0
Sulfuric Acid Analysis of Gaseous Olefins JIARYAS P. 3IATUSZ 4 K , Phillips Petroleum Company, Bartles, ille, Okla.
sorption products; (4) solubility of hydrocarbons in precipitated polymer products; and (5) liberation of unabsorbable gas by overstrong absorbents, including those containing silver sulfate. Suitable apparatus and technic for overcoming these effects are described. Within its natural limits of applicability, the improved method gives, for samples of all possible olefin concentrations, rapid and reliable results.
Principles and limitations governing the selection of conditions for determination of gaseous olefins by absorption in sulfuric acid are presented briefly. Data indicating the influence of the following previously uninvestigated factors upon analytical results are given: (1) reversibility of absorption; (2) solubility of gaseous paraffins in sulfuric acid; (3) increase in solubility of hydrocarbons because of acid-soluble ab-
S
concentration, the rate of absorption was constant. The observed constant rates of absorption are plotted against acid concentration in Figure 1. Dobryanski recommended 63 to 64 per cent sulfuric acid for absorbing isobutylene; 83 to 84 per cent acid for propylene, butadiene, and n-butenes; and 100 to 102 per cent acid for ethylene. Figure 1 indicates that 63 to 64 per cent acid absorbs isobutylene about 500 times as readily as propylene, and that 83 to 84 per cent acid absorbs propylene about 500 times as readily as ethylene. Contact of a gas sample successively with acid of these strengths thus removes in turn isobutylene, propylene, and ethylene. n-Butenes are absorbed together with propylene because their rate of absorption averages only about twice that of propylene (5, 6, 11). Separation can be effected readily by a preliminary fractional distillation into three-carbon and fourcarbon fractions which then are analyzed separately by sulfuric acid absorption. The diolefin butadiene, if present, is absorbed, for the most part, with the n-butenes. Its presence is indicated qualitatively by an intense yellow c o l o r a t i o n of t h e a c i d . Bllene appears to be absorbed at a rate similar to that for butadiene (3). If five-carbon vapors are present, isoprene and tertiary amylene5 (trimethyl ethylene and unsymmetrical methyl ethyl ethylene) are absorbed with isobutylene; n-amylenes (pentene-1 and pentene-2) and isopropyl ethylene are absorbed with propylene and n-butenes (4, 10). For separation from propylene and butylenes, fractional distillation and dilution with an inert gas, followed by sulfuric acid absorption, can be used ( 4 ) .
YSTEMATIC selection of analytical conditions in accordance with the principles and the limitations discussed in this paper makes the determination of gaseous olefins by absorption in sulfuric acid quantitatively accurate.
Principles
RATESOF
ABSORPTIOS. Figure 1, based primarily on the work of Dobryanski ( 6 ) , n-ho made the first of sereral recent studies of the absorption of gaqeous olefins by sulfuric acid (2, 3, 6, 11), indicates the relationship between acid concentration and rate of absorption. Dobryanski pafsed all of a 100-cc. sample of olefin from a gas buret into a cylindrical absorption pipet having no contact tubes. After permitting the acid on the pipet wall to drain for 5 to 10 minutes, he returned about 50 cc. to the buret. Then, after closing a special stopcock in the U-connection leading t o the acid-expansion chamber, thereby preventing disturbance of the acid surface, which had a constant area of about 19.9 sq. cm., be followed the absorption by direct buret readings. For each particular combination of unwturatetl gab and acid RELSTIYE
