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422
cold titration, but if the alkali used to neutralize it contains silicate, the silica will also become fixed aa potassium fluosilicate and high results for the subsequent hot titration ensue. The magnitude of this error depends upon the length of time the standard alkali had been stored in g l w and, of course, the amount used in the cold titration. To eliminate this source of error the standard alkali must be stored in a heavily waxed bottle (high melting point mineral wax) or better still, in a steel drum. If stored in a steel drum, it must be siphoned over through an iron or nickel tube. An alkali-filled scrubber should be attached to the drum to remove carbon dioxide from air entering as solution is removed. This procedure is a modification of the method given by Kolthoff and Furman ( 8 ) . The method is capable of giving results of considerable accuracy, aa shown by experiments in which known amounts of silica were added to reagent hydrofluoric acid. Optical quartz ground to IWmesh, wmhed with hydrochloric acid, and ignited was used. Harshaw reagent hydrofluoric acid uas added to
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the quartz and allowed to stand at room temperature with frequent stirring until the quartz dissolved before proceeding with the method. Results are shown in Table I. HYDROFLUORIC ACID. To obtain the hydrofluoric acid content ’ HzSiFI, deduct from the total acidity the sum of 0.8330 X % 0.4078 X % HBO,, and 0.6243 X % SO2. (Note. The factors used throughout this procedure and in t,he standardization of the alkali are based on rational atomic weights, 1, inasmuch as the atomic weights of hydrogen and fluorine are low.) Table I1 shows the results of duplicate analyses of eight different tank cars of anhydrous hydrogen fluoride by this method. LITERATURE CITED
(1)
Kolthoff and Furman, “Volumetric Analysis”, Volume 11, pp. 37, 512, New York, John Wilev B Sons. 1929.
(2) Ibid., p. 127.
Lange, N. A., “Handbook of Chemistry”, 5th ed.,p. 1175, Sandusky, Ohio, Handbook Publishers. 1944. (41 Scott’s Standard Methods of Chemical Analysis, 5th ed., p. 2209, New York, D. Van Noatrand Co., 1939. (3)
Infrared Analysis of Butadiene L. J. BRADY, Mcllon institute, Pittsburgh, PI.
IT HAS
been pointed out many times in the literature (1-5, Y, 8 ) that the infrared absorption spectrum of an organic molecule is unique, and that with few exceptions admixture with other compounds does not affect this property. As a result, the concentration of any component in a mixture can be determined by infrared analysis, provided it has a t least one absorption band at a wave length where the other components are relatively transparent. Within experimental error, the absorption measurements made on the usual research type of infrared spectrometer follow the familiar Beer’s law, I = Ioe-ky where ZO is the energy incident on the sample, I is the energy transmitted by the sample, k is the absorption coefficient a t wave length A, e is the concentration, and z is the length of the optical path through the sample usually equal to the length of the absorption cell. This expression can be put in the alternate form:
butene-2, are of the same order of magnitude (Figure 2). As a consequence, as long as the concentration of the combined impurities remains constant, fluctuations in their relative amounts will have no significant effect on the optical densities of reiined butadiene samples. I t is obvious, therefore, that the total im-
2
4
Le@
6 8 Absorption Cell Ilan.
IO
12
c
.-
-E
(.
a
0
where T i s the transmission fraction, c is the extinction coefficient, and D is the optical density. For monochromatic radiation the optical density of 8 mixture equals the s u m of the optical densities of the components-that is, at any wave length A:
D = di
+ d t + dr +. . . . . d.
where D is the optical density of the mixture and d, is the optical density of the nth component. In general, n such equations, one for each of n selected spectral positions, are. required t o determine the concentration of each component present. It frequently happens in industrial work that the concentration of only one component present in a mixture is of interest. In such casea the analytical procedures can often be simplified. This paper discusses such a problem, the infrared analysis of refined butadiene. Refined butadiene contains varying amounts of impurities such as butene-l, butene-2, perhaps traces of isobutylene and acetylenes, together with small amounts of saturates, the latter usually amounting to less than 5% of the total impurities present. These impurities not only absorb strongly at 6.9 microns (1450 om.-,) where butadiene is relatively transparent, as shown in Figure 1, but the optical densities of the major impurities, butene-l and
c
L 0
c
. 0 0
n
5
6
r
8
Wave Length in Micron8 Figure 1.
Inhrred Absorption Curves of Butadiene and Associated Impurities
ANALYTICAL EDITION
1944
423
Opticd Dansity
Figwe
P.
Trmrmiaion of ButadirnrBuhnrs Mixtures at 6.9 Mi"0"S
Figure 4.
Inhared Shutter Mechanism
For plant analysia the feed-back control on the photoelectric potentiometer is set cut a maximum. The source and slit widths are then rtdjusted until nearly full-scale deflection is obtained with butadiene in the sample cell or with dry air, in either the sample cell or dummy and !he absor tion screen in place. After standsrdieing the dmrption cells using the screen and dry air the analysis of butadiene is carried out ss follows: Continuously Bowing streams of gaseous butadiene are brought to a distribution manifold near the spectrometer, The sample passas from the manifold over solid potassium hydroxide into the sample absorption cell, thence exits through the barostat shown in Figure 5 to the atmosphere. The baroatst acts as an exit valve on the absorotian cell and a t the same time adjusta'the pressure of the butaditw to k ~ o n s t m tvalue. The Figure 3.
