Double Freezing-Point Method for Determination of Styrene Puri JOSEPH F. MAS1 AND RUTH K. CHENEY National Bureau of Standards, Washin@on, D. C. A m e t h o d is described for determining the purity of rcecyele a n d blended styrene monomer, in which the purity is from 87 to 98y' an,d the principal impurities are ethylbenzene, 4-vinJleyclohexene, and 1,3-butacliene. The d a t a required are the freezing points of the original sample and of a portion f r o m which the volatile impurities h a v e heen removed. The separaition of low-boiling hydrocarbons is accomplished by oomplete evaporation 01 F a portion of the sample u n d e r low pressure a n d condensation of the styrenc> and heavy impurities a s solid in a trap a t -65" C. The prohable error of t hbe method is estimated a t 10.15 weight 70of styrene i n samples of lower purity
'T
-lnNl=AAt(l+BAt...)
HE measurement of the freezing-point depression is one of
the most convenient ways of determining the purity of hydrocarbons in general. The method presented here demonstrates what may he accomplished by the use of liquid-in-glass thermometers for routine work in industrial laboratories where the time, personnel, and equipment may not he available for the more sccurate technique using phtinum resistance thermometers. The method also includes a simple and effective procedure for separating very volatile impurities when they are present in amounts up t o a few per cent. The method was developed for determination of the purity of recycle and blended styrene in the synthetic rubber plants under the control of the Officeof Rubber Reserve of the Reconstruction Finance Corporation.
(1)
where N , is 1,he mole fraction of styrene, At = lo - t,, the differenee between the freezingpoints of pure styrene andsolution,
and
(3)
(degrees Kelvin), molar hedt of fusion, and change of molar heat capacity on fbsion, of pure styrene. T e r n s of higher order than those indicated in theparentheses in Equation 1may be neglected. Sinee in plant practice it is required that the purity of the samTHEORETICAL ple he given in weight per cent, i t is necessary to have an evaluIf a solution which is predominantly styreue is ideal with reation of the number. nature. and relative amounts of the imuurispect to all the constituents, and if the impurities remain in the ties, in addition to &e eon;litions imposed in Equation 1. The liquid phase while the styrene is being frozen, We may Write ( 4 , 5 ) : Drineioai imDurities in recvele stmene are ethvlheneene. 4-vinylcyclohexene; and 1,3-butadiene (1, 2); it hssbeen assumed that others are present in negligible amounts. If it is further assumed that the ratio of ethylbenzene to 4-vinyleyolohexene is approximately constant, only two independent measuremenbs are needed to fi* the weight per cent of styrene. The last assumption is justified because the molecular weights of the two Gimpurities are so close together that for a given number of total moles of ethylbenzene and 4-vinylcyolohexene, even large variations in their molecular ratio will cause only small differencesin their total weight per cent actually present in the sample and also calculated from freezing-point depression if, again, the solution is ideal. The two independent measurements chosen are the freezing points of the sample before and after separation Of the light hydracmhons from it. It is obviously of practical advantage to have a single procedure for both measure J ments. By applying Equation 1 to both freesSTYRENE ing points, equations were developed for FREEZING POINT APPARATUS the weixht DW cent of Cs impurity in Figure 2. Apparatus for Separation Figure 1. Styrene Freezing Point the C,-free sample, the weight per cent of Low-Boiling I m p u r i t i e s Apparatus ~~
~
~~
.
n
321
.
