197. Alpha-D-Talose ANN VAN CAMP, Lilly Research Laboratories, Indianapolis 6, Ind. CHZOH
HoQH H 1
H
1
H
HE sugar a-D-talose has been isolated T f r o m the antibiotic hygromycin B (1). This sugar has been well known for years, but had not previously been isolated from a natural source. Crystals of a-D-talose suitable for crystallographic work were obtained by recrystallization from methanol solution. The x-ray powder diffraction data were obtained using a camera 114.6 mm. in diameter with chromium radiation and vanadium filter. A wave length value of 2.2896 A. was used in the calculations.
CRYSTAL NORPROLOGY Crystal System. Orthorhombic. Form and Habit. Prisms elongated parallel to the c axis.
Axial Ratios. 0.6211.
a:b:c
= 0.6656:l:
X-RAYDIFFRACTION DATA Cell Dimensions. a = 8.08 A., b = 12.14 A,, c = 7.54 A. Formula Weights per Cell. 4. Formula Weight. 180.16. Density. 1.603 grams per cc. (flotation), 1.620 grams per cc. (x-ray). Space Group. P212,21. OPTICALPROPERTIES Refractive Indices (5893 A,, 25" (3.). fl = 1.550, y' (in 110) = 1.556. Optic Axial Angle. 2V is large. Optic Axial Plane. 001.
X-Ray Powder Diffraction Data hkl d (Calcd.) d 1/11
6.74 5.05 4.74 4.08 3.84 3.63 3.57 3.33 3.27 3.07 2.85 2.66 2.63 2.15
0.40 1.00 0.53 0.07 0.07 0.13 0.27 0.07 0.07 0.07 0.13 0.07 0.13 0.13
110 111 021
121 210 130 031 220 112 221 140 141 310
6.74 5.02 4.73 4.08 3.83 3.62 3.57 3.36 3.29 3.07 2.85 2.67 2.63
FUSION BEHAVIOR a-D-Talose melts a t 128' to 132' and does not recrystallize on cooling.
c.
ACKNOWLEDGMENT
The author thanks Paul Wiley for supplying the crystals from which these data were obtained and Ralph R. Pfeiffer and Harry A. Rose for technical advice.
LITERATURE CITED
(1) Wiley, P. F., Sigal, M. V., J . .4m. Chem. SOC.80, 1010 (1958).
WITH this issue, ANALYTICALCHEMISTRY is closing out it5 series on crystallographic data. However, W. C. McCrone expects to continue to publish this material in his own News Letter. Any further correspondence relative to crystallographic data should be sent to W. C. McCrone, 501 East 32nd St., Chicago 16, Ill.
Improved Volumetric Determination of Boric Acid by Means of a pH Paradox
F. L.
Hahn, Angel Urraza 71 8-303, Mdxico 12, D. F., Mdxico
Translated by Ralph E. Oesper, Cincinnati, Ohio
of like p H are Iformed, combined and no precipitate is no change of p H is expected; F TWO SOLUTIONS
if the initial solutions are of different pH, it may be assumed that their mixture will have an intermediate value, whose actual magnitude will depend on the quantities of the respective solutions and their buffer capacities. If, contrary to expectation, the addition of an alkaline reagent lowers the p H instead of raising it, the effect may be called a p H paradox. A phenomenon of this type and its use for the determination of hardness in waters have been described (1); the calcium and/or magnesium to be titrated transforms a very weak acid [tertiary ion of the (ethylenedinitrilo) tetraacetic acid] contained in the reagent to a notably stronger one. 316
ANALYTICAL CHEMISTRY
In the case discussed here the general aspect is about the same but the mechanism is exactly reversed : The reagent, a mixed solution of sorbitol and sodium hydroxide, first transforms the acid to be titrated and then measures it. Other applications of the same principle are under study. Volumetric Determination of Boric Acid and Borates. When, in the usual procedure, a boric acid solution containing mannitol, glycerol, or invert sugar is titrated with pure sodium hydroxide, it is necessary, after the first appearance of an end point, to add more of the complexing agent to make sure that a sufficient excess is present. If the indicator color fades, more alkali is added until the color reappears, and this procedure is repeated until the color change is
permanent. This inconvenience is averted by using a combined reagent containing the complexing agent in a large excess over the titrating agent, NaOH. Since mannitol is expensive, glycerol is inefficient, and invert sugar is unstable in alkaline solution, sorbitol was studied, first as the pure compound and then in the form of its very cheap commercial 70% solution; both work perfectly. A mixed reagent, containing approximately 60% of sorbitol and 0.1N in sodium hydroxide, permits boric acid to be titrated as easily and exactly as acetic acid with ordinary sodium hydroxide. Figure 1 shows the general aspect of such a titration: A marked slope of the pH a t the beginning is followed by a flattening a t around 50% and the
