= Membrane osmometer
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A <membrane osmometer> connects <osmosis> to a visible liquid level or a measured balancing <pressure>. In a teaching version, a <solute>-containing pouch is joined to a narrow vertical tube and submerged in external <water>. Entry increases the enclosed <volume> and raises the meniscus. Both the column and any stretched pouch can contribute an opposing <pressure>.
The University of Toronto's <Osmosis (Brine) by University of Toronto Department of Physics> demonstration (activity Me-012b, Description, steps 1–14) specifies soaking a dialysis membrane tube, tying one end closed and fastening the other around a thistle tube's narrow end. A stand holds the tube. Colored brine is cooled to room <temperature> and injected using a syringe to reduce bubbles; the pouch is completely immersed in a $1$ or $2\,\mathrm L$ beaker of distilled <water>. A taped marker records the initial level; subsequent meniscus height is the readout. The protocol proposes another pouch containing distilled <water> as a comparison, and its older instructions explicitly check seals for leaks.<Osmosis (Brine) by University of Toronto Department of Physics>[Osmosis (Brine), Description steps 1–14, water-filled comparison and Old Description A.2: apparatus, readout and leak check]{ref}
A rise is consistent with net <water> entry. To infer <osmotic pressure> quantitatively, use a retained <solute> and establish a stable equilibrium after accounting for <dilution>. If both free surfaces are at the same atmospheric <pressure>, the liquids have approximately equal <mass densities> $\rho$, and the pouch adds negligible mechanical load, the <hydrostatic pressure> difference at membrane height is approximately $\Delta P=\rho gh$, where $h$ is the meniscus height above the external surface and $g$ is <gravitational acceleration>. This is the liquid-column inference, not a pressure calibration supplied by the Toronto demonstration. For unequal <mass densities>, compare the two column pressures separately.
Useful additional controls are matched initial levels and geometry, constant <temperature>, replicate runs, and monitoring external <solute> to test retention. These are proposed checks, not reported Toronto results. Leaks, trapped bubbles, <surface tension> in the narrow tube, <evaporation> and membrane stretching can alter the level. In particular, a dialysis membrane may pass dissolved salt: the brine setup illustrates transient movement but cannot be assumed to retain every dissolved <ion>. Color improves visibility without proving salt retention. A stationary level can reflect weakening <concentration> differences or leakage rather than osmotic <thermodynamic equilibrium>; a rising level alone supplies neither $n$ nor a verified value of $nRT/V$.
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