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NOTES ON DYNAMIC OSMOMETRY K. B. GOLDBLUM PZastica Laboratory, General Electric Company, PittsfLeZd, Massachusetts Received August $6, 1946
During considerable work on the determination of molecular weights by the dynamic osmometric method of Fuoss and Mead (l),a few peculiarities of the osmometer itself as well as the need for thermostatting the instrument were observed. The peculiarities and means for eliminating their effects are given below as helpful hints for those who might be troubled with the same di5culties. PLATE TEIICKNESS
The early osmometers used in the laboratory were built from +in. stainlesssteel plates. It was found that with the partial bolt circle of the Fuoss and Mead design, a bending of the plates could be detected when the bolts were tightened. Under this condition the membrane was free to move perceptibly, with the result that the determinations were uncertain. The changes suggested to correct this condition are the following: ( 1 ) to make the instrument fromplates a t least 1in. thick; (8) to design a complete bolt circle to insure uniform pressure on the membrane. However, from work with an osmometer made of heavier plate (lt in. thick), it has been found that the complete bolt circle is desirable but not absolutely necessary. VALVE AND STEM MATERIAL
The material making up the valve blocks and the stems in the early osmometers was brass for the blocks and stainless steel for the stems. Much difficulty was experienced in keeping this combination leak-tight. It was found that the brass seats were deformed easily and that good closure was almost impossible. A much better combination was obtained by the use of hardened tool steel for the blocks and by continuing the use of stainless-steel valve stems. The valve seats were ground and lapped, with the result that tight valves were realized. STAND PIPES
The fragility and frequent breakage of the glass stand pipes on the side arms of the osmometer were aggravating factors. The replacement of the glass with stainless-steel pipe spun to the same dimensions solved this problem. CONSTANT TEMPER.4TURE
It had been found that temperature control was one of the major problems in osmometric measurements. The need for holding a particular temperature within small limits is of twofold importance. In the first place, the temperature effect on ther/C us. C curve (K is the osmotic pressure and Cis the concentration) is quite marked, especially where the degree of solvation of the polymer is high and quite temperature dependent. In this case, the u / C us. C curve is not hori-
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zontal but has a positive slope. This slope will vary with temperature. Thus if the temperature of the instrument is different each time that measurements are made on the same solution, the T / C values will be different. So, in order to obtain a curve that can be extrapolated to a value of ( T / C ) Owhich will have some significance, it is necessary to make all measurements a t the same temperature. RAKGE O F TEMPERATURE VARIATIOS
The second point is the need to hold the constant temperature within certain small limits. The closed side of the osmometer acts as a pycnometer to temperature changes. These slight volume changes are reflected, highly magnified, in the movement of the level of the liquid in the closed-side capillary tube. Diffusion of the solvent through the membrane tends to cancel part of the change and restore the equilibrium conditions. However, if the temperature drift is rapid, the diffusion process is too slow to take care of it. Data were collected on a thermostatted osmometer in which the water-bath temperature was regulated to a range of 0.045"C. a t 30°C. and in which the heater was on for 6 min. and off for 6 min. These data showed that the closed-side capillary level varied about 1.2 mm. over the 0.045"C. range. The accuracy of reading the capillary levels was about in the range of 0.1 to 0.2 mm. Thus a temperature control within a range of 0.004°C. was indicated. MEANS OF THERMOSTA'ITING THE OSMOMETER
Sside from the necessity of holding a constant temperature within narrow limits, there were several mechanical considerations in the design of a thermostatted osmometer. The direct immersion of the osmometer in a bath was rejected, because of the necessity for a complicated system for the side arms through the walls of the bath container as well as the fact that each membrane change, when necessary, would be a time-consuming operation. The use of a water jacket covering each plate of the instrument seemed to be a solution that would be practical. The jackets could be designed so that the membrane could be changed without disturbing the water jackets and vice versa. OSMOMETER BODY DESIGN
Such an osmometer was designed (see figures 1 and 2). The body of the osmometer has several modifications of the Fuoss and Mead design. The full bolt circle which was displaced 22.5" has been already discussed. The other change concerned the welding on of a block of metal for the side-arm connections. This was done to allow more of the plate to be covered by the water jacket. This change necessitated the drilling of the outlet to the side arm a t an angle similar to that for the outlet to the capillary tube above. WATER JACKET DESIGN
The water jacket was designed (see figure 3) so that an area larger than that of the working area of the interior of the instrument was covered on the outer plate. The jacket had two sets of bolt holes around the periphery. The larger set
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corresponded to the bolts holding the osmometer plates together and was drilled large enough so that the heads of these (Allen) bolts go through freely. The smaller set of holes was for the bolts that hold the jacket and its gasket to each plate. Thus, if the membrane needs attention, it can be reached by unloosening the large bolts without interfering with the water jacket. Conversely, if one or
FIG.1. Osmometer plate
the other of the water jackets needs repair, it may be fixed without disturbing the membrane. In order to minimize the effect of changes in room temperature, the jacketed osmometer was packed in waste in a wooden box. This box had slots cut in the sides to allow for the side arms. The cover had holes and slots for the thermometer, capillaries, and the rubber tubing through which the circulating water was conducted.
