International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. C. ch. 3.2, p. 158
Section 3.2.2.2. Flotation method
F. M. Richardsa
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Although historically used much earlier, this technique is essentially an approximation to the gradient-tube method. The specimen is immersed in a liquid, and a denser or less dense liquid miscible with the first is added until the sample neither rises nor sinks in the solution (Wulff & Heigl, 1931).The density of the immersion medium is then determined immediately by standard techniques such as pycnometry, by the Westphal balance, or by refractive index (Midgley, 1951). The method is reported as capable of a probable accuracy as great as 0.02%.
The compounds listed in Table 3.2.2.1 are also useful in this method. With slurries or with specimens smaller than 1 mm3, a centrifuge must be used to achieve a reasonable rate of settling. As little as 0.05 mg of material has been used with good results (Bernal & Crowfoot, 1934). A modification of this method has been described in which the density of the immersion medium is varied by altering the temperature (Reilly & Rae, 1954; Wunderlich, 1957).
References
Bernal, J. D. & Crowfoot, D. (1934). Use of the centrifuge in determining the density of small crystals. Nature (London), 134, 809–810. Google ScholarMidgley, H. G. (1951). A quick method of determining the density of liquid mixtures. Acta Cryst. 4, 565. Google Scholar
Reilly, J. & Rae, W. N. (1954). Physico-chemical methods, Vol. 1, 5th ed., pp. 577–608. New York: van Nostrand. Google Scholar
Wulff, P. & Heigl, A. (1931). Methodisches zur Dichtebestimmung fester Stoffe, insbesondere anorganischer Salze. Z. Phys. Chem. Abt. A, 153, 187–209. Google Scholar
Wunderlich, J. A. (1957). Un méthode rapide pour mésurer la densité d'un cristal. Acta Cryst. 10, 433–434.Google Scholar