Analytical Methods for Butadiene Symposium on Analytical Methods in the Manufacture and Utilization of Butadiene, sponsored jointly by Technical Committee H, American Society for Testing Materials Committee D-2, and the Synthetic Rubber Division, Reconstruction Finance Corp., held February 6, 1952, in Washington, D. C.
Determination of Individual Acetylenes in Butadiene and in C, Hydrocarbon Fractions R. F. ROBEY, B. E. HUDSON, JR., AND H. K. WIESE Chemical Dicision, Esso Laboratories, Linden, .V. J .
As little as 0.029’0 of ethylacetylene (1-butyne) and vinylacetylene (1-buten-3-yne) can be determined separately and quantitatively in mixtures with other C4 hydrocarbons by a relatively simple procedure. Acetylenes are precipitated as mixed silver acetylides by treatment with alcoholic silver nitrate. The liberated nitric acid is titrated as a measure of total acetylenes. The acetylides are treated with aqueous potassium bromide to regenerate the mixed acetylenes in concentrated form. Analysis of the concentrate for the individual acetylenes can be conducted bromometrically as well as by physical methods. Each step w-as studied to determine conditions for attaining satisfactory accuracy.
t o the analysis of an acetylene concentrate, the authors’ euperience indicates that bromometry is the more convenient chemical procedure. The hydrocarbons of major concern, their structures, and boiling points are listed in Table I. Acetylene, biacetylene, and methylacetylene interfere with analyses for ethyl- and vinj-Iacetylenes. Any methylacetylene that remains in the sample appears as its equivalent percentage of ethylacetylene Biacetylene has not been found to date in commercial butadiene. However, simple qualitative tests for the presence of acaetylene and biacetylene are available (6). Volatile mercaptans (thiols), amines, and other low boiling substances that react with silver nitrate to liberate nitric acid or that deplete the acid liberated from silver nitrate by acetylenes, must be removed prior t o analysis. Dimethylacetylene, 1,2-butadiene, butenes, butanes, and 1,3-butadiene itself have no effect.
B
UT.iDIESE that meets the current specifications for synthetic rubber raw material contains no more than 0.10% by !\-eight of acetylenes. Solely on the basis of boiling points, the acetylenic components could be ethylacetylene or vinylacetylene or both. Because of the low concentrations generally present, direct application of established chemical or spectrometric methods of analysis fails t o distinguish between these two substances. The fact that ethylacetylene and vinylacetylene behave somewhat differently both in commercial butadiene purification processes and in polymerization operations was demonstrated ( 5 ) during the critical war years. Consequently, a suitable procedure was developed for quantitative determination of individual alpha-acetylenes in Ca hydrocarbon mixtures as well m in product butadiene. A4nalysiscan be carried out in three steps: (1) complete precipitation of acetylenes as mixed silver acetylides; (2) treatment of the segregated acetylides t o regenerate the mixed acetylenes in concentrated form; (3) quantitative determination of the individual acetylene8 in the concentrate by a special bromometric procedure. The authors have already described ( 1 ) an infrared spectrophotometric procedure for analysis of acetylene concentrates. It seems likely that the mass spectrometer could be applied also. Catalj-tic hydrogenation also differentiates ethylacetylene and vinylacetylene in C, hydrocarbon gases ( 7 ) . Large corrections must be made for hydrogen consumed by mono-olefins and diolefins As a result, the method is not well suited to small percentages of acetylenes. -4lthough hydrogenation can be adapted
PRINCIPLES INVOLVED
Alpha-acetylenes react with silver nitrate in alcoholic solution t o form the corresponding silver acetylides (insoluble complex) and nitric acid: HC=C-R
+ 2.4gN03 S AgC=C-R..4gN03
+ HSOj
(I)
An equilibrium is involved. To obtain complete reaction of the acetylenes it is necessary t o neutralize the nitric acid as it is formed. The volume of standard alkali required is a measure of the quantity of all acetylenes. The mixed acetylides, free of entrained hydrocarbons, are treated with aqueous potassium halide solution to liberate the acetylenes: AgC=C-R.AgNOa 2 AgX
+ 2 KX + HZ0 + + KOH + KNO3 + HCzC-R
(2)
Table I. Pertinent Hydrocarbons Geneva Name Ethyne Propyne 1.%Butadiene 1-Buten-3-yne 1-Butyne 1.3-Butadiyne
1080
Common Name
Structure
Boiling Point, 0
c.