LOGARITHMIC VARIATIOX ABSORPTION RATEWITH A c ID C oN cE N T RAT I ON. OF
FIGURE1. ABSORPTIONOF
~ S S A T l 2 R A T E D GASES BY SULFCRIC
354
ACID
Since the rate of absorption, as is indicated by the straight curves of Figure 1, increases logarithmetically with increase in
ANALYTICAL EDITIOX
JULY 15, 1938
concentration of sulfuric acid, use of the strongest concentration consistent with reliable results greatly shortens the time required for an analysis. ABSORPTIOK RATEPROPORTIOSAL TO ACID SCRFACE.The rate of absorption is independent of the volume of the acid but is directly proportional to its surface area (3, 6). L'BSORPTION
RATE PROPORTIONAL TO P a R T I A L PRESSURE O F
OLEFIN. Since the rate of absorption is proportional to the partial pressure of the olefin (3, 5 ) , the end of the absorption is approached s l o ~ d y . Limitations IKCOJIPLETE SPECIFICITY OF SULFURIC ACID. Because a small amount of propylene and n-butenes can be absorbed with isobutylene and a little ethylene can be absorbed with propylene and n-butenes, corrections for incomplete specificity of absorbents must be made. The usual method of making corrections is to cont'inue the absorption with one body of absorbent until a small constant absorption per pass is obtained and to multiply this by the number of passes (8-fI). Changes effected by the absorption-in the composition of the gas and in the composition and the effective surface area of the absorbent-and multiplication of small errors by the large number of passes required for complete absorption, especially for high olefin contents, make this method, if unmodified, uncertain and subject to large errors. REVERSIBILITY OF ABSORPTIOII. Although under gasanalytical conditions the absorptions of ethylene and of propylene are so nearly completeas to be justifiably regarded as being irrerersible, those of the butylenes-probably also of the aniylenes-are distinctly reversible. Unless more than one portion of acid is used, u p to more than 1 cc. of butylene remains unabsorbed. PHYSICAL SOLUTIOK OF PARAFFIXS IS SCLFCRIC ACID. Published data on the solubility of gaseous paraffins in sulfuric acid appear to be limited to the follom-ing for methane a t 20" C. and one atmosphere (1): Per cent H ~ S O I Cc. hleH per cc. acid
0.0 0.0350
35.9 0.0169
61 7 0.0131
95.6 0.0308
Table I presents approxiinate yalues obtained a t about 25' C. for the physical solubility of n-butane in sulfuric acid. Three or four 30-second passes of 90 to 100 cc. of the gas were made into the pipet described below, which contained about 2 cc. of acid. The absorption was usually complete after two passes; two-thirds of i t occurred in the first pass. After the residual volume was read, the acid abore the mercury in the pipet was replaced by a fresh 1.0-cc. portion. This procedure was repeated until a total decrease sufficiently large to be measured by the buret was obtained. Presence of up to 1.50 per cent silver sulfate in 96 per cent acid had no effect. SOLUBILITY OF n-BCT.kNE TABLE I. PHYSICAL ACID SOLUTIONS
HISOI
%
a 6
Soluhilltp,
Cc n-BuH cc, Ac,d
H2S0r
IN SULFURIC
SolubllltY,
Cc. n-BuH Cc. Acid
-
%
102.0% HI SO^ = 100 0 % HpSO, conzaining 2.0% free SO3 by weight. 104.0Y0 H2SOi = 1 0 0 . O ~ oH?SOa coniainmg 4.0% free SO3 by weight.
The data indicate that the error due to solubility of paraffin gases in 1-cc. portions of sulfuric acid of concentrations below
355
TABLE 11. ISFLUENCE OF PRESENCE OF DISSOLVED PRODUCTS ON OLEFIN DETERMINATIONS
H~SOI
Gasesa Taken
%
CC.
