Complete Apparatus for Determination of Osmotic Molecular Weights GEORGE D. SANDS AND B. L. JOHNSON Chemicd and Physical Research Laboratories, The Firestone Tire and Rubber Company, Akron, Ohio The procedure and the complete apparatus for measuring molecular weights are described. and illustrated in sufficient detail to make possible constructing the equipment. By using a dozen of the static osmometers simultaneously, a complete molecular weight determination can be made in 1 to 4 days. Representative data for standard government specification GR-S polymers and for samples of German Buna S-3 subjected to various treatments illustrate the results obtained. Reproducibility of osmotic heights is of the order of 5%.
A
NUMBER of devices for determining osmotic molecular
weights are described in the literature (4, 3, 7-10, 14, 13). Those employing valves, glass to metal seals, etc., not only require special techniques and a great deal of care in fabrication, but are susceptible to leaks and trapping of air because of their complicated nature. Some of the glass osmometers ( I f ? ) are of simple design and are relatively easy to make, but are, of course, less rugged than metal instruments. If breakage is avoided by cementing (5,13)instead of clamping the membrane to the osmometer, it is difficult to keep the membrane permeable while the cement is drying. In an effort to utilize the greatest number of edvantages of the osmometers which have been described previously and to produce an instrument which might be of more general use to those who have never had experience in the tech-
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0
I
CM.
Figure 1. Assembled Osmometer
niques of osmotic pressure, a metal osmometer body and clamp were made which permit the membrane to be manipulated under solvent a t all times. The design is of such a simple nature that the instruments can be made in any machine shop, since no special techniques in fabrication are required. The osmometers are very easy to use, and leaks are not often encountered. Since these osmometers are small and require little attention after a run is started, a dozen can be used simultaneously in a small cylindrical thermostat (22-liter) to reduce the total time required to make a complete molecular weight determination. OSMOTIC PRESSURE APPARATUS
Osmometers. A drawing of an osmometer is given in Figure 1 I t consists of an osmometer body, a membrane clamp, and a measuring capillary sealed to a female 10/30 standard-taper ground-glass joint. This female portion fits the male section of the joint which comprises part of the osmometer body. The body and membrane clamp are made of No. 416 stainless steel; brass is unsatisfactory because it catalyzes the decomposition of elastomer solutions. The osmometer body, except for the mercury cup, is turned OD the lathe from one piece of 4.45-cm. (1.75-inch) diameter stainless steel rod; the cup is turned separately and press-fitted into place. The male standard taper (0.1 taper) is cut in the lathe; the fit between the metal and glass halves of the joint is general1 good enough to obviate grinding the two surfaces together. TKe two surfaces adjacent to the membrane and the male taper are finished with a very fine lathe cut, and then polished in the lathe with fine emery cloth, care being taken to make all polishing peripheral and not radial. Brass stud bolts (three bolts spaced a t 120") and nuts were used, since they do not come into contact with the polymer solution. A measuring capillary is made by sealing a 50-cm. length of Pyrex capillary tubing (approximately 1-mm. bore) to a 10/30 joint, and attaching hooks for springs 7 cm. from the open end of the joint. A 15-em. length is cut off the end of the capillary t a which the joint is to be sealed; the short length of tubing serve% as the reference capillary. Identifying numbers are scratched with a Carborundum crystal on the measuring and reference capillaries, so that they can be matched properly when in use. Ta hold the reference capillary i n place, four turns of rather heavy steel spring are used (diameter of spring, 16 to 18 mm.; diameter of wire, about 1.5 mm.; distance between turns, 5 to 7 mm.). The measuring capillary is slipped between the first and second turns of the spring, and the reference capillary is lodged between the third and fourth turns, so that the capillaries are perpendicular to the axis of the spring and parallel to each other (see Figure 2). A spring of the above dimensions contacts each capillary a t two points, holding the reference capillary firmly to the measuring capillary, while giving an unobstructed view of both capillariee over their entire lengths. Membranes. Membranes were prepared according to t h e directions given by Wagner (14). Undried regenerated celluloee film 0.016 mm. (0.004 inch, wet thickness) was obtained from Sylvania Industrial Corp., Fredericksburg, Va., and stored until used in a weak formaldehyde solution which protects the film from bacterial attack. To condition the membranes, 10-cm. (4-inch' 261
262
ANALYTICAL CHEMISTRY
vant (11). It utilizes a rhyrrtton tube, tieneral J5leC%rlcI L z 3 1 , with the grid potential controlled hy the mercury-in-glass thermoregulator which contains 50 to 75 ml. of mercury. Since the tube is eapprtble of carrying an average current of 2.5 amperes dl the heatine current is uassed directly through the tube, elirkninating
.....
