richer in mercury where it can be determined quantitatively. Thus, this procedure allows us to assay low levels of mercury in many less contaminated foodstuffs and can also be employed to determine mercury in natural waters. Since mercuric ions are selectively extracted from the cupric ones at pH 0, the procedure was shown to be particularly useful for simultaneous colorimetric determina-
tions of both these elements in one sample of food. In many samples with mercury level less than 0.05 ppm, it was impossible to assay it quantitatively without the combining and back-extraction procedure. Received for review December 7, 1972. Accepted June 6, 1973.
Optical-Crystallographic Properties of P-D-Glucose George R. Dean1 Miles Laboratories, Inc., Elkhart, Ind. 46514
Although the optical-crystallographic properties of CY-Dglucose are listed by Winchell (I), corresponding data for the P isomer have never been published. Other properties such as phase relationships (2-4) and crystal structure by X-ray diffraction (5-7) have been described, as well as methods for preparing the compound in the laboratory and in industry (8, 9). Because D-glucose is such a common and important sugar, the additional data reported herein should fill a long-standing need of the microscopist and analyst.
EXPERIMENTAL Apparatus. A 1-liter resin flask was fitted with a vacuumsealed stirrer, condenser, solution inlet, thermometer, and electric heating mantle. Reagents. D-Glucose was the commercial hydrate form of the sugar. Seed crystals of 6-o-glucose were prepared by an accepted procedure (8). Procedure. Crystals of 6-D-glucose were prepared by slowly cooling a hot, concentrated aqueous solution of D-glucose. An aqueous solution containing 90% of the sugar was prepared separately and introduced into the flask a t 90 "C. After a few seed crystals of 6-D-glucose were added, the solution was stirred gently. After crystallization began, the solution was allowed to cool gradually t o 80 "C. At the same time, the concentration was slowly decreased by admitting small amounts of a more dilute solution of D-glucose. Vacuum was adjusted so that the solution boiled gently at all times. After 1 hour, the mixture was poured into two volumes of glacial acetic acid at 100 "C, quickly filtered, washed with hot acetic acid, and dried. By deliberately holding the.yield to a low level, comparatively large, well-formed crystals which were suitable for the present study were obtained. Otherwise, the product contained too many fine crystals. Initial specific rotation showed the product to contain 90% pand 10% a-D-glucose. Anal. [ a I z 0 ~ Accepted: , +18.7", Found: lRetired. Present address, 404 N. Buchanan St., Edwardsville, Ill. 62025. (1) A. N . Winchell, "The Optical Properties of Organic Compounds," Academic Press, New York, N.Y., 1954. (2) R. F. Jackson and C. G. Silsbee, Nat. Bur. Stand. ( U S . ) , Sci. Papers, 437, 715 (1922). (3) W. B. Newkirk, Ind. Eng. Chem., 28, 760 (1936). (4) F. E. Young,J. Phys. Chem., 61, 616 (1957). (5) 0. L. Sponsler and W. H. Dore, J. Amer. Chem. SOC., 53, 1639 (1931). (6) W. G. Ferrier, Acta Crystallogr., 13, 678 (1960). (7) /bid., 16, 1023 (1963). (8) R. L. Whistler and J. N. BeMiller in "Methods in Carbohydrate Chemistry," Vol. I, Academic Press, New York, N.Y., 1962, pp 130-131. (9) G. R. Dean and J. B. Gottfried, Advan. Carbohyd. Chem., 5, 136137 (1950)
2440
+28.0". The accepted value (10) had been determined only after repeated fractional dissolution in ice cold water and subsequent precipitation with cold ethanol. Because larger crystals from water alone were desired, they were not further purified.
RESULTS Optical-crystallographic properties, determined by common microscopic procedures, are summarized in Table I. Figure 1 is a photomicrograph of the crystals in contact with the mother liquor. In Figure 2 are diagrams of principal views of P-D-glucose, together with crystallographic axes, profile angles, and principal optical directions.
DISCUSSION Table I shows p and y to be equal within experimental error. Actually, they differ slightly as evidenced by the observed biaxial interference figure, the latter of which indicates (-)2V = 8". Calculation shows that for this angle P and y differ by less than one in the fourth decimal place. The crystals from water uniformly show positive elongation for all orientations with c parallel to the microscope slide except that represented by the second diagram in Figure 2. Here one is looking down the acute bisectrix and birefringence is extremely low. Since such is not observed with the CY isomer, it should be easy for the microscopist to recognize this exceptional case. Otherwise, it ought to be safe to consider sign of elongation as a useful analytical property. The common method for analyzing a mixture of 6- and a-D-glucose is to observe the changing optical rotation of a freshly prepared solution and then extrapolate to the moment of dissolution ( 1 1 ) . From known initial rotations of the P and CY forms, the composition can be calculated. This method, however, can be used only with chemically pure D-glucose. Optically active impurities, especially those that exhibit mutarotation, give inconclusive results. In such cases, the microscopic method is especially useful. Large, well grown crystals of P-D-glucose can be recognized in contact with the mother liquor and distinguished by crystal form alone. Small, irregular crystals or fragments can be detected by refractive index measurement. Since for this purpose, the crystals must be isolated in (10) H. S. isbell and W. W. Pigman, J , Res. Nat. Bur. Stand.. 18, 141 (1 937). (11) C. S. Hudson and J. K. Dale. J. Amer. Chem. SOC., 39, 320 (1917).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 14, D E C E M B E R 1973
C
C
I
I
-
- b
a
\
Figure 2. Diagrams of princirial views of fl-n-glutose
Table 1. Oplilcal-Crystallographic Properties of P-D-GIuicose
~
Figure
1.
-
-
Cryslai morpho1o w Crystal system Form and habit
Crystals of 0-D-glucose from water, Magnlfication, Axial ratio
70X
Orthorhombic Prisms elongated parallel to c and showing the forms: (101).loll),1001).{ZlOI.(0211. a:b:c= 0.734:1:0.530° = 0.73:1:0.5Zb
clean, dry form from very concentrated solution, the refractive index method is not convenient for general analytical use. Conoscopic observation of interference figures is possible in contact with the mother liquor, hut only on larger crystals that lie in a particular orientation on the microscope slide. The most useful property for distinguishing between the two isomers is sign of elongation. This is positive and negative for 0- and a-o-glucose, respectively. Results must be viewed with some caution, because crystal elongation is not an absolute property hut is a habit which is influenced by temperature, concentration, and, solvent. From aqueous solution, however, this work has shown the property to he uniform over the normal range of conditions under which 0-o-glucose separates as a stable solid phase. Small crystals can he identified in contact with the mother liquor. A quartz wedge compensator is suitable, hut for the smallest crystals a sensitive tint or first order
Optical properties
* 0.002 y = 1.554 * 0.002
Refractiveindices
a
(5893A; 25 'C)
0 = 1.554 f 0,002
Optic axial angle Vibration directions
1.544
(-)Pi = 8
* 4"
a = b p=a
y=C Sign of elongation Positive X-Ray diffraction (6,7). Micr red plate is ideal. With its use, crystals of thickness as little as 2 micrometers can he easily distinguished. Received for review June 1, 1973. Accepted July 13, 1973. The author wishes to thank the management of Miles Laboratories, Inc., for permission to publish this work.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
2441
