Diffusion measurement water using method

As with diffusion coefficients in air, a referencing method can be used to estimate the diffusivities in water using a reference compound when measured values for the... 

Water Transport. Two methods of measuring water-vapor transmission rates (WVTR) ate commonly used. The newer method uses a Permatran-W (Modem Controls, Inc.). In this method a film sample is clamped over a saturated salt solution, which generates the desired humidity. Dry air sweeps past the other side of the film and past an infrared detector, which measures the water concentration in the gas. For a caUbrated flow rate of air, the rate of water addition can be calculated from the observed concentration in the sweep gas. From the steady-state rate, the WVTR can be calculated. In principle, the diffusion coefficient could be deterrnined by the method outlined in the previous section. However, only the steady-state region of the response is serviceable. Many different salt solutions can be used to make measurements at selected humidity differences however, in practice,... 

An infrequently used method (in pharmaceutical research) for determining the UWL permeability involves measuring transport of molecules across a high-porosity microfilter that is not coated by a lipid. The molecules are able to diffuse freely in the water channels of the microfilter. The filter barrier prevents convective mixing between the donor and acceptor sides, and an UWL forms on each sides of the microfilter. Camenisch et al. [546] measured the effective permeabilities of a series of drug molecules in 96-well microtiter plate-filterplate (Millipore GVHP mixed cellulose ester, 0.22 pm pore) sandwich where the filters were not coated by a lipid. The permeabilities were nearly the same for all the molecules, as shown in Fig. 7.8a. Our analysis of their data, Fig. 7.8b, indicates / aq = 460 pm (sandwich stirred at 150 rpm). We have been able to confirm similar results in our laboratory with different microfilters, using the lipid-free method. 

The most numerous researches have been carried out upon water-membrane systems, for which a summarising paper by Carson (1) has outlined the main methods of measurement. To prevent lateral diffusion of water through the edges of the specimen, or between it and its supports, various devices have been adopted. The edges have been moulded in wax, or compressed with a mixture of beeswax and resin. Shellac, petrolatum, and rubber have been used for the same purpose or the cell may be mercury sealed. Films of lacquers and paints may be made by painting the lacquer on suitable surfaces, such as amalgamated tin-plate (2), and then peeling off the dried membrane. 

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