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..
the cabinet (see Figure 2): A 1 W w s t t incandescent limp, covered with copper fail to block the light, supplies adequrtt; energ. to maintain the bath a t the operating temperature of 30.2 * 0.005" C. Stirring is provided by a Ceneo cone drive stirrer (Cataloe No. 18.805) with a metal shaft and propeller
Figure 2.
Apparatus in Operation
pasition over the bolts. The osmometer is Bled immediately with toluene to keep the membrane thoroughly wet. The three nuts are tightened successively very gradually, first with the .. - . ~ ~
PREPARATION OF POLYMER SAMPLES
~~~~
Since even small percentages of extremely low molecular weight material can give misleading number average molecular weight . , >> ./ I . ~ values, these small moLeCuLeS are enrn3naLeLI oy & preclpm.aon from mixed solvents.
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1
1
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.
~
Six g r a m of polymer are cut into 1 X 1 X 3 mm. strips, placed in ahout 1liter of toluene, and allowed t o stand a t least 48 hours. The solution is then poured through several layeyers of cheesecloth t o remove gel. This method of removing gel is necessary to obtain conveniently sufficient amounts of polymer for osmotic pressure work, although the possibility of including dispersed gel in the sal is greater than is the case when a screen-rack type of cell (8).is used for sf"all amounts of polymer, Since the molecular weight of such dispersed gel IS very large, I t would he expected to cause hut little error. The total volume of the filtrate is measured, its "inherent" (4) viscosity is determined in an Ostwald viscometer, and its eoneentration is evaluated. The precipitation is carried out hy the addition of a volume of methanol equivalent to one half the volume of the filtrate. While the solution is being stirred mechsniedly, the methanol containing 10 mg. of sodium iodide is added dropwise from a separatory funnel. The electrolyte is added to cause more rapid agglomeration of the polymer. The precipitated polymer is allowed to settle severil hours, the supernatant liquid is decanted, and its volume and concentration are determined. The residue is washed quickly with a portion of toluene, and then ahout 800 ml. of toluene are added and allowed to stand at least 48 hours t o take the polymer hspk into solution. Since osmotic rises are measured on this solntipn and on 0.75, 0.50, and 0.25 dilutions, its concentration and inherent viscosity are carefully measured. To evaluate all concentrations mentioned above, 25 ml. of the solution are evaporated t o dryness (8 to 12 hours) in 9.n oven -..sir ..~ ~ a t 70" C. Heating a t 70" C. is continued until the residue reaches c(mstant weight ~
UREMENT OF OSMOTIC PRESSURE
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a wad of absorbent cotton wet with toluene is placed in thecavity underneath and held in place with a holder made from a coil of heavy copper wire and two small springs which hook over the lip of the mercury cup. The osmometer is emptied and rinsed rapidly three times with solution, and then the osmometer is filled and the mercnrv CUD is Dartiallv filled with solution. A measur......