99.7 99.7 99.7
E, 4 9 . 6 0 ; N2, 50 10 E, 5 7 . 0 0 ; N?, 50.20 E, 79 70; n-BuH, 19.80
I-cc. Max. Acid 30- Absorbed Por- See. per tions Passesh Portion Olefin
% 1
1 1
9 1 . 0 P, 79 45; S2, 20 10 9 1 . 0 P,4 9 . 8 0 ; n-BuH, 50.00 8 8 . 5 P, 4 9 . 6 0 ; N2, 49.90 8 8 . 5 P,80 15; N r , 1 9 . 7 0 8 8 . 5 P, 8 0 . 2 0 ; n-BuH, 19.80 8 8 . 5 P. 4 9 . 9 0 ; n-BuH, 49.75 8 4 . 0 P,5 0 . 1 0 ; S ? ,49.70 84 0 P,50 10; n-BuH, 49 60
20 28 32
49.50 106.33C 80.00
99.8 99 8 100.4
6 6 8 12 16 10
97 4 97.8 97 4 97.4 97.6 97.5 97.4 97.5
Error
% ... +0.'6
16
78.40 42.55 24.85 32.85 34.35 23.25 48.76 48.85
9 9
48 48
11.90 13.50
99 6 99 7
+0:1
n-2, 49.60; N?, 4 9 . 5 5
4 11
16 47
47.90 14.60
99.8 100 0
+0:2
N?, 50 35
8 5 8 8
79.9 79.9
B-I, 49 50; N2, 4 9 . 5 0 B-I, 49.40; n-BuH, 50.20
8 4 ,0 79.9 70 2 70 2 70 2 64.7 64.7 64.7 64.7 64.7
i-B, 5 0 . 4 0 ; i-B, 5 0 . 0 0 ; i-B. 5 0 . 3 0 ; i-B, 49.05; i-B, 79.50; i - B , 4 9 50; i-B, 80.15; i-B, 48.90;
B-2, 49.50; n-BuH, 5 0 . 2 3
16
+0:4
...
+Or2 +0.1 +0:1
24 16.95 99.6 n-BuH,40.75 24 20.50 99 9 +Or3 n-BuH. 49.50 24 25.45 99.6 0.0 N2, 49.90 28 25 40 99.6 ,.. 99.6 ... 28 22.80 ti?.19 90 n - B u H , 4 9 80 i 36 48.55 99 6 0.0 n-BuH, 1 9 . 7 0 10 32 26.15 99.6 0.0 n-BuH, 49.65 9 28 13.80 99.6 0.0 a E = 99.8% ethylene;, P = 97.4%,propylene; B-1 = 99.6% butene-1; B-2 = 99.8% butene-2; ?-B = 99.6% Isobutylene. b The sample was kepi in the pipet for 30 seconds a t each pass, exclusive of in0ow and outflow ( 5 t o 7 seconds each). C The same portion of acid was used as i n the preceding analysis.
TABLE
111.
5
ISFLUESCE O F PRESESCE O F VSDISSOLVED POLYMER ON BUTYLEXE DETERYIS 4TIONS Max. ib30- sorbed See. per tions Passes Poi tion l-CC. Acid Por-
HlSO;
Gases0 Taken
%
cc.
SS 5 19.9 88.5 88.5
B - I , 50.30; S ? .50.13 B-1, 49.50; Nv2, 49.45 B - I , 49.95; n-BuH, 49 90 B-1, 50.55; n-BuH, 4 9 . 3 0
84 0 89 3 88 5 86.0 84.0 84 0 84 0 82 0 82 0 79.9
B-2, 4 9 . 60; B-2, 48 40; B-2, 50.35; B-2, 49.55; B-2, 49.45; n-2, 49.90; B-2, 49. 90; B-2, 50.50; B-2, 50.80; B-2, 49.50;
5 '3 1
G
N2,4 9 . 5 5 n-BuH, 4 8 . 5 3 n-BuH, 4 9 . 5 0 n-BuH, 49.60 n - B u g , 49 25 n-BuH, 50.10 n-BuH, 49.00 n-BuH, 49.45 n-BuH, 49.20 n-BuH, 5 0 , 2 5
Olefin
%
,..
12 48 12b 16
34.35 11.90 51.20 34.70
99.6 99.6 102.5 100 8
20 7 125 11 12 c 17 13 28 27 47
47 45 52 42 49 29 20 41 18 14
99 102 103 101 100 100 100
90 45 05 40 70 30 00 45 30 60
Error
%
8 2 4 3 5 3 2 100 3 100 2 100 0
+2:9 f1.2 +2:4 f3.6 +1.5
+0.7 +0.5 +0.4 +0.5 +0.4 +0.2
44.80 99.6 ,.. 49.10 99.6 53.35 104.4 46.90 101.1 + 1 . 5 0 B-1 = 99.6% butene-I; B-2 = 99.8oJ, butene-2; i-B = 99.6% isobutylene. b Absorption was incomplete; additional passes removed ahout 0.2 cc. per pass. c Additional passes removed about 0.05 cc. per pass. d Additional passes removed about 0.3 cc. per p a s .
