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INDUSTRIAL AND ENGINEERING CHEMISTRY
Comparisons with commercial cell assemblies show that the proposed unitized cell assembly allows considerable saving in time required for cleaning and assembling for successive determinations; on the average, five or six determinations are possible within one hour. The precision of the results obtainable by the photoelectric recording polarograph and unitized constant-temperature cell assembly is indicated in Table I. With wave heights of 4 to 5 cm., the average deviation is * 1% and the maximum deviation is +3%, regardless of the changes taking place in room temperature.
Vol. 17, No. 11
LITERATURE CITED
(1) Cohn and Kolthoff, J . Biol. Chem., 147, 705-19 (1943). (2) Hoekstra, Rec. trav. ehim., 50, 339-42 (1931).
(3) Kolthoff and Lingane, “Polarography”, New York, Interscienoe Publishers, 1941. ( I ) Muller, “Polarographic Method of Analysis”, Easton, Pa., Journal of Chemical Education, 1941. (5) Pompeo and Penther, Rev. Sci. Instruments, 13,218-22 (1942). (6) Semerano, “I1 Polarografo”, Padova, -4.Draghi, 1932. PRESEWTED before the Division of Analytical and Micro Chemistry at t h e 108th Meeting of t h e A Y E R I C CHENICAL .~~ SOCIETY, New York, N. Y .
Apparatus and Procedure for Technical Butadiene Analysis CHARLES L. GREGG, M a i n Laboratory, The Dow Chemical Company, Midland, Mich.
For the gas volumetric determination of butadiene with molten maleic anhydride, a bead-packed Hempel pipet type of absorber i s used in place of the conventional U-tube absorber. This pipet type of absorber provides an apparatus much simpler and easier to manipulate and permits more rapid determination of butadiene. Accuracy equals that obtained with a U-tube absorber when equilibration of reagent to gas is properly maintained. The factors affecting equilibrium between reagent and gas are discussed, and a procedure is described which conforms to the requirements of these factors.
IiY ORDER
to sustain research requiring the analysis of a large number of butadiene samples, an apparatus was desired with. which the butadiene content could be determined with facility. .It the time work was started in 1937, previously described equipment (3, 4 ) did not seem to lend itself well to the task. The apparatus adopted for this purpose is shown in Figure 1. In this apparatus the butadiene is absorbed in a small amount of molten maleic anhydrlde retained in the bead-packed absorption chamber over mercury. The entire sample comes in contact with the reagent spread as a film over the surface of the exposed beads. Agitation of the gas, imparted through the mercury, ensures intimate contact of gas and absorbent. Because of the greater contact, the butadiene is absorbed more rapidly in this pipet than in the U-tube type of absorber devised by Tropsch and Mattox (5). The apparatus is designed to eliminate stopcocks where they are likely to be troublesome. There is only one connection to the pipet-that of the buret-and the gas path is only between the absorber and the buret. This eliminates an additional gas reservoir with its accompanying mercury leveling bulb. The assembled butadiene analyzer requires the manipulation of only one mercury bulb. The resulting simplified manipulation is the chief advantage of the equipment, enabling an analyst to operate the equipment for a day without undue strain and with an efficiency which accomplishes an increased number of analyses. The characteristic long hairpin-bent capillary of the Hempel pipet, with the improvement suggested by Lucas ( 1 ) provides some check against accidental withdrawal of the reagent from the absorber. With this equipment, as with all apparatus for gas volumetric analysis of butadiene employing molten maleic anhydride, the procedure must provide for the very appreciable physical absorption of the C, hydrocarbon gases other than butadiene in molten maleic anhydride. Presaturation of the reagent to an equilibrated condition with the gas constituents of a sample is necessary before actual analysis of the sample. This equilibration of the reagent to the gas must be recognized to be labile. The effect of each of the following variables must be controlled
to accomplish satisfactory equilibration: temperature; quantity of reagent; pressure; and composition of sample. Temperature of the reagent is kept constant by immersing the absorption pipet in a boiling water bath. It is important that the mercury reservoir as Fell as the absorption chamber be kept at constant temperature. The quantity of reagent is kept small to minimize the amount of gas required to saturate it. The shape of the absorption chamber and the mercury reservoir, as well as the location of the reservoir, is such as to keep small the change of mercury head of pressure on the absorber with the admission and withdrawal of gas. The specified procedure avoids pressure disturbances of equilibration. When the reagent is equilibrated to a particular sample, it will not be in equilibrium with a gas of different composition. The reagent must be equilibrated to each gas composition analyzed. With this apparatus the first two samples carried through the procedure are required to equilibrate fresh reagent to the gss being analyzed. Two more samples must be run to provide check analyses. Thus, four samples must be run through the CONNECTION TO
SAMPLE VENT
EXPANSION
BLADDER
Figure 1.
Assembled Butadiene Apparatus
ANALYTICAL EDITION
November. 1945
- 1
729
/50mm
I.
' 1
I
The dimensions of the buta-. diene pipet are such that it will
/ I
Figure 2.
