SOP: Determination of Sucrose in Sugar by Polarization
GIFSQ — Global Institute for Food Safety & Quality
Principle
Solutions of sucrose rotate the plane of polarized light. The angle of rotation varies with the concentration of the optically active substance, the length of the light path through the solution, the wavelength of the light, and the temperature. Sucrose in sugar is therefore determined with a saccharimeter fitted with the International Sugar Scale: the normal weight of pure sucrose (26.000 g) made to 100 ml in water and read in a 200 mm tube with white light at 20°C gives a reading of exactly 100°S (degrees sugar). Invert sugars, invariably present, affect the reading; for routine control the polarization is corrected with the invert-sugar content.
Apparatus required
- Saccharimeter fitted with the International Sugar Scale (ICUMSA), standardized at 20°C
- 200 mm polarimeter tube
- Dry beaker
- 100-ml volumetric flask
- Long-stemmed funnel
- Filter paper
- Potassium dichromate light filter (when white light is used)
Reagents
a. Alumina cream: dilute 500 ml of 30% (w/v) aluminium sulphate solution to 2000 ml. Add 10% (w/v) ammonia solution gradually, with constant stirring, until no more precipitate forms. Allow the precipitate to settle and decant the clear liquid. Shake the precipitate with 1 litre of water, let it settle and decant again. Repeat the washing and decantation until the supernatant liquid is neutral to litmus and gives no precipitate with barium chloride solution. Pour off the excess water and store the residual cream in a glass-stoppered bottle.
b. Anhydrous lead subacetate (alternative clarifier, about 0.6 g) — used in small quantities only when clarification with alumina cream is difficult.
Procedure
Weigh accurately 26 g of sugar in a dry beaker
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Transfer quantitatively to a 100-ml volumetric flask with the help of a long-stemmed funnel; rinse the beaker and the funnel with water and transfer the washings to the flask, keeping the total volume below 80 ml
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Bring the sugar into solution by gently swirling the flask
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Add 1 to 2 ml of alumina cream until nearly all the impurities are precipitated (if clarification is difficult, repeat using small quantities of anhydrous lead subacetate instead — about 0.6 g is usually required; avoid excess)
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Make up the volume to 100 ml
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Filter through a filter paper, rejecting the first 20 to 25 ml of the filtrate
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Polarize the filtrate in a 200 mm tube
Calculation
Sucrose is calculated from the polarization and the invert-sugar content:
S = P + 0.22I
Where,
- S = sucrose
- P = polarization
- I = invert sugar
Note: the printed expression reads “0.221”; carried here as 0.22I (0.22 × invert sugar), which is the relationship the text defines.
When the polarization is carried out at a temperature other than 20°C, correct the saccharimeter reading to 20°C:
S₂₀ = S_t[1 + 0.0003(t − 20)]
Where,
- S₂₀ = saccharimeter reading at 20°C
- S_t = saccharimeter reading at t°C
- t = temperature (°C) at which the polarization is carried out
Note: the formula is applicable only up to 30°C. Keep the instrument in an air-conditioned room adjusted to 20°C, bring the solution to 20°C, and measure at the same temperature whenever possible.
Whenever white light is used, pass the light through a potassium dichromate solution of such concentration that the per cent K₂Cr₂O₇ multiplied by the length of the column of solution in cm equals 9.
The quartz-plate equivalents of the International Sugar Scale are:
- Normal quartz plate = 100°S = 40.690° ± 0.002° (λ = 5461 Å) at 20°C; i.e., 1° (λ = 5461 Å) = 2.4576°S
- Normal quartz plate = 100°S = 34.620° ± 0.002° (λ = 5892.5 Å) at 20°C; i.e., 1° (λ = 5892.5 Å) = 2.8885°S
Note: the printed wording (“eee quartz plate”, “Normalpiste plate”, “Normalpiste”) is garbled in the extraction; rendered here as the normal quartz plate equivalents the passage defines.
Precautions
- The saccharimeter is standardized at 20°C — measure the solution at the same temperature; if that is not possible, apply the temperature correction.
- Avoid using excess lead subacetate when it is used as the clarifier.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
DeWhalley, H.C.S., ICUMSA Methods of Sugar Analysis, Elsevier Publishing Co., New York, 1964 — cited in the book.
Browne, C.A. & F.W. Zerban, Physical and Chemical Methods of Sugar Analysis, 3rd edn, 1941, p. 548, John Wiley & Sons, Inc., New York — cited in the book.