7 7 . 5 i-B, 50 20; N?,4 9 . 9 0 7 7 . 5 j-B, 49 45; N?, 5 1 . 1 0 7 7 . 5 I-B, 51.10; n-BuH, 49.30 7 7 . 5 i-B, 49.90; n-BuH, 4 9 . 8 5
5 3 1 6
10 10 10d 10
44,'s
about 90 per cent is negligibly small. I n stronger acids it can be fairly large. Obviously, the use of small portions of acid (7) is superior to that of the large volumes sometimes employed. PHYSICAL SOLCTIOSIX ABSORPTIONPRODUCTS. In the analyses of synthetic mixtures given in Table 11, the errors noted were due to the increase produced in the solvent power of the acid for n-butane by dissolved olefin-absorption products, Acid portions of 1.0 cc. were used; but as approximately 1 cc. could not be removed and replaced because of wetting of the walls of the pipet and the contact tubes, a total of 2 cc. was present. The effect of solubility in the absorbent itself was eliminated by previous saturation of the firat portion with n-butane and by use of the solubility data already presented for estimation of corrections for subsequent portions. The magnitude of the error decreased with de-
VOL. 10, NO. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
356
~
TABLE Iv.
HzS04 %
Gases5 Taken
cc.
E, 79 60; Nz, 20 20 E, 50 i o ; Na, 49 65 E, 50 5 5 ; Nn, 49 60
96.3
P, 49.75; Kz,49.55
1
96.3 96.3 94.0 94.0 94.0
P, 51.45; P, 49.45: P, 49.75; P, 4 9 . 6 0 ; P, 49.35;
W z , 49.90 Nn, 50.15 Sa, 49.80 Ka, 50.95 Iiz, 49.60
2 3 1 3 3
E. 79 80; E , 50 60; E , 79 70; E , 50 20; E, 49 45; E, 49 60; E, 49.60; E, 57.00; E, 49 90;
n-BuH, 19 , 8 0 Tz,49 10 Iiz, 19 70 Nz 49 80 n-BuH, 491.30 N n , 50 25 Nn, 50 10 Nz, 50 20 Tn, 50 00
1 1 5 1 1 1 5 5 6 1 1 4 I
96.3 B-1, 49.50; Kz,50.20 9 6 . 3 B-1, 50.40; Sn, 49.60 96.3 B-1, 50.45; Nz, 49.70 9 4 . 0 B-1, 50.75; Nt, 49.30 94.0 94.0 91.4
E
B-1, 49.70; Nz, 50.40 B-1, 50.85; Sz, 49.75 B-1, 49.35; Nz, 50.65
!I 2 3 1 1 2 3 1 1
~~
IKFLOENCE O F OVERSTRONG ACID O S OLEFIN DETERMINATIONS
Max. I-cc. AbAcid 30sorbed Por- Sec. per tions Passes Portion
104.0b 104.0 104.0 102.0d 102.0 102.0 102.0 102.0 102.0 99.2 99. I 99.7
~~
6'
4c
1oc 10 8C
9 7 8e
20 "8 19 2< 4c 2 3e 3c 3 3 8
2c 4c 2 3e 2c 4C 2 3e 2C 4'
- .
%
Error %
i5.10 47.55 28.90 77 95 49 45 77.90 23.00 30.30 13.70 49.50 106 45/ 17.55
94 4 93 7 97 0 97 7 97 7 97 8 97 8 98 5 99.8 99.8 99.8 99.8
-5.4 -6.1 -2.8 -2 1 -2.1 -2.0 -2.0 -1.3 0.0 0.0 0.0 0.0
48 30
97 1 96.9 97.3 97.4 97.4 97.4 97.4
-0.3 -0.5 -0.1 0.0
97.0 95.Q 97.4 97.1 97.6 96.7 97.8 98.7 98.2 97.6
-2.6 -3.7 -2.2 -2.5 -2.0 -2.9 -1 8 -0.9 -1.4 -2.0
50:05 42.80 48.45 47.75 42.55 48.00
4a:;a 46 95 49.60 ,
.