322
ANALYTICAL CHEMISTRY
of butadiene, and the weight per cent oi styrene. The assumption of ideality, on which these equations we:e based, is invalid, however, for solutions of ethylbenzene and butadiene in styrene. Since it was found that an error of as much as 0.7%, in 90% styrene solutions, can be introduced by the use of the theoretical equations, they were discarded in favor of empirical relations obtained from experimental data, as described below. APPARATUS AND MATERIALS
The freezing-point apparatus (Figure 1) consists of a 665-m1. unsilvered Dewar vacuum flask of Pyrex, a nest of three test tubes, and a stirrer operated by a reciprocating vacuum-type stirrer motor. (This apparatus is substantially the same as that designed by Irving Madorsky, Rubber Section, National Bureau of Standards, for freezing-point purity of fresh styrene.) The temperature of the styrene is measured with a mercury-thallium-filled thermometer, de-
50 to 60 mm., and a diameter of 6 to 7-mm. The apparatus for separation of low-boiling impurities (Figure 2) consists of an evaporating flask and a cold trap, connected as shown by glass tubing (8 to 10 mm. in outside diameter) and heavy-walled neoprene connectors. The bath surrounding the cold trap is contained in a Dewar flask, preferably silvered. The vacuum pump used with this apparatus is capable of reducing the pressure to less than 0.5 mm. of mercury and has a capacity sufficient to maintain the pressure below 10 mm. of mercury when pumping on the impure styrene solution. The materials necessary for an analysis are dry ice and a freezing mixture made of equal parts by volume of chloroform and carbon tetrachloride. The following materials were used in making up the solutions for investigation of the method:
OB
-32.5EXTRAPOLATION
J
FREEZING CURVE
OF
OF A
TYPICAL
STYRENE BLEND
A
-33.0-
OBSERVED FREEZING POINT, -32.89
I1
OC.t
\\
-33.5-
A
-34.0 -
, !
L
I
0 2 . 2 : l RpTlO !F
In W
K
a
3
I
ETHYLE!ENZENE
,
I
I
T?
I
4-VINYLCYChOHEXENE
PLATINUM RESISTANCE THERMOMETER A THEORETICAL 8 EQUATION 4
E l N W W
a
L L ,
WEIGHT PERCENT Cg
IMPURITY
Figure 4. Experimental Freezing Point Depression of Styrene by Ethylbenzene and 4-Vinylcyclohexene
Styrene. Two lots of styrene monomer were specially purified by the Dow Chemical Company. The purities of the two lots, by freezing point, were 99.80 and 99.90 weight %. Ethylbenzene. A sample purified by the Petroleum Laboratory of the Pennsylvania State College, a t least 99.5% pure by melting point, was used. 4-Vinylcyclohexene. The sample used was purified by fractional distillation by the Koppers Company. From refractive index and specific gravity measurements, its purity was judged to be better than 95%. 1,J-Butadiene. Phillips Petroleum Company's Pure Grade butadiene, 99% pure by freezing point, was used without further purification. PROCEDURE
The following procedure, although describing operations on plant samples, is also applicable to the synthetic mixtures discussed later. The first step in a n analysis is to determine the freezing point, tl, of a portion of the original sample. The next step is to remove the light hydrocarbon impurities from another portion of the sample and determine its freezing point, tz. From t? is computed the weight per cent of heavy impurity, p , , in this sample from which the light hydrocarbons have been removed. Finally the weight per cent of light impurity, p p , in the original material is computed from the difference in the two freezing points, t p - tl.
Samples of recycle and blend styrene to be analyzed are obtained in clean screw-cap bottles of about 1-pint capacity. These samples are chilled to 4O0 F. (4.4O C.) before analysis in order to minimize thk loss of light hydrocarbons by evaporation, Determinationof Freezing Points. The freezing mixture in the Dewar flask is cooled with dry ice to -43O C. Approximately 10 ml. of the sample are placed in the innermost test tube and the apparatus is assembled as in Figure 1. The thermometer is immersed a t least 2 cm. above the bulb and the bath level is about 2 cm. above the sample. Throughout the determination the sample is stirred a t the rate of about 150 strokes per minute, and the bath is maintained (by addition of small pieces of dry ice) within 0 . 5 O of a temperature selected so that the rate of cooling of the sample, while freezing, is near 0.1' C. per minute. This rate of freezing is produced by temperatures from -42" to -44" C. with the apparatus shown in Figure 1. The temperature of the sample is read to 0.01" C. a t 30-second intervals; a t least three readings are taken before freezing begins and from 20 to 25 after freezing begins. The time-temperature readings are plotted on a graph which is scaled so that 1' (on the ordinate) is equal to 10 cm. and 10 minutes are equal to 10 cm. The uncorrected freezing points are determined by extrapolat-
a
V O L U M E 20, NO. 4, A P R I L 1 9 4 8
5d to.10 I-
p.o.00.
v
cb 0
I
0 0
u a
-DEVIATION
0"-0.10 0
" (ha.