Detail of Pipet
procedure to complete the first analysis. Each run ai11 require 10 to 12 minutes, so that about an hour is needed for the first analysis. Subsequent similar samples can be analyzed in one half to three fourths of this time with two to three runs. Dissimilar samples usually will require four runs, occasionally five, for analysis. Accurate results are dependent upon the maintenance of equilibrium during the analysis. When proper procedure is adhered to, results that check closely are obtained. An operator's checks should be within 0.2%; those of different operators within 0.4% as a maximum. Usually analyses of the same sample by different operators will not differ by more than 0.2%. Samples containing from 0.5 to 100% butadiene can be analyzed. The apparatus can readily be combined with other gasabsorption analysis equipment, so that additional analyses can be performed on the residual gas after the butadiene determination.
t h l buret stopcock through R mslifold. In the one farther from the buret the diluent gas is stored; the nearer one is used as a mixing chamber for sample and diluent. auxiliary absorption apparatus connects t o the manifold beyond the reservoirs. A gas microburner is kept a t hand to warm the capillary of the pipet a t the time of changing reagent. A 5-ml. beaker is used to charge reagent to the pipet, and a 100-ml. beaker is provided to catch discharged reagent. This latter is held by an adjustable clamp. so that it can readily be moved aside while the pipet is being charged. REAGENTS
Maleic Anhydride. Eastman Kodak Co. No. 1226 is satisfactory. The reagent should be freshly distilled. Di-n-amylamine. Eastman Kodak Co. No. 2353 is satisfactory. PREPARATION OF APPARATUS FOR ANALYSIS
The butadiene pipet is charged with approximately 175 cc. of mercury. The rubber expansion bladder is connected. The bath is brought to boiling and the capillary above the bath IS warmed with the microburner until a little too warm to hold with
APPARATUS
The gas buret is of 100-ml. capacity, graduated in 0.1 ml There is considerable advantage t o having this buret equipped with but a single stopcock designed for the apparatus, so as to provide connections between the buret and (1) the butadiene pipet, (2) the sample source, and (3) auxiliary mixing and analysis equipment. Figure 3 shows the design of this stopcock (fabricated by the H. S. Martin Co., Evanston, Ill.). The buret is protected from fluctuating temperature by an ample water jacket. Mercury is the confining liquid used in the buret. Figure 2 shows the absorption pipet, which is packed with 6.5mm. lengths of fire-polished 6.5-mm. glass rod. The presence of a small space above the beads permits them to shift a little with the rising of the mercury confining liquid, thus preventing any trapping of gas among the beads. The mercury reservoir half of the pipet is centered with the absorber half so as to reduce to a minimum the change in mercury head of pressure on the absorber with the admission and withdrawal of gas. The hairpin-bent capillary of the pipet connects directly to the buret with a butt-to-butt joint held by a short section of rubber tubing. rl small enlargement in the capillary (1) prevents droplets of reaqent from working up the capillary towards the buret, and a reference mark is provided to which the reagent is drawn when gas is measured in the buret. Reagent is charged to the pipet through a side arm on the capillary. The length of the capillary ensures ease of operation without drawing, the reagent over into the buret. A rubber expansion bladder IS connected with rubber tubing to the reservoir half of the pipet. It 1s convenient to have a tee in this connection t o which is attached a vent closed by a pinchclamp and an aspirator bulb, so that the amount of air in the expansion bladder can readily be varied.
il
EL c n
Figure 3.
Detail of Stopcock
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
730
the fingers. The capillary between the side arm and the buret is filled with mercury from the buret. The bladder is filled with air and is squeezed to force any spent reagent from the pipet into the beaker. Air is admitted to the pipet and again the mercury is forced up slowly through the beads to work the reagent from their surfaces and finally out through the side arm. By placing a pinchclamp on the bladder tubing the mercury is held so that it fills the absorber. A 5-ml. beaker held by a test-tube holder is filled with molten maleic anhydride and two drops of di-n-amylamine are added as recommended by Robey, Morrell, and Vanderbilt (2). *he beaker is held so that the reagent is drawn into the absorber by the receding mercury when the clamp is removed from the bladder tubing. The side arm and its short section of rubber tubing are filled with mercury from the buret and the rubber is clamped with a screwclamp. If necessary, a little mercury is run from the buret over into the pipet to force the last drops of reagent from the capillary into the pipet and leave the capillary clean. Then the reagent is drawn up to the reference mark. The expansion bladder is now deflated. PROCEDURE
The bath should be kept boiling during analysis. 1. A sample closely approximating 100 ml. is taken into the buret and measured to the nearest 0.1 ml. 2. The gas is passed into the butadiene pipet for from 30 to 60 seconds. 3. While the gas is in the pipet, it is kept in very gentle motion by alternately gently squeezing and releasing the expansion bladder. 4. The gas is returned to the buret to change the reagent surface on the beads. Steps 2, 3, and 4 are repeated until absorption is complete. Readings on the buret need not be taken until the absorption is small. The final volume is recorded when the volume remains constant during a half-minute period in the pipet. Then repeat samples are run exactly as the first as promptly as possible until closely checking results are obtained. It is essential for the maintenance of equilibrium that sample sizes vary not more than 1 ml. The reagent should be changed when about 500 ml. of butadiene have been absorbed in it, or when it has been kept a t 100’ C. for about 4 hours. If the mercury in the absorber becomes separated into small globules, or if the reagent becomes viscous
Oven For Measurement
OF
Vol. 17, No. 11
on the surface of the beads, the absorber should be flushed with a charge of fresh reagent. The condition is usually due to too long use of reagent or the absorption of oxygen in the reagent. Only moisture-free gas should be admitted to the reagent if its ability to absorb butadiene rapidly is to be maintained. SUMMARY
For technical butadiene analysis with molten maleic anhydride, an apparatus employing a mercury-filled pipet type of absorber in place of a U-tube absorber has certain advantages: The apparatus may be much simpler. The absorption of butadiene is more rapid because of the much greater contact between reagent and gas. The manipulation is much easier and relatively trouble-free. Operating efficiency is enhanced through reduction of strain on operator. Less dexterous operators can perform the analysis. The number of analyses an analyst can accomplish is increased. The apparatus is adaptable t o combinations mith other apparatus for more complete analysis of samples.
