31 1
V O L U M E 20, NO. 4, A P R I L 1 9 4 8 DISCUSSIOh
The A.O.A.C. method ( 1 ) of determining nicotine has disadvantages, as pointed out by Avens and Pearce ( 2 ) , who described an apparatus and method for eliminating some of them. Although the Avens-Pearce method is faster than the A.O.B.C. method, it is too slow for rapid routine analysis of many samples. The apparatus requires too much attention during operation to permit the simultaneous use of several units. Bowen and Barthel ( 5 ) describe an apparatus well suited to the use of several in a bank; however, there is considerable increase of sample volume during distillation and a great tendency for frothing, and thr distillation time is about the same as with the Avens-Pearce apparatus. The conventional steam-distillation apparatus as used in the A.O.A.C., Avens and Pearce, and Bowen and Barthe1 methods must be disassembled for sample introduction, removal, and cleaning. The apparatus described herein is far more rapid; there is no tendency to froth, and sample introduction, removal, and cleaning are merely a matter of turning a stopcock and removing rubber stoppers. The apparatus requires a minimum of desk space and because of the speed of distillation the need of a bank of stills is eliminated. The method for determining nicotine in green samples eliminates drying and grinding the sample and gives a more ac-
curate value for the nicotine content of green tissue, since 10 to 30% of the nicotine is lost in the common methods of drying. Although drying a t low temperature prevents this loss, such drying methods are slow and require equipment not available in most laboratories. This method also permits the simultaneous determination of nicotine and chloroplast pigments &s described by Griffith and Jeffrey (6). The acetone-extraction method yields quantitative results if the nicotine is distilled immediately, but nicotine is lost if the acetone extract is allowed to stand. This loss occurs even a t temperature as low as 7 " C. but can be prevented by the use of concentrated sulfuric acid. LITERATURE CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis,5th ed., p. 64 (1940) (2) Avens, A. W., and Pearce, G. W., IND. ESG. CHEM.,A m L . ED., 11,505(1939). (3) Batson, D.M., Chemist Analyst, 35,45 (1946). (4) Bowen, C. V.,J . Assoc. Oficial Agr. Chem., 28,578 (1945). (5) Bowen, C.V.,and Barthel, W.F., IND. EXG.CHEM.,ANAL.ED., 15,596 (1943). (6) Griffith, R. B., and Jeffrey, R. N., IND. ESG.CHBM.,A m L . ED., 17,448(1945). I
RECEIVED June 2,1947. The investigation reported in this paper is in connection with a project of the Kentucky Agricultural Experiment Station and is published by permission of the director.
Determination of lithium Aluminum Hydride in Solution JOHN A. KRYNITSKY, J. ENOCH JOHNSON, AND HOMER W. CARH.IRT ,Vaz.al Research Laboratory, Washington, D . C .
A rapid method for determination of the concentration of lithium aluminum hydride solution is based on the liberation of hydrogen by the hydrolysis of this reagent. An apparatus is described in which this is accomplished at constant temperature and the evolved hydrogen is measured by change in pressure.
S
00s after the discovery of lithium aluminum hydride by Finholt, Schlesinger et al. (1, 2) it was found that this reagent is of great value as a synthetic tool for the reduction of many organic compounds ( 3 ) . In using this reagent a t this laboratory a rapid quantitative method for its determination was needed. Because the lithium aluminum hydride was prepared and used in ether solution, a method was sought for the determination of its concentration without the removal of the solvent. It has been shown ( 1 ) that lithium aluminum hydride is decomposed by water to liberate hydrogen quantitatively according to the equation
+ 4H20 -+
LiA41H4
LiOH
+ Al(OH)? + 4H2
The present method of analysis is based on this reaction and an apparatus was driigned and constructed in which a known volume of solution i i hydrolyzed and the evolved hydrogen is measured by change 111 prrssure. Inasmuch as the hydrogen is determined by pressure change, the error due to variation in the vapor pressure of ether with temperature n a y be considerable. This error is practically eliminated by maintaining the decomposition flask a t 0' C. with crushed ice and nater. dlthough it would be desirable to keep the entire apparatus a t 0" C., it was found that by reducing the volume of the expozed portion t o a minimum, sufficiently satisfactory results are obtained. Use of the apparatus with the flask mzintained at a higher temperature led t o erratic results. APPARATUS
The apparatus used is shown in Figure I The decomposition flask I $ a 2-liter round-bottomed flask with 3 35/20 spherical
socket joint. A IO-ml. buret having a pmssure-equalizing bypass and take-off arm is attached t o the flask. The buret is closed by a tightly fitting rubber stopper. A standard ball-andsocket locking clamp is used to hold the joint between the flask and buret in order to prevent leakage. The take-off arm on the buret is connected through a T-tube to a manometer by means of
Table I. Soh No.
Analyses of Lithium Aluminum Hydride Solutions
Vol. of Sample
m.
Molarity Net Free Pressure Molarity b y -41 Volume Increase Found Rlethod
MZ.