0,
323
% 0-
"
A
V"
OF OBSERVED AT FROM EQUATION 4 02.2:1 RATIO OF ETHYLJENZENE 70 4 - V I N Y L C p 3 H E X E N E
I
a 1:2
PLATINUM RESISTANCE THERMOMETER
I
I
0
2
4 5 6 7 8 WEIGHT PERCENT C g IMPURITY
3
I
I
9
IO
II
and are good to 0.02" C. If the bulb volume changes, the calibration a t all points on the scale will be changed by a constant amount; this correction is obtained by takingice points weekly duringthe first six months of the life of the thermometer, or until it appears that only small changes are taking place. Ice points are checked every month or two after that. Good ice points are obtained by crushing clean, clear ice, washing it, and making a thick slush with distilled water. The thermometer is immersed to the 0" mark in this slush, in a vessel protected from radiation.
A further control measure is freezing of a standard material of known freezing point. A stable reference substance, having a freezing point close to that of styrene, is bromobenzene. A sample of this substance, with a freezing point certified by the Xational Bureau of Standards, is frozen in the same way as a sample of styrene, except that freezing is continued until the stirrer begins to labor or until the sample has been freezing about 20 minutes. The points are plotted with a temperature scale about 5 times as sensitive as that for styrene. In following the extrapolation procedure given above, it is important to disregard the initial flat portion of the freezing curve, by choosing points G, H , and I (Figure 3) so that G is on the downward-sloping portion of the curve. Errors of several hundredths of a degree were discovered after failure to observe this precaution on two occasions. By making use of these checks, an accuracy of +0.02" C. can be obtained. Separation of Low-Boiling Impurities. Since the low-boiling impurities (principally butadiene) differ so greatly in volatility from the remainder of the impure styrene mixture, they may be effectively removed by the following simple procedure.
Figure 5. Deviation of Experimental Freezing Points from Empirical Equation 4 for Mixtures with No Butadiene
A-THEORETICAL B-EXPERIMENTAL
1
-4
-0
1
/
D
I 2 WEIGHT PERCENT BUTADIENE
Figure 6. Freezing Point Depression by 1,S-Butadiene of Styrene Containing 6.37 Weight % C8 Impurity ing the time-temperature curves by a graphical method described by Taylor and Rossini (4). This method is illustrated in Figure
3. Bfter the liquid cooling curve, OP, and a vertical line, A B , are drawn a t the estimated time, Z,, that freezing would have begun in the absence of undercooling, three experimental points G, H , and I are chosen about equally spaced and as far apart as possible; G is sufficiently far from the beginning of freezing so that essential equilibrium has been established. A horizontal line is drawn through I , DE is drawn through H and G, J Z is drawn through H and I , and J K is drawn parallel to DE. The extrapolated freezing point, F , is obtained by drawing a line from K through G to intersection with A B . The corrected freezing points are obtained by adding algebraically to the uncorrected values, the calibration correction, the change in ice-point correction since the original calibration, and the correction for emergent stem, for the thermometer used. The corrected freezing point of the original sample is called t,, and the corrected freezing point of the sample after removing butadiene is called tP. The respective freezing-point lowerings, At, and Atz, are obtained by subtraction from -30.63' C., the freezing point of pure styrene ( 3 ) . The per cent styrene is calculated from Equations 4, 5, and 6, or from a nomogram prepared from them. In order to realize the greatest possible accuracy with liquid-inglass thermometers it is necessary to check their calibration and the freezing-point procedure being followed. These thermometers are prone to change in bulb volume and rise in ice point, particularly during the-first few months after manufacture. I t is not unusual for this change to be as great as 0.1" C. The thermometers are calibrated before use by the National Bureau of Standards. Calibration corrections are obtained under total immersion conditions a t -40", -35', -30°, and 0 " C., are stated to 0.01' C.,