47.75 47 30 48.55
...
lTa.t. 1 :-' c A\-4cid 30- scrbed PorSec. per tions Passes Port1 Jn Olefin
Olefin
0.0 0.0 0.0
HnSO1
Gasesa Taken
%
cc.
9 1 . 4 B-1, 49 65; N s , 50.00 9 1 . 4 B-I, 50.20; Sn, 4 9 . 3 5 9 0 . 3 B-1, 49.80; Xa, 5 0 . 2 0 9 0 . 3 B-1, 4 9 . 0 0 ; N2, 49.60 9 0 . 3 B-1, 49.80; S?,4 9 . 1 5 96.3 96 3 96 3 94 0
B-2, 49.30; n-2, 50.30; B-2, 49.65; B-2, 49.73;
Tn, 5 0 . 6 5
N ~ so , 05 S ? ,49.70 ?i?,50 60
49.70 9 4 . 0 n-2, 49.65; sn, 94,O B-2, 49.70; 50.30 9 1 . 4 B-2, 5 0 . 4 5 ; hr,49 80
N?,
9 1 . 4 B-2, 5 0 . 2 0 ; 9 1 . 4 B-2, 4 9 . 8 0 ; 9 0 . 3 B-2, 50.15: 9 0 . 3 B-2, 50.20; 9 0 . 3 n-2, 49.55; 96.3
Sa. 49.45 Nz, 4 9 . 1 5 Sn, 49 73 Nn, 49 50 x n ,4 9 . 3 5
i-B, 4 9 . 8 0 ; S?,5 0 . 7 5
2 3 1 3 4
2 3e 3C 3 48
48 20 44.15 49.25 46.80 39.40
98.9 98.8 98 6 98.8 99.1
-O.i -0.8 --I 0 -0 8 -0 5
! I
2 C
48.65
98.4 97.5 98.8 99 2 98 9
-2.3 -1 0 -0 6 -0 9
7
2 1 1 2 2 1 1
4c 2 26
2C
49:OO 46 90 49 40
4c
48:40 4 7 , 20 50.30 49'00 46 90 50 05 48 80 45 i o
2 3 1 3 4
48.65
1 1
,..
1
96 3 91 4 91.4 91.4 90.3 90.3
i-B, 50.00: S2, 49.30 i-B, 49.90; Nz, 50 80 i-B, 50.15; i-B, 51.20; i-B, 49.55; i-B, 49.05;
Error
4 7 0 %
48: 20 49.25 I
h*z,50.30 Nn, 49.80 N?.49.90 Nz,49.50
48: 8 5 48.70 49.20 48.00
2 3 1 3
-1
1
9
Q7 9
-1
99.4 99.4 98.7 99.3 99.5 99.5 99.6 99.7
99.0
-0 8 -0.4 -0.4 -1 1 -0.5 -0.3 -0.3 -0 2 -0 1
97.5 96.7 94.5 97.2 98.5 97.8 99.0 99.1 99.0 99.2
-2 1 -2.9 --b. 1 -2.4 -1.1 -1.8 -0 6 -0.5 -0.6 -0.4
P = 97.4% propylene; B-1 = 99.6% butene-I; B-2 = 99.8% butene-2; i-B = 99.6% isobutylene. b 104.0% HnSOd = 100.0% HnSOi containing 4.0% free SO3 bv weight C Additional passes increased t h e residual volume.
d 102.0Y HzSOr = 100.0% HLSOI containing 2 . 0 7 free SO3 by weight. Speciaf short passes, about a seconds long, exc1;sive of Inflow and outflow, were used. I Same portion of acid mas used as in preceding analysis.
