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Organic colloids must have high molecular weights; thus, gum arabic, although possessing the empirical formula C12H22O11 is acidic, and the very small amount of base required for its neutralisation shows that its molecule is much more complex: (C12H22O11)7. By-the method of depression of freezing point applied to colloidal solutions, high molecular weights have also been found: starch, 25,000: tannin, 1100; silicic acid, 49,000; rubber (in benzene), 6500. The slowness of diffusion and dialysis is readily understood when one considers that with such enormous molecules (often ultra-microscopically visible) the molecular movement must be very slow, since the square of the velocity is inversely proportional to the molecular weight. The osmotic pressures of colloidal solutions are, as would be expected from the large molecular weights, very small but they appear to be definite. Pfeffer obtained the following values with i per cent, aqueous solutions:
| Pressure cm. Hg. | Molecular weight | | Potassium nitrate | 178 | - | | Cane-sugar | 47 | 342 | | Dextrin | 16.5 | 975 | | Gum arabic | 7.2 | 2230 [(C12H22O11)7 = 2394] |
Since the molecular weights are inversely proportional to the osmotic pressures (except in the case of potassium nitrate, which is an electrolyte and is abnormal), the figures in the third column may be calculated from the osmotic pressures and the molecular weight of cane-sugar, 342.
Linebarger (1892), using a parchment-paper membrane, found the molecular weight of colloidal tungstic acid by the osmotic method to be 1720, which corresponds with (H2WO4)7 = 1750.
Graham's suggestion that colloids as a class have high molecular weights, and complex molecules, possibly formed by the association of a number of crystalloid molecules (e.g., in the case of tungstic acid), has therefore been generally confirmed.
The molecular weights of colloids have also been determined from the rate of diffusion; the latter is inversely proportional to the square-root of the molecular weight. In this way Herzog (1908) found the molecular weight of albumin to be 17,000; Sabanejeff and Alexandroff found 13,000-14,000 by the freezing-point method; Sorensen found 34,000 by the osmotic pressure method, and this value was confirmed by Svedberg, using a centrifugal sedimentation method. Svedberg found 375,000 as the molecular weight of casein. The satisfactory agreement between modern results obtained by different methods seems to indicate that colloids possess definite molecular weights, which may vary with the method of preparation.
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