Scientific Accel 250 LC laboratory refractometers utilize this principle, chiller. but may differ in their optical design.
1 Index of Refraction of Water, Alcohol, and Carbon Bisulfide Relative to Air for sodium light,λ = 0.5893* In addition to these offices, 99.8% Carbon Thermo Fisher Scientific Temp. (°C) Pure Water Ethyl Alcohol Bisulfide maintains a network of representative organizations 20 1.33299 1.36048 1.62546 throughout the world. 22 1.33281 1.35967 1.62387 24 1.33262 1.35885 1.62226 26 1.33241 1.35803 1.62064 28 1.33219 1.35721 1.61902 30 1.33192 1.35639 1.61740 Benelux Tel. +31 (0) 76 579 55 55 32 1.33164 1.35557 1.61577 [email protected] 34 1.33136 1.35474 1.61413 China Tel. +81 3 5826 1616 36 1.33107 1.35390 1.61247 [email protected]
38 1.33079 1.35306 1.61080 France Tel. +33 (0) 1 60 92 48 00 40 1.33051 1.35222 1.60914 [email protected]
Table 1: Effect of temperature fluctuation on the index of refraction Germany Tel. +49 (0) 721 4 09 44 44 * Information in this table was obtained from the Handbook of Chemistry and Physics, 59th Edition, [email protected] CRC Press, 1979. Accel 250 LC Laboratory Chiller India Tel. +91 (22) 27 78 11 01 Conclusion [email protected] When utilizing a refractometer for Japan testing samples, the most effective Tel. +81 45 453 9220 and consistent results are obtained [email protected] by removing variables from the USA process. Liquid temperature control Tel. 603 436 9444 improves the reliability of the [email protected] results by removing inconsistencies www.thermoscientific.com/tc such as correction factors and rounding errors from the process.
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