METHODS FOR MEASURING OSMOTIC PRESSURE
and REVERSE OSMOSIS

Osmotic Pressure via an Internal Measurement Mechanism
Normallly when doing dialysis one fills the tube as full as possible so as to have the greatest surface are for osmosis. The process will thus occur as rapidly as possible. Afterall, osmosis is not an overly fast process in the first place!
When the contents are hypertonic, the tube becomes pressurized. In order to measure that osmotic pressure, or turgor pressure, one is wont to think about affixing a manometer or other pressure gauge to the system. But how can one do that and yet maintain of tight seal that can withstand the expected pressures? Thus we move from science to technology!
However, if one were to fill the tube only one third full with a large balloon of trapped air above it, a barometer of sorts is created because that air is compressible (see left figure to the right). The higher the osmolarity, the more water will try to flow into the tube, the liquid volume expands and that compresses the trapped air. All one needs to do is note the amount of that compression, which is quite easy since the tube is a cylinder and ruler measurements are not difficult to make.
So much for the theory of this technology. Let us now turn to making it happen - the tricks of the trade, as it were.
1. Make up a hypertonic solution. Table sugar is inexpensive and can form 40% solutions. However, because it is a small molecule, it will slowly drop in concentration as osmosis continues. It would be better to use something like PEG (polyethylene glycol; aka "carbowax") having such a high molecular that it cannot escape the tubing.
2. How to entrap so much air in the tubing. After pouring in the desired amount of hypertonic solution, blow a stream of air down into the tube, and then close the very mouth of the tube. By spinning the tube around by the bottom end, a twist in the tubing works its way down such that the trapped air fills out the "balloon." Do not twist so much as to start compressing the air. All you want is to have the air take the wrinkles out of the tube. Now tie your knot and work that knot down to bottom of the twist, and pull the knot tight.
3. The tube at this time might still be a bit limp. Using a few turns of a string around and around the tube well below the meniscus, tighten the string so the tube is just barely rigid, and tie the string.
4. It helps to keep the whole dialysis "sausage" submerged so that the upper part does not dry out and become brittle. There are two ways to keep the "sausage" totally submerged:
  1. tie two heavy lead fishing "sinkers" to each end of the sausage
  2. take a long container (perhaps a flower vase), fill it with water by submerging it horizontally in a bucket and then inserting the sausage into it. Now lift the bottom of the "vase" and stand it up in the bucket. You will see the trapped sausage bumping its "head" upon the bottom of the completely filled vase.
Your system is ready to use, after you make two measurements:
1. In millimeters, measure and record the distance between the top and bottom knots in the tubing. This measurement will later be used to adjust for the elasticity of the tubing as pressure increases and the tubing stretches.
2. In millimeters, measure and record the distance between the top knot and the meniscus. This measurement corresponds to the starting volume of the trapped air at one atmosphere of pressure.
Using this simple apparatus, there are now two approaches to determining osmotic pressure.
1. One is simply to immerse the tube in water and follow the compression of the trapped air (above figure). The problem with this is that the concentration of the liquid inside the tubing is continuously being diluted with water as the run proceeds. Only when the compression ceases can one then determine to concentration of solute that brought about that amount of compression. Often the solutes have no simple method for quantitation.
2. Another approach (right) is to compress the trapped air a known amount by adding more belts around the bottom portion of the tubing (below the meniscus). When this pressurized system is immersed in water, and the meniscus neither rises nor falls, then the osmotic pressure is known WITHOUT changing the concentration. But this will require a number of trials, as one might try a series of set-ups each at a different pre-pressurization. Then one looks for the tube with "no change."
Calculation:
Pressure = (T_o/T_x) x (B_o/B_x) x 14.2 psi
Atm pressure x 14.2 = psi (pounds per square inch)

REVERSE OSMOSIS

This section is under contruction.