Sucrose (table sugar) is produced by evaporating and crystallizing the juice extracted from sugar cane or the sugar beet. It is monoclinic, and crystallizes from water as colorless, transparent, birefringent, thick tablets. Some distinct (100) cleavage is evident, but ground crystals fracture conchoidally. The crystals, which may be equant (001000) or flakelike (001010), have a specific gravity of 1.588. Sucrose melts with decomposition at about 190°C. [Winchell (1954) reports 160-190°C; Merck Index (1989) reports 160-186°C; Kofler (1954) reports 189°C; Kuhnert-Brandstätter (1971) reports 188-190°C; using a heating rate of 5°/minute, the melting point was determined at McCrone Associates, using a Linkam Hotstage, to be 189°C]; at decomposition, sugar chars, and emits characteristic odor of caramel. The refractive indices are nD = 1.540 (α), nD = 1.567 (β), nD = 1.572 (γ); (-)0.032, 2V = 48º. The extinction angle is 23º on the (100) view.
Figure P1-1 is a 100X view of sugar seen in plane-polarized light; many of the crystals are very well-formed, and quite attractive. This particular sample has a granular surface, which will not be true for all sugar samples. Figure P1-2 is the appearance of this sample using slightly uncrossed polarizers; Figure P1-3 is the appearance of sugar using fully crossed polarizers. These two photomicrographs illustrate the interference colors that result from rather thick crystals with a birefringence of 0.032; namely, all colors from low first-order at thin edges, through first- second- and third-order colors, up to high-order white for the thickest particles. Figure P1-4 is the appearance on adding the first-order red compensator. Sucrose is highly soluble in water, slightly soluble in methanol, very slightly soluble in ethanol, and insoluble in ether or chloroform.
No fluorescence was observed with narrow-band 365 nm ultraviolet light.Microchemical / Spot Test:
To distinguish between Sucrose (cane sugar) and Dextrose, a modification of the tests described by Reich in 1847 (Arch. Pharm. 50 293) can be used: (1) Place a drop of water near one end of a glass microscope slide, and add enough sucrose to make a concentrated solution. Add several small crystals of potassium dichromate. Heat the drop to boiling, using an alcohol lamp. Sucrose produces a dark olive green color; dextrose is indifferent, or chars, or the solution remains orange. (2) Add a small drop of potassium hydroxide solution to a drop of concentrated sucrose solution near one end of the glass slide, and heat to boiling. Add a drop of cobalt salt (chloride, nitrate, etc.) solution. With sucrose, a blue-violet precipitate is produced; with dextrose, a dirty green precipitate is produced.
Determining Extinction Angle:
In a different sample of sugar, viewed between fully crossed polarizers at 100X, notice the fairly well-formed crystal in the very center of Figure P1-5, the elongated bright one; it is vertically aligned with the N-S crosshair, but is not at extinction; here a stage reading is taken from the vernier scale on your rotating stage. Next, the stage is rotated until the crystal goes to extinction (Figure P1-6); seen also with the first-order red compensator in Figure P1-7, and the reading is again taken at the rotating stage vernier. The angular difference was 24º (the literature value is 23º); this is the extinction angle; the angle between the extinction position and this prominent crystal face/length.
At 200X magnification, Figure P1-8, the relationship between the crystallographic faces is more evident, and interference colors of 8 to 10+ orders can be seen, increasing with crystal thickness.
These monoclinic crystals will have a biaxial interference figure, but textbook, centered figures do not just happen. Figure P1-9, for example, is a 400X view of a particular crystal, with a well-formed face. Figure P1-10 is the interference figure presented by this face; we are almost looking down one of the two optic axes. The curvature indicates that β is NE-SW to the left of the “eye”, so that when the first-order red compensator is introduced, Figure P1-11, β is seen to go downscale on the Michel-Lévy chart, indicating that it is the fast (low index) direction; therefore the perpendicular direction (NW-SE) must be slow (high index); only γ is higher than β, so we must be looking at a gamma-beta plane, and the only way to do that is to be looking down alpha (α); that is, alpha is the acute bisectrix, which, by definition, indicates that the crystal is optically negative.
Even more tilted axes are common, Figure P1-12, but now and then, by chance, or because of crystal rolling, a centered or nearly centered interference figure, Figure P1-13, can be obtained, so that the optical axial angle, 2V, can be determined [reported by Winchell (1954) in the text, as 48º, but in his diagram key as 42º]. Another different sample of table sugar, seen using slightly uncrossed polarizers at 100X (Figure P1-14) shows larger particle size, and grainy faces.
If sucrose crystals are ground to a powder, the well-formed crystals are mechanically damaged and reduced in size. Figure P1-15 is a 200X view of pulverized sugar, as seen using slightly uncrossed polarizers; Figure P1-16 is the same field of view seen using fully crossed polarizers; and Figure P1-17 is the view on adding the first-order red compensator. More yellows and blues of the lower orders are now more evident because of the much reduced thickness of the pulverized material.
Finding good interference figures is now less common than with the larger, well-formed tablets, and solubility, recrystallization, and microchemical testing may be needed. Commercial powdered sugar may have starch added: ultrafine 6X confectioners sugar usually contains 4% cornstarch as an anticaking agent. Block sugars may contain starch as a binding agent. [See Powdered Sugar (AMP:11)]. Natural brown sugar contains about 2% of the minerals found in the sugarcane plant, including calcium, iron, phosphorous, magnesium, and potassium, which are lost in the refining process.PLM References:
Chamot, Émile Monnin and Mason, Clyde Walter (1940). Handbook of Chemical Microscopy, Volume II: Chemical Methods and Inorganic Qualitative Analysis; Second Edition. John Wiley, New York and London.
Kofler, Ludwig and Kofler, Adelheid; Brandstätter, Maria (1954). Thermo-Mikro-Methoden zur Kennzeichnung organischer Stoffe und Stoffgemische. Universitätsverlag Wagner, Innsbruck, Austria.
Kuhnert-Brandstätter, Maria (1971). Thermomicroscopy in the Analysis of Pharmaceuticals. Pergamon Press, Oxford and New York.
McCrone, Walter C. and Delly, John Gustav (1973). The Particle Atlas Edition Two, Volumes I-IV. Ann Arbor Science Publishers, Ann Arbor, Michigan. Volume V, McCrone, Walter C., Delly, John Gustav, and Palenik, Samuel James (1979). Volume VI, McCrone, Walter C., Brown John A., and Stewart, Ian M. (1980).
The Merck Index; An Encyclopedia of Chemicals, Drugs, and Biologicals. Eleventh (Centennial) Edition. (1989). Budavari, Susan Editor. Merck & Co., Inc., Rahway, New Jersey.
Winchell, Alexander N. (1954). The Optical Properties of Organic Compounds. Second Edition. Academic Press, New York.