Polarimetry and the International Sugar Scale – Official
2.Field of Application
The Standard is used in statutory analyses, trade analyses and for factory control. It is applicable to raw materials, intermediate and final products of the sugar industry.
3.Definitions
3.1 Normal sugar solution
The "normal sugar solution" is defined as 26.0160 g of pure sucrose weighed in vacuo and dissolved in water at 20.00℃ to a final volume of 100.000 cm3. This corresponds to 26.000 g weighed in air under normal conditions (1013 mbar, 20℃, 50% relative humidity) and dissolved in water to a final volume of 100.000 cm3. Alternatively, ICUMSA proposes the following gravimetric method to achieve the same results with greater precision: The "normal sugar solution" is defined as 26.0160 g of pure sucrose weighed in vacuo and dissolved in water at 20.00℃ to a final (vacuo-)mass of 109.7639 g. This corresponds to 26.000 g weighed in air under normal conditions (1013 mbar, 20℃, 50 % relative humidity) and dissolved in water to a final mass value of 109.6604 g.
3.2 The basis of the 100°Z point of the International Sugar Scale
The basis of the 100°Z point of the International Sugar Scale [3,4] is the optical rotation of the normal solution of pure sucrose (3.1) at the wavelength of the green line of the mercury isotope 198Hg (546.2271 nm in vacuo) at 20.00 ℃ in a 200.000 mm tube. For other wavelengths, the formula for the rotatory dispersion of sucrose solutions [5] is valid.
3.3 Effective wavelengths
For quartz wedge instruments the effective wavelength has been fixed at 587.0000 and/or 882.600 nm [3,6].
For the double line of yellow sodium light the mean effective wavelength has been fixed at 589.4400 nm [3,7].
7. Apparatus
The specifications apply to visual and photoelectric polarimeters. Visual polarimeters shall be of the half-shadow type. If the angle of the half-shadow is fixed, it shall be between 5° and 10°, if variable, adjustable between 0° and 10°.
Instruments with a rotating analyser or polariser, the angle of which is measured, shall be illuminated with a monochromatic light of wavelength between 540 nm and 633 nm. Instruments with quartz wedge compensation may be illuminated by a monochromatic light source or by an incandescent lamp filtered to produce light with an effective wavelength of 587 nm.
NOTE – In all polarimeters the scale depends upon the wavelength of the light being used. The knowledge of the wavelength of the instrument is, therefore, indispensable for the user in order to apply the correct "sugar value" of the quartz control plates (see 7.5), with reference to the certified values for these wavelengths.
The scale of polarimeters must be linear within their class of accuracy.
Polarimeters equipped with the International Sugar Scale in °Z are called "polarimetric saccharimeters". Results of polarimeters with other scales, e.g. circular scales (360° for the full circle) can be transformed into °Z using a quartz control plate (see 7.5).
According to the precision required, polarimeters have three classes of accuracy:
Class 0.2 maximum error over the scale ± 0.2 °Z
Class 0.1 maximum error over the scale ± 0.1 °Z
Class 0.05 maximum error over the scale ± 0.05 °Z
NOTE – Most automatic polarimeters used for sugar analysis fulfill the demands of Class 0.05.
After the first half hour of operation the systematic error of automatic polarimeters for sugar analysis due to "drift" should not exceed 0.05 °Z over 24 h.
The definitive indication shall be obtained within a maximum of 30 seconds, whatever the value of this indication, on automatic polarimeters for sugar analysis. This is important because the temperature of the cell compartment is distinctly higher than the room temperature. Therefore, when using tubes without thermostatic temperature control, the reading should be taken within 1 min of placing the tube in the instrument, i.e. before the temperature of the solution begins to rise.
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