poises. The solute was tagged with the isotope sodium-22.The transient profile in the diaphragm, which is predicted by the solution of the appropriate equations, results in a flux that was monitored using a scintillation detector and an electronic scaler. The difhsivity of this high-viscosity system can be measured in a matter of hours with this technique. Figure A diagram of the experimental setup appears in general nature of the setup is a diaphragm with side from which liquid is drawn out, circulated through the counting chamber, and returned to the cell. The cell is a Pyrex fine frit with a porosity of 4 to 5p. The total volume of the circulation system including the upper reservoir is 250 CC. DESCRIPTION OF APPARATUSThe mixing apparatus, in conjunction with the circulation of the liquid, rapidly disperses the solute throughout the system as it enters the upper reservoir. The Paddle on the mixing rod, which turns at about 1 rev./sec., is placed as dose to the diaphragm as possible. This prevents a concentration buildup in a stagnant layer above the diaphragm.The counting chamber (about 20 cc.) is streamlined to prevent stagnant liquid pockets from being formed. The disk shape of the glass chamber results in a good geometric efficiency for a maximum counting rate. A lead shield houses the counting chamber to lower the background count and thus increase the sensitivity of tracer detection.A thermostatically controlled heater and a continuously operating fan maintained the constant-temperature Plexiglas box at 32 e 0.5"C. A desiccant was placed in the box, and all penetrations were sealed. The peristaltic pump is of the pulsating positive displacement type with capacities from 0.5 to 250 cc./min.A scintillation counter was used. This consisted of a 1.25-in. thallium-activated sodium iodide crystal, a photomultiplier tube, preamplifier, an amplifier, single-channel analyzer and a scaler. The analyzer threshold was set high enough to suppress noise yet low enough to record the gammas following the decay of sodium-22. A second, shielded scintillation unit was used to continually monitor the background while the main counter was PROPERTIES OF TRACER SOLUTIONThe selection of the sodium-22 isotope in the form of sodium chloride was based on its hard gamma emission, known difFusivity in water, ability to dissociate completely in glycerol at a concentration of 1.25 X loT4 normal, and a long half-life (2.7 yr.) . PROCEDUREFor standardization, the prepared aqueous solution is pumped into the lower reservoir until it contacts and rises into the diaphragm. Bubbles are removed from under the diaphragm through the side arm. AU tubing and the counting chamber in the circulation system are initially filled with distilled water. Water is then added to the upper reservoir to bring the total of the circulation system to 250 cc. Circulation and mixing are started immediately.The glycerine run is similar. When filling the lower reservoir, care must be used to get as little as possible of the glycerol solution through the diaphragm...