crease in concentration of acid and with increase in frequency of replacement of acid. -4s illustrated by the similarly obtained analyses given in Table 111, a similar but much larger error was caused by precipitated polymer products. These, present as a cloudiness or as definite droplets, absorbed n-butane with avidity. iilthough the absorption was one of physical solution, i t did not reach completion during the analysis period because of continued precipitation of polymer. The effect mas largest when only one portion of absorbent was used throughout the analysis. For such analyses Table 111 gives the result after twelve 30-second passes; four additional passes increased the apparent percentage of olefin from 102.5 to 104.3 for butene-1 and from 103.4 and 100.5 to 108.9 and 101.3 for butene-2, respectively. KO attempt was made to correct the data for the increase in solvent power of the acid caused by dissolved products. The data of Tables I1 and I11 indicate that, if the acid is not too strong and if it is replaced so frequently that absorption is limited to less than about 15 cc. per portion, such physical solubility errors become practically negligible. EXABSORBABLE GASLIBERATED BY OVERS~ROSG ABSOEBENTS. The analyses in Table IV, for which 30-second passes were used except as otherwise stated, illustrate the large errors produced by liberation of gas or vapor unabsorbable in fresh acid or alkali when overstrong acids were used. T h e data indicate that, in the analysis of ethylene, if absorption is limited to not more than 15 to 20 cc. per portion, acid up to a t least 100 per cent in strength can be used safely. Similarly, the upper limit of safe acid concentration is about 95 per cent for propylene and about 90 per cent for butylenes. Discoloration of the absorbent, except when butadiene is present, indicates. because of liberation of gas, a low analytical result. The analyses in Table V show that forination of unabsorbable gas is promoted by the presence of silver sulfate. The data indicate that in the analysis of ethylene, with sufficiently frequent replacement of absorbent, silvei d f a t e
TABLE5'.
a
= 99.8% ethylene;
IXFLUENCE OF SILVER SULF.4TE ON ETHYLENE DETERMISATIONS Max. 1-cc. AbAcid 30sorbed Por- Sec. per H B 0 r AmSOr Gases5 Taken tions Passes Portion Olefin Error
7 99.7 99.7 99.7 99.7 99 7 99.7 99.7 96.0 96.0 96.0 96.0 96.0 96.0 96.0 96.0 96.0 91.0 90.0 90.0
0
% 0.00 0.00 0.50 0 50 0.50 0.50 0 50 1.50 1.50 1.50 1.50 1.50 1.50 1.00 1.00 1.00 1.50 1.50 1.00
cc. E , 49.60; Nr, 50.10 E , 57.00: Xi,50.20
E , 79 95; S?,1 9 . 8 5 E, 8 0 . 1 5 ; x>,1 9 ,i o E, 7 9 . 3 0 ; Pi?,20.30 E , 7 9 . 4 0 ; s2,20.60 E, 81.20; n-BuH, 18. 30 E, 79.55; Iia, 19.70 E , 68.80; Ii?,3 2 , 4 5 E, 49.60; n-BuH, 49 90 E , 70.85; NI, 29.20 E, 68.35; Nz, 31,15 E , 66.25; N?, 35 35 E, 7 9 . 4 0 ; XI, 20.15E , 45.80; n-BuH, 4 1 ,80 E , 81.00; n-BuH, 19. 70 E , 69.75: Nz, 3 0 . 2 0 E , 80.15; Nn, 19.60 E, 80 45; N2, 2 0 . 2 0
% 1 1 1 3
4 6 9
1
1
6
6 7 11 1 8 10 16 15 22
20 28 12c 12 15 14 18 160 8C 9 10 12 Ild 36 15 17 42 36 44
49.50 106.45b 78.30 54.30 38.65 28.55 27.70 77.10 66.75 19.95 24.10 24.80 11.30 77.0: 13,l5 24.60 8.90 9.40 9 4.3
99.8 99.8 97.9 99.4 99.6 99.6 99.7 96.9 97.0 98.1 97.5 98.1 99.6 97.0 98.9 98.1 99.7 99.3 99.6
%
...
...
