Sugar solutions, refractive indices

ICUMSA (1) has adopted tables showing the relationship between the concentration of aqueous solutions of pure sucrose, glucose, fmctose, and invert sugar and refractive index at 20.0°C and 589 nm. 

The concentration of a pure sugar solution is determined by measurements of polarization (optical rotation), refractive index, and density. 

Refractive Index. The refractometric value of sugar solutions is used as a rapid method for the approximate determination of the soHds content (also known as dry substance), because it is assumed that the nonsugars present have a similar influence on the refractive index as sucrose. Measurement is usually carried out on a Brix refractometer, which is graduated in percentage of sucrose on a wt/wt basis (g sucrose/100 g solution) according to ICUMSA tables of refractive index at 20.0°C and 589 nm. Tables are available that give mass fraction corrections to refractometric values at temperatures different from 20°C. 

The refractive index also varies with the amount of substance in a mixture. Most often, refractive index is used to assess the concentration of sugar in wine, soft drinks, cough medicines and other preparations having relatively high concentrations of sucrose. Refractive index is also used to determine the concentration of alcohol in fermented products. For sucrose solutions the refractive index varies from 1.3330 (pure water) to 1.5033 when the solution contains 85% sucrose. This is an increase of approximately 0.0002 in the refractive index for each 0.1%... 

Small amounts of impurities have a significant effect on the refractive index. In fact, the refractive index for many binary mixtures changes linearly with concentration over a wide range of concentrations. A calibration curve of refractive index vs. concentration along with the refractive index of a sample can be used to find the concentration of a species in the sample. For example, the food and beverage industry uses the refractive index to find the concentration of sugar solutions. Table 15.1 lists several additional applications for refractive index. 

The refractive index detector operates by comparing the refractive index of the mobile phase prior to the column with the refractive index of the column eluate. This detector responds to nearly all solutes but it is highly temperature-sensitive (Skoog et al., 1998). This type ofdetector can be used for sugars and fatty acids. 

However, samples that have no UV absorbance, exist in ionic form, or require structural information have to couple with other types of detectors. Refractive index (RI) detector is almost a universal detector in that it responds to almost any solute (UV-absorbing molecules, sugars,... 

Since the concentration is proportional in many individual cases to an easily delermined physical constant, such as specific gravity (e.g.. of solutions), the index of refraction, specific rotatory power (e.g., sugar solutions, tcrpcncsi. such constants are frequently used to ascertain and express concentration data. 

Two long-established methods for the estimation of solute content are based upon the observations that both the solution density and the refractive index of a solution change as the solute concentration of the solution changes. The methods that have been developed for estimating solution concentrations based upon these properties, and the underlying assumptions and limitations of these methods, can be best demonstrated by looking separately at how each method can be used to estimate the sugar content of solutions, and hence, by difference, their moisture content. 

The refractive index of aqueous saccharose solutions varies with the concentration, and on this is based the determination of the sugar-content of a solution by means of the refractometer. 

The refractive index of a pure sucrose solution is an accurate measure of the concentration of dissolved substance. Stolle (39) found little variation in the refractive index of solutions of different sugars (sucrose, dextrose, levulose and lactose) at the same concentration. Investigations by Main (40), Schonrock (41) and Landt (42) resulted in the adoption of a standard table of refractive indices of sugar solutions by the International Commission for Uniform Methods of Sugar Analysis in 1936. 

Another parameter to be considered in scattering is the ratio of the refractive index of the suspended particles to that of the medium. When the two refractive indices are equal, suspensions of particles exhibiting no scattering are possible. In commercial sugar solutions, the ratio of refractive indices may be of the order of magnitude of 1.1. This value is considerably lower than that encountered in aerosol experiments,26 and is typical of many turbidity problems in solutions.27... 

The rate of the interconversion may also be followed by measuring the change in volume or in refractive index. Such measurements give rate coefficients identical with those obtained by the polarimetric method. In Table XVIII, rate coefficients for the mutarotation of a number of sugars are listed. The rates of mutarotation of several sugars (for example, D-ri-bose, D-galactose, and all the ketoses) do not obey the first-order law. Their complex mutarotations result from the presence in solution, in appreciable concentrations, of more than two species. In addition to pyranoses, there must be present either furanoses or acyclic forms, or both. 

Evaluation of the sensitivity of vaiious waveguide/substrate systems is easily accomplished by simply miming a salt, sugar, or other known solvent or solute system that can alter the refractive index in a known or measureable way over the waveguide and measuring the phase change that results interferometrically. As the concentration of the salt solution increases, so does the refractive index of the... 

Appearance of solution Acidity or alkalinity Refractive index Aldehydes Related substances Halogenated compounds Limit of chlorinated compounds Sugars Chloride Heavy metals Water Sulfated ash Specific gravity Sulfate Esters... 

Refractive index detectors are mostly used for sugar and lipid analyses. The presence of dissolved solutes in the mobile phase will cause a change in the refractive index. However, it is important to realise that the intrinsic characteristic makes up for the fact that gradient elution is usually not employed with a refractive index detector, as a change in the mobile phase implies a change in the refractive index which can barely be differentiated from that resulting from the presence of dissolved solute(s). 

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