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GUIDE PINS
The original osmometer design showed guide pins to assist in putting the two plates together without cutting the membrane. In the modified design there
FIG.2. Section of osmometer plate through A-A
FIG.3. Water jacket plate
was no space on the bolt circle to have guide pins without weakening the construction. Therefore, two pins were made of steel threaded a t the ends to fit into the tapped holes for the large bolts. The shanks of these pins were made without heads and with rounded tips so that they could be screwed into holes in the bolt ring 180' apart and used as guide pins. After several of the large bolts were tightened in place, these pins could be removed and the bolts put in their places.
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K. B. GOLDBLUM CONSTANT-TEMPERATURE BATH AND CONTROL
A large Pyrex jar 12 in. in diameter and 18 in. high filled with distilled water containing a slightly alkaline sodium chromate solution to minimize corrosionwaa
0. I 2
3 4 5 6
7 8 SCALE IN INCHES
FIG.4. Glass thermoregulator
HEATING
LOAD
FIG.5 . Thyratron control circuit set up with a laboratory-size Lightnin Mixer for good agitation. The circulation to the osmometer jackets was maintained by a small Eastern Laboratories centrifugal pump. The bath was covered with a %-in.Textolite panel, suitably
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cut out for the various connections. This minimized evaporation and helped to maintain constant conditions. The temperature control was maintained by means of a specially designed Pyrex-glass thermoregulator (see figure 4),which can be built by a glassblower. Contact in the capillary was obtained through a 0.010 in. tungsten wire mounted on a threaded screw for ease in positioning. The other contact is through the platinum wire shown in the side arm. In conjunction with the thermoregulator, a thyratron tube control circuit' was used (see figure 5). The tube was a GE FG 57 thyratron tube. The filament transformer can be a 5-volt or 6.3-volt filament transformer. The one actually used was a Utah Radio Products 6.3volt make. The battery used xas a No. 5156 Burgess 45-volt type with a center tap to giw the plus and minus 22.5 volts. The resistors and capacitor were of the conventional radio type. In figure 5, A, G, C, and F are the anode, grid, cathode, and filament, respectively, of the tube. The thermoregulator is denoted by T, and the short-circuit switch to protect the tube during the warm-up period (300 sec.) is shot1-n as S. A Cenco 125-watt knife-edge heater in the bath in series with a variable resistance was connected to the load side of the thyratron circuit to furnish the necessary heat. The bath v a s maintained a t 30.00"C. on a thermometer graduated in 0.1OC. and calibrated by the National Bureau of Standards. The choice of 30.00"C.mas made so that, except in rare instances in the summer months when room temperature might exceed this, no internal cooling coils would be necessary. SUMMARY
The complete installation as outlined above has functioned satisfactorily. The temperature control exceeded expectation; the range found was 0.002"C. Many difficulties of osmometry that plague the operator in making determinations have been removed or their effect has been minimized. REFERENCE (1) Fuoss, R. M., AND MEAD,D. J.: J. Phys. Chem. 47,59 (1943). 1 Modeled after data in a written communication from Dr. D. J. Mead, General Electric Research Laboratory, Schenectady, New York.