About 15 ml. of the original sample are poured into the flask of the evaporating apparatus (Figure 2), and the apparatus is assembled as shown and connected to the pump and a small manometer. Before the clamp above the flask is opened, the system is pumped out to less than 0.5 mm. and the cold trap is cooled with a carbon tetrachloride-chloroform mixture maintained a t -65' C. with dry ice. While the flask is shaken gently, the clamp is opened slowly enough so that the pressure does not rise above 10 mm. rlfter the clamp is open, a beaker of warm water placed around the flask aids evaporation. The styrene, ethylbenzene, and 4-vinylcyclohexene are condensed as solid on the walls of the trap, while the light hydrocarbons pass through as vapor. When the sample has completely evaporated, the cold trap is detached, the contents are allowed to melt, and the freezing point of the condensate is determined. DATA ON SYNTHETIC MIXTURES
In order t o investigate the method and develop the required empirical relationships between freezing point and composition, a series of experiments was made on 68 synthetic mixtures ranging from 87 to 987, styrene. The mixtures were made up with known composition and their freezing points determined by the procedure described above. The first thirty of these styrene solutions contained the solutes ethylbenzene and 4-vinylcyclohexene in ratios of 2.2 to 1 and 1 to 2. At a much later time, freezing points of eight solutions containing ethylbenzene and 4vinylcyclohexene in various ratios were obtained. Some freezing points were also obtained with a platinum resistance thermometer on solutions containing ethylbenzene and 4-vinylcyclohexene in 1 to 1 ratio. The freezing-point lowering is plotted against weight per cent, for all these solutions containing no light hydrocarbons, in Figure 4. Various equations were fitted to these points by the method of least squares. Separate equations through the points representing different ratios of the impurities could be obtained, but a t 90% styrene, the difference in per cent styrene found by using 2.2 to 1 ratio of ethylbenzene to 4-vinylcyclohexene instead of 1 to 2 was only 0.16%. Since there does not exist a t this time a good independent method for determining
324 the ratio of these two impurities, and t'here is some evidence that the ratio is usually in the neighborhood of 1 to 1 (e), a least square fit xvas obtained using a quadratic function of Atafor all the points together. This equation is 2.389Atz
pi
+ 0.003At:2
- 11) - 0.025(t2 - ti)'
(5)
The deviations of the experimental values of tz - tl from those calculated by Equation 5 are plotted against weight per cent of butadiene in Figure 7. S o correlation with content of hea. impurities is observed.
DEVIATION OF OBSERVED T2
- TI FROM E Q U A T I O N 5
BUTADIENE ADDED TO STYRENE CONTAINING:
0 1.82 WEIGHT PERCENT Cg IMPURITY c)
0'
8
co.1 -
II
3.72
I,
I' 80
a6.02
'I
06.37
''
89.12
"
os.I S
"
I,
0
t-
I
vi
o-
m
," -0.1 a
0
I
2 3 4 WEIGHT PERCENT BUTADIENE
The weight per cent of styrene in the original sample was obtained from the equation =
(100 - pJ(100 100
-
n
0
8
0
0 0
0
o 0
0
0
-0.1 -
* 000
0
I
*
*
0
I
2
3
4
5
6
7
8
9
IO
WEIGHT PERCENT G8 IMPURITY
Figure 9. Change in Freezing Point of Impure Styrene Solutions Caused by Adding and Removing 1,3-Butadiene, Plotted against Weight Per Cent of CS Impurity
The weight per cent, of styrene in all solutions was calculated by use of Equations 4 , 5 , and 6, and compared with the actual values; the average deviation in p , was *0.06. The efficiency of the separation of lowboiling impurities is shown by the results obtained on seventeen synthetic mixtures, which are plotted in Figures 8 and 9. The difference in freezing point of the sample before adding butadiene and after removing it by vacuum evaporation is plotted against per cent butadiene in Figure 8. N o correlation is observed. In Figure 9 the change in freezing point of the same samples, due to the addition and removal of butadiene, is plotted against per cent heavy impurity in the sample. The positive side of the At, axis is the direction of increase of purity upon separation. From these two graphs, it is evident that the greater the percentage of ethylbenzene and 4vinylcyclohesene, the more of these substances are removed in the separat.ion, regardless of the amount of butadiene. This observation seems to indicate that the error is caused by distillation of the heavier fraction rather than by the sweeping of solid particles through the trap. The latter effect would be the first error suspected in the arrangement of this apparatus, but such effects are partly dependent upon the amount of gas passing through.
