The porous diaphragm method of measuring diffusion velocity, and the velocity of diffusion of potassium chloride in water

The velocity of diffusion of substances in dilute solution is a very informative property, but one of which it has been possible in the past to make but little use, on account of the very low accuracy of all but the most extended and laborious experiments, as is well seen in the collection of data KCl in water from various authors plotted by McBain and Dawson (1935). The chief experimental difficulty has been the elimination of the convection which will be produced to some extent by any temperature fluctuations or vibration however small, and particularly by the process of dividing up the diffusing solution when this necessary for analysis. Convection is least serious when concentrated solution of a heavy solute are examined, because the large density gradient has a stabilizing influence: it is also less serious the more rapidly the solute diffuse. Unfortunately, however, there is as yet no satisfactory method of interpreting results in concentrated solutions, and the method of diffusion is most in demand for slow diffusing colloidal solutes. The technique, introduced by Northrop and Anson (1929), and later applied by several other workers (McBain and Liu 1931; Laing McBain 1933; McBain and Dawson 1935; Valkó 1935; Cole and Gordon 1936; Mouquin and Cathcart 1935), of allowing the diffusion to take place only within the diaphragm is almost completely eliminated and is generally utilized outside the diaphragm to maintain the two outer reservoirs of solution uniform in composition. As a consequence of this uniformity we have an approximately steady state in the diaphragm. The method therefore includes the advantage in mathematical interpretation of the steady state method developed by Clack (1908-24), namely, that the rate of diffusion across the diaphragm is an explicit function of the concentrationc and the diffusion coefficientD for each concentration, being equal tok ∫c c 'D dc (1) wherec' andc are the concentrations in the reservoirs andk is a constant of the dimensions of length. In Clack's apparatusk is the ratio of cross-sectional area to length of the diffusion tube. In the diaphragm apparatus the value ofk can only be determined by calibration with a solute of known diffusion coefficient.