Stokes nitrogen density measurements

A dispersion of the sample is placed on top of a liq of greater density. The rate of sedimentation is detd by measuring the sediment vol at fixed time intervals. The results are converted to a size distribution by Stoke s Law Nitrogen Adsorption. The amt of N adsorbed on a sample is detd by carefully measuring the press change of a known vol of N exposed to a known wt of dry mat at constant temp. The info is used to detn the surface area which is converted to a particle size distribution Turbidometric Methods. The absorption of a beam of light passing thru a suspended sample in a suitable liq is measured as a function of time. 

This equation defines the Stokes diameter Xstokes on the basis of the density contrast Ap = ps - pm between solid and fluid phase. It should not be confused with the density contrast Ap between the porous particle phase (with Pp.etr) and the fluid. Such a definition might be meaningful for dense agglomerates with defined porosity, which can be measured by nitrogen adsorption (Staiger et al. 2002), or may be deduced from the powder bulk density (Barthel et al. 1998) or from sediment properties (Katzel 2007, pp. 183-187). However, it should be avoided for fractal aggregates, where the concept of uniform porosity fails for several reasons (no defined surface, size-dependent and locally varying porosity). 

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