-1.9 -0.4 -0.2 -0.2 -0.1 -2.9 -2.8 -1.7 -2.3 -l., -0.2 -2.8 -0.9 -1.7 -0.1 -0 .i -0 2
in concentrations varying from 0.50 per cent in 100 per cent sulfuric acid to 1.0 per cent in 90 per cent acid can be used. KOexperiments were made with other catalysts beyond the observation that neither mercury sulfate nor nickel sulfate appeared to accelerate the absorption of ethylene by 96 per cent wlfuric acid.
Basic Scheme of Improvement In the past, the sulfuric acid method has been capable of giving accurate results only for relatively small concentrations of olefins. Large concentrations, because of the limitations described, have led to positive and negative errors of unpredictable magnitude. The chief improvement to be made is the autoinatic minimization of these errors to the point a t which they are negligible.
.4SA4LTTICALEDITION
JPLI- 15, 1938
357
11
FIGURE 2.
APPARATUS FOR S U L F U R I C
Although in many cases the obvious expedient of dilution of the sample with an inert gas is applicable, a generally more satisfactory scheme, in combination with the technic of frequent replacement of absorbent, is the use of a preliminary absorbent so weak that it requires no correction for incomplete specificity, to reduce the concentration of the olefin to the point a t which a stronger final absorbent can be uied reliably.
ACID AXALI-SISO F
G A E ~ UOLEFINS ~
Apparatus
The apparatus shorn in Figure 2, designed for rapid replacement of small portions of absorbent, is highly satisfactory. A contact-type absorption pipet is connected at its top through a three-way stopcock to the manifold of a gas-analysis assembly and at its bottom through a rubber tube to a leveling bulb containing mercury. Small siphons lead from its bottom t o sulfuric acid reservoirs, of which two types suitable for acids of up to 100 per cent in strength are shown. When the siphon stopcock for one is open or nhen the plunger for the other is in the elevated pozition shown, acid is dran-n into the pipet by lowering of the leveling hbsorbents bulb. Because mercury slowly displaces silver from silver sulfate, For determination of gaseous olefins to xithin 0.2 per cent, sulfuric acid containing this catalyst is kept in glass-stoppered the absorbents specified in Table 1-1are satisfactory. bottles from which portions are dran-n directly into the pipet through the discharge arm of the pipet stopcock. If all absorbTABLE VI. ABSORBESTSR E C O M h f E S D E D FOR DETERXISATION ents are handled in this way, acid reservoirs need not be incorporated in the gas-analysis assembly. S o lubricant other than O F G.4sEocs OLEFISS the acid itself is used for the stopcocks. Preliminary Final -4s is indicated by Figure 2, the manometer stopcock customary Olefin Absorbent Absorbent to Orsat-type assemblies can be omitted. Isobutylene 6 0 4 2 % H&Oa C S - i O % H280a n-Butenes, propylene Ethylene
80-82% H2S01 88-90% H9SOa o.9-1.0yo Agr>Or
+
88-90% HISO; 98-100% HISO&i0 4-0 ,570 .kg&Oi
The higher figure given in each range is considered to be the maximum permissible value. Intermediate and lower concentrations can be used. For example, concentrated sulfuric acid containing up to 0.7 per cent silrer sulfat'e can be used as a final absorbent for ethylene. The solutions are prepared in accordance n-ith the equality A = TV#/(C - S), in which A is the n-eight of concentrated acid containing C per cent sulfuric acid to be added to the n-eight, W , of ice to yield a solution of S per cent sulfuric acid. For acid of specific gravity 1.84, C varies from about 94 to over 96; its exact value can be calculated from the strength, found by specific gravity measurement, of a solution prepared from known weights of acid and ice. The 98 to 100 per cent acid is prepared by the addition of fuming acid to concentrated acid. A portion should be tested with a 100-cc. sample of air; if it increases the volume of air in t r o passes by more than 0.05 cc., it should be diluted with a little concentrated acid.