5
Figure 7. Deviation of Experimental Freezing Point Lowering by Butadiene from Empirical Equation 5
P3
0
0' O-
(4
and is shown by curve B in Figure 4. Curve A is obtained from the laws of ideal solutions. The deviations of observed freezingpoint depressions from those calculated by Equation 4 are shown in Figure 5. As expected, most of the points representing 2.2 to 1 ratio of ethylbenzene to 4-vinylcyclohesene give negative deviations, n-hile those of the inverse ratio are positive. By low-temperat,ure condensation in vacuum, butadiene in various amounts was added to various mixtures of st,yrene and the 1 to 1 mixture of ethylbenzene and 4-vinylcyclohexene. The nonideality of the butadiene-styrene solutions is shonm by the points of one series of solutions in Figure 6. The solutions in this series were made by adding butadiene to styrene containing 6.37 weight yoof ethylbenzene and 4-vinylcyclohexene (1 to 1). Other series containing different amounts of heavy impurities gave similar curves. After the freezing-point determinations, the added butadiene was removed (from fresh portions of the solutions) by the vacuum evaporation procedure given above, and freezing points ( t z ) were determined. It was found that the weight, per cent of butadiene could be represented within the esperimental error by the equation p , = 1.366(tz
+0.1-
- PP)
The average of the deviation of all the solutions from the calcu1ated;value of pl obtained by Equation 4 was +=0.06 weight 70. The average deviation of p,, in the synthetic mixtures containing butadiene, from the value calculated by Equation 5 , was t0.05.
ACCURACY
The error in the freezing-point determinations which is caused by the use of a liquid-in-glass thermometer should not exceed 0.03" C. Reproducibility of freezing points in this laboratory, using different thermometers and different operators, has been of the order of 0.02" C. There is no evidence that there is any error in the first freezing point (tl) caused by evaporation of light .hydrocarbons from the sample in the freezing tube, at least for samples containing less than 376 butadiene, provided the sample is thoroughly chilled before testing. If the first freezing point is correct, any error in the second freezing point ( t z ) such as indlcrtted by Figures 8 and 9, will result in a portion of the heavy impurity's being calculated as butadiene, or the reverse. Thus the error in per cent styrene is
325
V O L U M E 20, NO. 4, A P R I L 1 9 4 8 essentially the difference between the error in p l and in p?, or approximately the value of the error in t?. The error which can be made by assuming that the ratio of ethylbenzene to 4-vinylcyclohexene is 1 to 1 may be as large as 0.5%if the impurity is entirely one or the other. Hovever, those recycle styrenes which have been investigated have contained these impurities in ratios from 1:2 to 2: 1. .in appreciable error in the method is introduced by the assumption that the only impurities are ethylbenzene, &vinylcyclohexene, and 1,3-butadiene. The presence of C4 hydrocarbons other than butadiene is not important, but there is recent evidence that recycle styrene contains Cg hydrorarbons, particularly isopropylbenzene, in amounts which niay be as large as 1.5c0(W). An error of as niuch as 0.2 in the veight per cent of styrene may ihus be made. iifter consideration of all the above sources of error, the probable error in iveight per cent of styrene as determined by the double freezing point method is set at h0.15, for samples of 90% purity.
ACKNOV-LEDG\IEZIT
The authors wish t o express their appreciation t o Daniel R. Stull and the Doiv Chemical Company for the specially purified lots of styrene which they furnished for this investigation. LITERATURE CITED
(1) Frieden, E.. Freeman. L.. and Ehrey. G . O . , Technical Report to Reconstruction Finance Corp., O f f i c ~ of Ruhher Reserve,
OPS-24. Aug. 20, 1945. (2) S a t i o n a l Bureau of Standards. Therniochemistry 5ection, unpublished d a t a contained in analytical reports t o Reconstruction Finance Corp., Office of Ruhher Reserve. (3) Rands, R . D.. and Brirkwedde. F. G . , recent unpublished woi.k a t S a t i o n a l Bureau of Standards. ( 4 ) Taylor, TI-, J.. and Rossini. F. D.. J . Rcserfrcic .Yatl. B u r . ,Standcirds. 32, 197-214 (1944). (5) K a s h b u r n , E. IT-.. J . Aim, C h o n . Soc.. 32,666 (1910). ~~
K E c L l v E D A u i r u a t 16, 1947. Prekented before the Ijivision of Analytical and Micro Chemistry at the 1 1 1 t h lI(>eting of t h e AMERICAK CHEMICAL SOCIETY, . i t l a n t i c C i t y , S . .J.