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the osmotic rise expeoted, so that the'ieverin the measuring capillary will he a t a convenient height. The capillary is seated firmly, and the retaining springs are applied. The outside of the osmometer is rinsed thoroughly with pure toluene tlnd mercury is poured into the cup. Reagent grade toluene or once-distilled commercial toluene which has been refluxed 2 hours with phosphorus pentoxide and redistilled may be used as solvent. The reference capillary is attached by. mema of % spring described above, and the whole is immersed in 8. tube of toluene, after the cotton wad has been removed. The ?ir trapped an the under side of the membrane is removed hy tilting the tube at ahout 45" and then moving the osmometer up and down &her rapidly. It must he remembered thst the membrane 1s to he exposed to air a minimum length of time; therefore the osmometer is kept in a flat dish containing toluene ahout 1 cm. deep during most of the ~~~~~~
~~
lary through the notched cork which closes'the soivent tibe. Since four osmometers are generally filled with the same solution, i t is desirahle to set two of the levels about 0.5 cm. above the expected rise, and the other two ahout the same distance below. giving an approach t o equilibrium from both sides. The osmotic rise is read two or three times a day until the level is essentially constant over several hours. Time for equilibrium to he established may vary from 1 to 4 days, depending on the permeability of the membrane and the proximity to the equilihrium value with which the level WBS set originaliy. After equilihrium has been reached for one solution, the osmometers may be rinsed and reused; the same membrane may he used for several
V O L U M E 19, N O . 4, A P R I L 1 9 4 7 7.0
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263
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R = 0.08205 liter atmosphere degree-' mole-' W = concentration in grams per liter T = absolute temperature of thermostat
1
v
60 50
For a temperature of 30.2" C., and when concentrations are expressed in grams per 100 ml., Equation 1 may be developed as follows:
40
s'
2
30
w
M
20
I O
1 M =
(c
x
-
T'
1 X-RT H
1
10)
(0.08205) (303.4)
(76.0) (13.60) Concentration, grama per 100 d.
Figure 3.
1
(0.08205) (303.4)
Osmotic Rise as a Function of Concentration
(76.0) (13.60)
K
=
3.004 X 10'
TYPICAL RESULTS ON POLYMERS
Concentration, grama per 100 ml.
Figure 4.
Extrapolation to Zero Concentration
weeks, after which time it appears to become somewhat impermeable. Measuring capillaries and the corresponding reference capillaries when immersed in toluene should give capillary rises xhich agree within 0.02 em. before they can be used without correction. Calculation of molecular weight values from the osmotic rise data is carried out as follows: The values of h in centimeters of toluene (direct cathetometer measurement) for the four different dilutions are plotted against concentration in grams per 100 ml., and a smooth curve is drawn through these four points and the origin; Figure 3 shows a typical h vs. c curve. Values of h a t 10 or 12 concentrations are read off the curve, h and these values are used to plot an - us. c curve. Extrapolation of the second plot (Figure 4) t o zero concentration gives the constant, A , from which the molecular weight is obtained directly by substitution into the equation:
M.W. =
Data on German Buna S-3 obtained by use of the described method appear in Table I. This material is a 70130 butadienestyrene copolymer (14) prepared using Xekal emulsifier and 0.065 to 0.090% Diproxid modifier, which was added in three equal batches a t the first, third, and fifth of eight reactors. Sample Buna 5-3 in Table I is the sol portion of the polymer. Heat softening to the point of maximum intrinsic viscosity with complete solubility was effected by heating strips 2 X 2 X 100 mm. in a 135" * 1' C. circulating air oven for 35 minutes. The milled sample was subjected to 60 passes on a cold hand-tight mill, which was sufficient to break down the gel completely. Heat softening reduces the molecular weight of the polymer to 98,200, which is lower than that of the unheated sol portion (166,000). Reduction of gel by milling produces a material with a number average molecular weight higher (224,000) than the sol portion (166,000). This effect indicates that the gel has been converted into soluble material of high molecular weight ( I ) '
Table I.
hfolecular Weights of Representative Polymers
Polymer Per cent sol Viscosity before precipitation Viscosity after precipitation % polymer in mixed solvents (low molecular weigh:) Concentratlon of dilution 1 , grams Per 100 ml. Osmotic height, cm. Dilution 1
K A
Dilution 0.75
(f);
where A = and K is defined below. This relation is a simplification of the van't Hoff law:
aV where
"at*.