.irm. HQ
1
8.35 8.05 10.00
1992 1984 1974
210 204.5 252.5
2
9.83 9.90 9.67 8.32
1990 1980 1971 1962
236 238 231.5 199.5
3
10.20 10.55 10,15 10.40 10.80
1990 1979 1969 1995 1984
162 170 169 168.5 173.5
4
11.40 10.15 10.50 10.85 10.55 10.70
1989 1978 1968 1957 1946 1936
Difference
0.735 0.740 0.732
0.732 0.714 0.723
Av. 0.736
0.723
0.701 0.699 0.693 0.691
0,710 0,707 0.710
Av. 0.696
0.7OY 0,474 0.479
0.013
0.477 0,314 0.319 0.315 0.312
0.006
0.315
0.005
0.464 0.468 0.481 0.474 0.468 hv. 0 . 4 7 1 122 0.312 108 0.309 112 0.308 117.5 0 311 114 0.309 116.3 0,309
hv. 0 . 3 1 0
0.013
312
ANALYTICAL CHEMISTRY
small-bore tubing. h drying tube is attached to the T-tube with a short piece of rubber tubing which may be closed by a pinchclamp. Before use, the volume of the entire system is measured to Tvithin a few milliliters and the volume of the ungraduated lower portion of the buret above the stopcock is determined accurately. PROCEDURE
The decomposition flask is clamped into place and surrounded rvith crushed ice and ice water. A mivture of 160 ml. of cold 10% sulfuric acid and 40 ml. of cold ether is placed in the flask and the remainder of the apparatus is assembled. The apparatus is allowed t o stand for 5 minutes with the pinchclamp closed. If a change of pressuie is observed, the pinchclamp is opened momentarily and the process repeated until equilibrium is reached Approximately 10 ml. of the lithium aluminum hydride-ether solution are added to the buret and its volume is estimated t o 0.01 to 0.02 ml. The system is closed and allowed to stand for a few minutes to ensure that it is a t equilibrium. The solution is run into the flask slowly until the buret is drained. After equilibrium is reached (usually 5 t o 10 minutes), the increase in pressure is read to 0.5 mm. The system is readied for a subsequent analysis by opening the pinchclamp and re-equilibrating to s t mospheric pressure. The concentration of hydride is calculated from the equation: Molarity =
Stopper
To Manometer
c-
-
Pinch Clomp
Drying Tube
Ice Bo.th
-
pressure increase (mm. of Hg) X net free volume (nil.) volume of sample (ml. ) X 68,100
The net free volume is the total volume of the system less the volume of all solutions added. The factor 68,100 combines R, T , and the fact that 4 moles of hydrogen are liberated per mole of lithium aluminum hydride.
Figure 1. .ipparatus for Analysis of Lithium .4luminum Hydride
RESULTS
LITERATURE CITED
Solutions a t four different concentrations of lithium aluminum hydride in ether 1vei-e analyzed by this procedure. As a check method, samples of these same solutions were decomposed by 1 S hydrochloric acid and the aluminum was determined by the. method of Snyder (4). The results of these determinations, given in Table I, indicate that the method has good reproducibility and for most purposes is sufficiently accurate for the quantitative determination of lithium aluminum hydride in ether.
(1) Finholt, A . E., Bond, A. C., and Schlesinger, H. I., J . Am. Chem. SOC.,69, 1199 (1947). (2) Finholt, A. E., Schlesinger, H. I., and Wilabach, K. E., paper presented before Division of Physical and Inorganic Chemistry, 110th Meeting of AM.CHEM.SOC., September 1946. (3) Nystrom, R. F., and Brown, W.G., J . Am. Chem. Soc., 69, 1197 (1947). (4) Snyder, L. J., ISD. EXG.CHEM.,.IXAL. ED.,17, 37 (1946). RECEIVED July 17, 1947.
Determination of Monomer in Polystyrene Spectrophotometric and Solubility Methods J. J. MCGOVERN, J. 31. GRIM,
AND
W. C. TEACH, MeZZon I n s t i t u t e , Pittsburgh, P a .
KXOWLEDGE of the amount of monomer left after polymerization is of importance in the manufacture of polystyrene. The conventional method of determining the monomer by ascertaining the methanol-soluble fraction of the product gives satisfactory results, but the time involved is lengthy, a matter of several hours. The fact that a spectrophotometric procedure will reduce the time required for an analysis t o approximately half an hour is of interest for process control purposes. This paper describes a spectrophotometric procedure and also one dependent on solubility in methanol, as \Tell as a comparison of data obtained by the two methods. RESIDUAL MONOMER STYRENE BY SPECTROPHOTOMETRIC METHOD
An examination of the absorption spectra of monomeric and polymeric styrene revealed that, while the former possessed bands a t about 282 and 291 millimicrons, the latter shon-ed only slight general absorption in these regions (Figure 1). Owens (3) had utilized this fact to analyze partially polymerized styrene samples with a medium quartz spectrograph. The authors have adapted the method to a spectrophotometer and further studied
the variables affecting the analysis. The elimination of photographic detection greatly simplified the labor involved and reduced to about 30 minutes the total time required for a single determination. The presence of dimer styrene and other polymers of low molecular weight would tend to give high results for the monomer analysis, inasmuch as their spectra overlap that of the monomer (Figure 1): However, after the polymerization reaction, it is believed that little material of such low molecular weight remains. AS regards higher polymers, it vias thought worth while to investigate the dependence of the monomer absorption upon the molecular weight of the accompanying polymer. Accordingly, the absorption of 1 and 5y0monomer styrene solutions, also containing 99 and 9570, respectively, of polymer fractions of varying molecular weights, was measured near 282 and 291 millimicrons. Six samples u ere investigated which contained polymer whose specific viscosities (1) divided by the concentration ranged from 0.5 to 5.4. Kithin the experimental error, constant absorption at each said wave length was observed (Figure 2). These results are in agreement with the work of Smakula ( 4 ) and Meehan ( 2 ) , who found that polystyrenes of varying molecular weights showed
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