Temperature Control With Abbe Refractometers

Refractometry is a simple analytical tool for assessing the concentration of solutions used in the food manufacturing industry.

Food manufacturing processes rely on quality control at all levels of production.  Refractometry is a simple analytical tool for assessing the concentration of solutions used in the food manufacturing industry. With applications in both the quality assessment of individual ingredients and of the final foodstuffs, this technique is quick, accurate and easily applied to both manufacturing processes and for producers of fruits and vegetables.

Refractometry Through The Ages

The technique of refractometry was first developed by Ernst Karl Abbe as a result of his studies into the improvement of microscopes and optical technology in 1869 [1]. The first refractometer was a simple modified microscope incorporating two prisms, which measured the angle of total internal reflection of the thin layer of sample. Modern Abbe refractometers follow a very similar structure, containing an illuminating and a refracting prism, in addition to a strong light source [1]. When a thin film of liquid sample is placed between the two prisms, the angle at which light is internally reflected is accurately determined and can be used in downstream concentration calculations.

Measurements at the Speed of Light

The speed of the light is constant in a vacuum; however, within a substance the light is hindered by the atoms and molecules of the material. Waves of light transitioning between the interface of materials with different densities change their direction and speed. This effect is amplified when the difference in density of the two materials is greatest. The angle at which the wave of light hits the interface (incidence) directly influences the angle to which it will be refracted. As the angle of incidence is changed the degree to which the light is refracted will also change, until ultimately the critical angle is reached. At the critical angle no light can leave the second material as it is reflected from the boundaries of that material, and the sample appears darker. Measurements of the critical angle allow analysts to calculate the refractive index of the sample.

It is this change in angle that analysts exploit when making measurements to assess the concentration of the sample. As the concentration of most solutes increases, the viscosity of the liquid solution increases. This alteration in the density of the sample impacts the refractometric measurements and after subsequent comparison to standard curves, produced using solutions of known concentration, can be used to calculate the concentration. This method is very commonly used by providers of soft and tropical fruits, where the sugar concentration of the fruit will impact greatly on its shelf-life and marketability.

A Question of Heat

The temperature of a liquid vastly influences the density of the substance, and hence how much the light is slowed when moving through it. As the temperature increases, the density of the liquid decreases and the light therefore travels more easily through the sample, hence the refractive index of the sample also decreases. This reduction is typically within the range of 0.0005 for every 1°C increase in temperature. Most refractive index measurements are made at 20 or 25 °C, with equipment being fitted with coarsely graduated thermometers.

Index of Refraction of Water, Alcohol, and Carbon Bisulfide Relative to Air for sodium light, ? = 0.5893*

Temp. (°C) Pure Water 98% Ethyl Alcohol Carbon Bisulfide
20 1.33299 1.36048 1.62546
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
32 1.33164 1.35557 1.61577
34 1.33136 1.35474 1.61413
36 1.33107 1.35390 1.61247
38 1.33079 1.35306 1.61080
40 1.33051 1.35222 1.60914

Figure 1: Effect of temperature fluctuation on the index of refraction

Even relatively small variations in temperature can lead to a significant alteration in the measured refractive index of a sample. When used in a food manufacturing setting, this can mean that the concentration and purity of a product can vary, reducing quality and creating inefficiency. Table 1 highlights the variations in refractive index of pure water, 98.8% alcohol and carbon bisulpide. These data clearly show a reduction in the refractive index of all three solutions as the temperature increases from 20 – 40 °C. 

The measurements of concentration require comparison of the refractometry readings with those of standard samples. Variations in the refractometric readings due to temperature changes can introduce critical errors into this quality control process and could have deleterious effects on the final product.

A Cool Operation

Recirculating chillers are commonly found in analytical laboratories. One of the most common requirements is in maintaining the low temperature of an instrument during its operation, either to ensure correct operation or the integrity of the sample. Recirculating chillers use water to remove heat from the internal environment and can easily be applied to most laboratory refractometers, such as the ABBE-3L Refractometer.

Guaranteeing the internal temperature of the refractometer is a simple and effective method of ensuring consistent and accurate results. Liquid temperature control technologies, such as the Thermo Scientific Accel 250 LC recirculating chiller, are easily incorporated into existing systems and can provide an ideal solution to obtain more reliable refractometric measurements.

For more information contact Scott at scott.pratt@thermofisher.com.

[1] http://www.rsc.org/chemistryworld/Issues/2008/November/AbbesRefractometer.asp

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