=
W -
M
=
RT
Dilution 0.50
( 1) Dilution 0.25
hd (76.0) (13.60)
d = density in grams per ml. of toluene a t temperature of thermostat h = osmotic rise in cm. of toluene M = molecular weight
HeatSoftened Milled Standard GR-S Buna S-3 Buna 5-3 Buna 5-3 A B 95.5 89.0 75.8 93.2 94.7 2.29
2.29
2.31
2.60
2.68
2.67
2.54
2.05
2.80
2.22
7.9
7.5
4.3
0.730
0.846
0.856
0.505
0.506
3.56 3.61 3.52 3.60 2.22 2.29 2.26 2.29 1.16 1.17 1.15 1.19 00 .. 44 08
5.54 5.54 5.64 5 63 3.47 3.58 3.51 3.47 1.94 1.95 1.95
4.55 4.47 4.42 4.39 2.59 2.63 2.58
2.40 2.31 2.45
2.38 2.25 2.29
1.58 1.65 1.55
1.57 1.57 1.54
1.34 1.36 1.29 1.33 0.48 0.48 0.47
0.89 0.94
0.90 0.91 0.86
...
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K hIolecular weight
17.4
...
...
...
...
...
-
..
0.91 0.37 0.88 0.41 0.45 0.87 0.38 0.88 1 81 3 06 1 34 2 60 2 55 3 004 X 3 004 X 3 004 X 3 004 X 3 004 X 105 103 105 105 105 166,000 98,200 224,000 115,000 116,000
...
A
10.7
ANALYTICAL CHEMISTRY
264 probably with little change in the sol portion.
ACKNOWLEDGMENT
In contrast to
this effect, natural rubber milled to the same extent has a lower molecular weight than the untreated sol, probably because milling breaks down both sol and gel. Two separate samples taken from the same piece of government specification GR-S (Table I, standard GR-S, A and B ) were carried through the complete osmotic pressure measurement procedure to establish the reproducibility of results. The last two columns of Table I show the precision obtained. One determination gave a molecular weight of 115,000 and the second yielded a value of 116,000. While these results are undoubtedly fortuitous, in view of the many possible sources of error, they indicate that good precision can be expected. SUMMARY AND CONCLUSIONS
Osmotic pressure apparatas and technique have been developed which can be used to determine the number average molecular weights of polymers in the range 50,000 to 500,000. Simplicity of construction and over-all convenience in use, combined with reliability of results, are the outstanding advantages. Reproducibility of the results obtained (of the order of 5 % ) is best judged from the osmotic rise values of the four different measurements for each of the four concentrations given in Table I. The molecular weights of German Buna S-3 and GR-S treated in various ways are reported: Buna S-3 sol, 166,000; heatsoftened Buna 5-3, 98,200; milled whole Buna S-3, 224,000; GR-S, A 115,000; and B 116,000.