Determination of Silicon in Organosilicon Compounds J. A. MCHARD, P. C. SERYAIS,
AND H. A. CLARK Dou* Corning Corporation, Midland, Mich.
Silicon has been determined in organosilicon compounds by methods ranging from acid digestion with sulfuric acid to newly developed procedures particularly adaptable only, to these compounds. The methods are review-ed and evaluated.
T
HE recent rapid advances in the commercial production and use of organopolysiloxanes have prompted a revien. and further development of the methods of silicon determination. .%long with the development of the chemistry of organosilicon compounds, investigators have had to establish the technique of analysis of these materials. The approaches used in determining silicon can be generally classed in three distinct groups: wet oxidation, dry combustion, and fusion. Early investigators niet viith many failures in their tests for silicon because of the high volatility of some of t,he compounds. Another source of error has been the tendency of the compounds to form some silicon carbide instead of silicon dioxide on combustion. Knon-ledge of past applications of the various techniques Kill place the chemist in a better position t,o select the nwthod best suited to his particular purpose. Long ago, Friedel and Crafts ( 2 ) employed fuming nitric acid in a wet oxidation procedure followed by solution in alkali and reprecipitation. Polis ( 2 1 ) achieved some success by decomposing the sample in a mixture of sulfuric acid and potassium permanganate, folloTved by leaching the manganese oxides with hydrochloric acid and filtering the silica. Kipping and coworkers (8,9 ) eliminated the permanganate and substituted nitric acid as an osidant which, being itself completely volatile, allon-ed final ignition to silicon dioxide without further transfer of the sample. More recently Hyde and DeLong ( 6 ) were able to analyze some organosilicon polymers in a similar fashion. Bygden ( 2 ) accomplished a wet, oxidation in a Kjeldahl-type flask, using mercury and sulfuric acid with subsequent filtration of the silicic acid. Perchloric acid has been substituted for sulfuric acid in a wet oxidation procedure proposed by Gilman, Clark, Kiley, and Diehl (4). Sone of these procedures has been successfully applied to the analysis of compounds of high volatility. Materials boiling below 200" C. are likely to give low results, and care must be
taken that, silicon carbide does not form in the final ignition step. The present authors have found that ignition in an adequate supply of oxygen at 800" C. almost completely eliminates the formation of silicon carbide. In a recent monograph on silicone chemistry, Rochoiv ( l e ) suggests a dry combustion method for the determination of carbon, hydrogen, and silicon. The procedure, however, is reported to be slo~v,conditions depend on the type of compound to be anal\-zed, and the formation of silicon carbide cannot always be prevented. Successful fusion of organosilicon derivatives in dry reagents had t o aviait the developnient, of the Parr bomb technique as described by Tseng and Chao (13), Khitmore and eo-workers ( I d ) , anti Gilliam, Liebhafsky, and Kinslow (3). By this technique the more volatile samples are best handled. The present paper is an attempt by the authors to sort, out of the present knowledge, the te.chniques best suited for the deterniination of silicon in the various types of o1,ganosilicon compounds. DISCUSSlOS
The most successful aiialysis of volatile samples has been accomplished by the use of Parr-type bomhs. Generally the silicon is determined, after peroside fusion in a bomb, by the classical dehydrat,ion with hydrochloric and/or perchloric acids. Howcver, the unavoidably large volume of salts resulting from the fusion makes evaporation and washing a tedious procedure, and unless the second prccipitation and evaporation step is undertaken, results are unreasonably low. On some samples, hydrofluoric acid correction necessitates the use of platinum, crucibles. This niethod of analysis requires on the average 24 hours for completion, which is excessive both from the standpoint of a good control method arid from equipment tie-up.