- 84
-23.2
+- 485 ... 407 10.5
1081
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 The mixed acetylenes can then be analyard either tironiometrirally or by other methods already mentioned. Under carefully selected catalytic conditions, vinylacetylene adds 2 moles of bromine:
+ 2 BrO --+ HC"r=C'Br-CII?Br
HC-C-CH=CH2
(3)
Ethylacetylene adds only one mole: HC=C-CHO-CH~
+ Bra +HCBr=CBr-CEIp--CH3
(4)
The relat'ive proportions of vinyl- and alk>-lacetylenes can be calrulat,ed from the consumption of broniinc' and the determined total quant,ity of acet'ylenes. Because of the instability of the compound? involved, an analysis onre st,arted should be completed n-ithout undue delay. PRECIPITATIOR- OF ACETYLIDES
Apparatus. A diagram of a typical arrangement is shon-n in Figure 1. T o accommodate a maximum of 0.05 gram of Cq acetylenes (0.001 mole; about 25 ml. measured as gas under room conditions) the two test tubes should be about 25 X 250 mm. and 19 X 180 mm. in size, respectively. The t.himbles should, of course, extend close to the bottoms of t,he tuhes. K i t h these provisions, a buret of 25-ml. capacity is suitable. Reagents. SILVERKITRATESOLUTION.Dissolve 2.0 grams of C . P . silver nitrate in 1.5 ml. of dist'illed n-ater and make up to 80 ml. with absolute methanol. Immediately disca1.d any portion not used. MIXEDISDICATOR. Dissolve 0.1 gram of methyl red and 0.05 gram of methylene blue in 100 ml. of 95% ethanol. STANDARD ALCOHOLIC ALKALI SOLUTIOS, 0.05 -1-sodium hydroxide in methanol. Procedure. Add 65 ml. of silver nit'rate solution to the first test tube and 15 ml. to the second. Add 4 or 5 drops of mixed indicator to the first and 2 drops to the second. Place the apparatus in an efficiently ventilated hood and behirid a safety glass shield away from all sources of ignition. Slowly pass sufficient gaseous sample to contain 0.025 to 0.05 gram of acetylenes through the test tubes or bubblers. It is convenient to vaporize liquefied sample from a chilled heavy-walled filter flask or from a small liquefied-gas cylinder. Keep t,he solution in the first bubbler essentially neut,ral by careful periodic addition of standard alcoholic sodium hydroxide solution from the buret. It is best to keep the solution a little on the ,,acid side," because the solution becomes very dark when basic and the end point may prove difficult to detect. The total tit.er should amount to a t least 10 ml., but riot more than 20 ml., in a valid run. Furthermore, no prwipitate should be evident in the second bubbler. Larger amounts of acetylenes can be accommodated if proportionately larger bubblers and volumes of reagents are used. I n any case, after enough sample has been passed, pour the contents of the first bubbler (silver acetylide slurry) into a 250-ml. flask and use the contents of the second bubbler to rinse the first. Complete the analysis as directed below. Calculation. Mole per cent total alpha-acetylenes in sample,
T =
A X N X At 10
x It'
where -1 = ml. of standard alkali -\- = exact normality of standard alkali -11 = average molecular weight of sample TI' = grams of sample BROMOMETRIC AX4LYSIS
Apparatus. A Gooch filtering assembly, several pipets, and a