A disadvantage of such equipment is the ease with which the tquilibrium between reagent and gases of the sample other than 1,3-butadiene may be disturbed. This can be overcome by proper analytical procedure without loss of the advantages of the apparatus. Accuracy of results is about the same as obtained with a LT-tube absorber. LITERATURE CITED
(1) Luoas, G.H., IND.ENO.CHEM.,Ax.4~.ED.,1, 79 (1929). (2) Robey, R. F., Morrell, C. E., and Vanderbilt, B. M., Papers Presented before the Petroleum Division, of the 10lst Meeting of the A.C.S., 8t. Louis, Mo., April, 1941. (3) Tropsch, H.,and Mattox, ’CV. J., IND.ENO.CHEM.,ANAL.ED., 6, 104 (1934). (4) “U.O.P. Laboratory Test Methods for Petroleum and Its Products”, Universal Oil Products Co., Chicago, p. 135,1937. PWBLICATIOX of this article delayed at the request of the Office of Censorship
Volatility
OF
Plasticizers
In Polyvinyl Chloride and Chloride-Acetate Copolymer Compositions D.
K. RIDER AND J. K. SUMNER, Resinous Products and Chemical Co., Philadelphia, Pa.
A new oven for use in the measurement of volatility of plasticizers in polyvinyl chloride and chloride-acetate copolymer compounds has been designed and built. Using this oven, a reliable test method has been developed which i s capable of distinguishing between various plasticizers on the basis of small differences in volatility.
0
N E of the basic requirements of a good plasticizer for polyvinyl chloride or vinyl chloride-acetate copolymers is
permanence. I n the ideal case the plasticizer should have a volatility of such a low order of magnitude that it will not leave the plasticized composition to any appreciable degree during expected serviceable life. The great majority of the commonly used plasticizers fall short of the ideal in this respect. Many are used in spite of their relatively high volatility because of other desirable properties, such as good efficiency or ability to impart improved low-temperature properties to the compositions in which they are used. Other things being equal, however, the more desirable of two plasticizers will be the one with the lower volatility. This is particularly true with nonrigid plastics, such as polyvinyl chloride or chloride-acetate copolymer, which depend entirely on plasticizers for their flexibility. I n compounds such aa celIulose acetate, which are rigid even when highly plasticized, some volatilization of plasticizer is sometimes permissible after
fabrication of the plastic article, provided that the accompanying shrinkage can be tolerated. Plasticizer loss from a resin composition proceeds by (1) diffusion of the plasticizer to the surface; and (2) evaporation of the plasticizer from the surface. Under special conditions such M elevated temperature, high vacuum, and thick samples the rate of loss of plasticizer would be largely controlled by the rate a t which the plasticizer diffused to the surface, Under usual service conditions, however, such as moderate temperature, atmospheric pressure, and relatively thin films the rate of evaporation from the surface is undoubtedly the dominant factor, unless there is a “case hardening” or skin effect. A volatility test which is designed to simulate service conditions preferably should be set up so that loss of plasticizer is largely determined by rate of evaporation from the surface. Several workers have investigated the volatility of plasticizers. Verhoek and blarshall(3) measured the vapor pressures of several plasticizers in the neighborhood of 100”C., and also that of trimcresyl phosphate in a polyvinyl chloride composition. I n the latter, the plasticizer had the same vapor pressure as the ure material when a t least 15% plasticizer was used. Liebhagky, Marshall and Verhoek ( 1 ) measured loss of plasticizers from polyvinyi chloride compositions in vacuum between 110’ and 155’ C. They concluded that the rates of diffueion of dibutyl phthalate, tricresyl phosphate, and dibenzyl sebacate were the