Infrared A b o r p t i o n Cells and Shutter Mechanism
puritias a n be determined from the transmission of the sample a t 6.9 microns, the butadiene being determined hy dserence. This procedure is to be preferred over that of determining the hutadiene concentration directly because a consideration of the exponential nature of Beer's law make8 it evident that the precision of B direct rtnalysi-, for a component falls off as its concentration approaehea 100%. EXPERIMENTAL
The results were obtained with two infrared sDectrameters. A large Littrow-typc, automatic rwording ipertr6metcr of I-meter focal length sbich has a 100 X 150 mm. rork eali prism was uwd tQ devrlup the analytical m e t i m l . This instrunietit ubc'z a photoelectrir rmtcntioroetcr(C)und n Leds & Surthrup Spedoman to w o r d t l r tranmii4un CUIVCJ. l'hr plant control hstrument has a M X 79 mm. rork salt "rim.is iiot automatie. h u t was de-
t h m 10 minutes
Figure 5. BSrO5t.31 for Regulating But.diene Resure in Abrorplion Cell
A comparison of the analytical results which were obtained using infrared anaIy& and the gravimetric d e i c anhydride method is set forth in Table I.
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424
Vol. 16, No. 7
Table I.
Analyses of Butadiene by Infrared and Gravimetric Maleic Anhydride Methods Butadiene Butadiene Gravimetria Gravimetric Infrared maleic anhyInfrared maleic anhyBsmpla analysia dride Sample analysis dride % % % % 1 99.30 99.5 A 99.30 99.3 2 3 4 5
97.20 98.87 98.90 99.43
97.0 98.8 98.9 99.3
B C D E
99.1 99.2 98.7 98.7
99.0 98.9-98.5 98.7-98.8 98.6-98.9
6 7 8 9 10
98.90 99.38 99.64 98.52 99.50
98.9 99.0 99.5 98.6 99.2
F G
98.7 99.1 98.7 98.3 98.3
98.7-98.7 98.8-98.6 99.0-98.8 99.1-99.2 98.1-98.398.4-98.4
13 14 15
98.87 99.42 99.53 99.30 99.60
98.9 99.4 99.6 99.6 99.5
99.1 99.2 98.7 99.0 99.1
99.1-99.1 99.1-99.2 98.9-99.0 98.9-98,9 99.0
16 17 18 19
98.7 98.7 99.1 98.7
98.9-98.6 98.7-98.7 98.8-98.6 99.0-98.9
99.2 99.2 98.7
98.9-98.5 98.7 98.7-98.8
l12 1
0.7000
0.8OoO
0.9006
Log Deflection of Meter
Figure 6.
Work Cune for Butadiene Analysis DISCUSSION
Beer’s law is an expression involving the number of moles present in the absorption path; consequently, an error will be introduced by substituting weight percentage for mole percentage in the work curve for butadiene. Other errors introduced through the differences in the extinction coefficients for the various impurities make it inadvisable to extend this method of analysis to butadiene concentrations below 90%. The 37 different high-purity butadiene samples shown in Table I have an average absolute deviation of 0.21% between the infrared and gravimetric maleic anhydride methods of analysis. Other studies revealed that the average absolute deviation for a number of infrared determinations made a t various times by different operators was 0.040/, while the gravimetric maleic anhydride method for the same series of analysis had an average absolute deviation of 0.17%.
n I J
K
L M N
0
A1 A2 A3
The gravimetric maleic anhydride method requires from 5 to 6 hours as compared t o the 10 minutes needed to make duplicate infrared determinations. As a result of its rapidity and precision this infrared method of analysis is particularly valuable for industrial work. LITERATURE UTED
(1) Avery, J. Optical SOC.Am., 31, 633 (1941). ENQ.CHEM.,ANAL. ED., 15, (2) Barnes, Liddel, and Williams, IND. 83 (1943). (3) Ibid., 15,659 (1943). (4) Brattain and Buck, J. Applied Phys., 13, 699 (1942). (5) Brattain, Rasmuasin, and Cravath, Ibid., 14,418 (1943). (6) McAlister, Matheaon, and Sweeney, Reo. Sci. Inst., 12,314 (1941 1 . (7) Nielson, O i l Gae J . , 40,34 (1942). (8) Wright, IND.ENQ.CHEM.,ANAL.ED.,13, 1 (1941).
C O R ~ ~ B I J T Ifrom O N the Koppers Cornpanfa Fellowship on Coal Products Analysis, Mellon Institute, Pittsburgh, Pa. The procedure deaaribed in this contribution wan originally evolved for and applied with sueceas in the butadiene plant at Kobuta. Pa.
Identification of Natural and Synthetic Rubbers H. P. BURCHFIELD Development Department, Naugatuck Chemical Diviaion,
U. S. Rubber Co., Naugatuck, Conn.
A method i s described for identification of the types of elastomers mod frequently encountered in the rubber industry. The Initial test depends on qualitative measurement of the pH and specific gravity of the pyrolysis producta. It can be carried out in a field laboratory in 3 to 4 minutes and will provide sufficient information for a classification of the sample.
lations necessary for a complete analysis. The method was d e signed for the identification of soft-rubber vulcanieates based on the polymer types represented by natural rubber, Buna 5,Buna N, Butyl, Neoprene GN, chloroprene-nitrilepolymers, polyvinyl chloride, and polyvinyl acetate. Distinctions within types are not possible. The polysulfide ruhbers are not included, as they can usually be recognized by odor.
THE
Identifications made on the basis of physical appearance and flame tests are discussed by Kluckow (4) and Nechamkin (8). Mark (6)describes the application of some simple chemical tests such as those for nitrogen, chlorine, and sulfur. The chromic to acetic add is described by acid oxidation of natural Kuhn and L’Orsa (6): Burger. Donaldson, and Baty (8) de-
use of synthetic elastomers as substitutes for natural rubber has given rise to B need for a rapid method by which thwe matefials can be distinguished from one another. The methods of identification available can be divided into two crows: those which deDend on the personal j u d m e n t of the