The authors wish to thank F. R. Stavely for his continued interest and suggestions throughout this work. Grateful acknowledgment is made to F. S. Grover and L. 0. Stauffer of the Laboratory Machine Shop for fabricating the osmometers Appreciation is expressed to the Firestone Tire and Rubber C‘o for permission to publish this work. LITERATURE CITED
(1) Baker, Walker, and Pape, GR-16; PB9687,Office of Technlral Services, U. S. Department of Commerce (June 29, 1944). (2) Boissonnas and Meyer, Helv. C h i m . Acta, 20, 783 (1937). (3) Carter and Record, J . Chem. Soc., 1939,660. (4) Cragg, J . Colloid Sci., 1, 261 (1946). (5) French and Ewart, IND. ENG.CHEM., ANAL.ED.,19, 165 (1947. (6) Fuller, Bell S y s t e m Tech. J., 25,374 (1946). (7) Fuoss and Mead, J . Phys. Chem., 47,59 (1943). (8) Herzog and Spurlin, 2. p h y s i k . Chem., A, Bodenstein-Festhann 239 (1931). (9) Montonna and Jilk, J . Phys. Chem., 45,1374 (1941). (10) Schulz, 2. physik Chem., A176, 317 (1936). (11) Sturtevant, “Weissberger’s Physical Method8 of Organic Chemistry”, Vol. I, p. 327, New York, Interscience Puh. lishers, 1945. (12) Wagner, IND. E m . C H E M . , ANAL.ED., 16, 520 (1944). (13) Wall, Banes, and Sands, J . Am. Chem. Soc., 68, 1429 (1946) (14) Weidlein, Chem. Eng. N e w s , 24,771 (1946). PRESENTED before the Division of Rubber Chemistry a t the 110th Meeting SOCIETY,Chicago, Ill. Investigation carried of the AMERICAXCHEWCAL out under the sponsorship of the Office of Rubber Reserve, Reconstructlor Finance Corporation, in connection with the Government Synthetic Rubbe* Program
Apparatus for Rapid Conductometric Titrations Determination of Sulfate d
d
LLOYD J. ANDERSON1 AND ROGER R. REVELLE*, Scripps Institution of Oceanography, La Jolla, Calif. This paper describes the development and construction of electrical apparatus suitable for conductometric titrations, and presents a method of using the apparatus for macro- and microdeterminations of sulfate. When the precipitation is controlled by seeding, amounts of sulfate as small as 1 mg. can be determined to *l%even in the presence of more than fifty times as much chloride.
N
UMEROUS attempts have been made to develop simple and rapid procedures for determining dissolved sulfate. Simple and direct titration procedures with barium solutions have been unfeasible because of the lack of reliable color-changing end-point indicators. As a result, a great many indirect methods have been devised in which the unknown sulfate is precipitated by a suitable reagent and a n equivalent amount of some other ion, easier to determine, is liberated. The methods of Muller ( 7 ) , Hinman (S),and Webb ( I d ) , though usually less time-consuming than the conventional gravimetric procedure, leave something to be desired in the way of reliability and accuracy. One color indicator for direct barium titration has been used by Robertson and Webb (9), who state, however, that “the simplicity of the method is to some extent outweighed by the capricious behavior of the indicator”. Several electrometric methods have been developed in which the end point is recognized by the aid of potentiometric or conductometric indicators. Reversible sulfate electrodes are not suitable for routine work because they require extreme care, in both construction and use, in order to prevent “oxygen poison1 Present 9
address, Present address,
U. s. Navy Electronics Laboratory, San Diego, Calif. U. S. Navy Bureau of Ships, Washington, D. C.
ing”. Bimetallic electrodes (IS) are often suitable for pure solutions but become insensitive in mixtures. Dutoit ( 2 ) was the first to apply the conductometric titration technique to sulfate determination and numerous others have reported using the method. Kolthoff and Kameda ( 5 ) investigated the reliability of the conductometric titration for sulfate and concluded that the procedure is too inaccurate for most analytical work, but that coprecipitation of sulfate, the usual nemesis of sulfate determinations, does not occur and hence cannot account for their low results. They also found that the mere presence of the barium sulfate precipitate greatly affected the conductance changes meaeured. during the titration. I n the present method, the precipitation is seeded with a small amount of pure barium sulfate. The seed crystals were precipitated from a solution of C.P. barium hydroxide by adding a slight excess of C.P. sulfuric acid and allowing the washed precipitate to stand for several weeks. These factors tended to produce comparatively large seed crystals. With such crystals as precipitation nuclei the effects of adsorption should be minimized. One would also expect seeding to produce precipitates that were much more uniform from one titration to the next. Perhaps the more uniform results reported herein are due in large part t o
