Tables for
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 3.2, p. 158

Section Flotation method

F. M. Richardsa Flotation method

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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[link]).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[link]). The method is reported as capable of a probable accuracy as great as 0.02%.

The compounds listed in Table[link] 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[link]). 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[link]; Wunderlich, 1957[link]).


Bernal, J. D. & Crowfoot, D. (1934). Use of the centrifuge in determining the density of small crystals. Nature (London), 134, 809–810.
Midgley, H. G. (1951). A quick method of determining the density of liquid mixtures. Acta Cryst. 4, 565.
Reilly, J. & Rae, W. N. (1954). Physico-chemical methods, Vol. 1, 5th ed., pp. 577–608. New York: van Nostrand.
Wulff, P. & Heigl, A. (1931). Methodisches zur Dichtebestimmung fester Stoffe, insbesondere anorganischer Salze. Z. Phys. Chem. Abt. A, 153, 187–209.
Wunderlich, J. A. (1957). Un méthode rapide pour mésurer la densité d'un cristal. Acta Cryst. 10, 433–434.

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