Choice of Analytical Conditions SIZEOF ACID PonTIoh-s. The amount of absorbent taken a t a time should be about 1 cc. As is illustrated by the tnw sets of analytical details in Table VII, one obtained with replacement of acid (1.0 cc.) above the mercury after every two passes of 30 seconds each and the other with one portion of 5 t'o 6 cc. of each acid, exclusive of acid clinging to the contact tubes, larger portions do not increase the absorption rat'e appreciably. In order that the absorbent may spread over the contact tubes sufficiently rapidly, each successive small portion should not be less than 1 cc. As the absorbent clinging to the walls of the pipet and the contact tubes approximates 1 cc., the total volume in the pipet is about, 2 cc. The top of the pipet should be marked at the 1.0-cc. point; but no great striving to obtain portions of exactly this size is necessary. Incidentally, the first analysis in Table VI1 illustrates the uncertainty involved when a single acid of the strength usually recommended is used for the determination of an olefin at as
INDUSTRIAL AND ENGI NEERING CHEMISTRY
358
TABLEVII.
IXDEPENDENCE O F A4BSORPTIOK
VOLUME
No. of
Passes 0 2 2 2 2 2 2 2 2 2 2 2 84.3
Many 1.0-Cc. Portionsa Absorbed Reading
cc.
100.35 85,60 74.25 67.15 63.30 61.40 60.50 60.00 59.70 59.40 59.20 58,95 27.00 21.90 20.65 20.30 20.20 20,20 20.20
14:+5 11.35 7.10 3.85 1.90 0.90 0.50 0.30 0.30 0.20 0 25 31.95 5.10 1.35 0.35 0.10 0.00 0.00
RATE AND ACID
One 5- t o 6-Cc. Portionb Reading Absorbed CC .
24.80 21 35 20.50 20,20 20.10 20.00 19.95 19.90 19.80
33.10 3.46 0.85 0.30 0.10 0.10 0.05 0.05 0.10
Corn osition of sample, cc.: a Isoiutylene 39.40, butene-2 40.70, n-butane 20.25. b Isobutylene 38.70, butene-2 40.45, n-butane 20.35.
high a concentration as 40 per cent: the isobutylene found was (100.35 - 58.95) - 11 X 0.25 = 38.68cc., whereas 39.40 cc. were taken; the butene-2 found was (58.95 - 20.20) 11 X 0.25 = 41.50 cc., whereas 40.70 cc. were taken. In the second analysis the isobutylene found was (99.50 57.90) - 11 X 0.30 = 38.30 cc., whereas 38.70 cc. were taken. This result, because of a fortuitous compensation of errors, is better than that in the first analysis; it should not be interpreted as indicating that the use of a large portion is superior to the use of several small portions. The inferiority of the use of the single portion is shown by the butene-2 determination, in which the absorption did not reach the sharp end point obtained when fresh acid was always present. The small final absorption rate, like those mentioned in the footnotes of Table 111, was due to physical solution of butane by the polymers that were being formed. After polymer formation had continued overnight, the residual volume of 19.80 cc. decreased in t x o passes to a constant value of 14.00 cc.; thus, the polymers from 40.45 cc. of butene-2 physically dissolved 6.35 cc. of n-butane. As ethylene was known t o be absent, the absorption might have been continued to this ultimate end point, which would have indicated a butene-2 content of (57.90 - 14.00) 11 X 0.30 = 47.20 cc., whereas 40.45 cc. were taken. If ethylene had been present, differentiation of the absorption of ethylene from the solution of butane by the polymers would have been impossible-a dilemma avoided by the use of multiple portions.
+
+
DURATIOP; OF PASSES.The passes of the gas sample into the absorption pipet must be made in a standard manner, with all passes alike. Very short passes, because of the large number required, produce a troublesome desiccation of the upper part of the buret. Very long passes, because of drainage from the contact tubes, produce a decreased over-all rate of absorption. This effect is illustrated by bhe absorption data in Table VI11 for 50 per cent isobutylene and 69.5 per cent sulfuric acid. Replacement of acid was made after each buret reading. Inclusive of the average of 5 seconds spent in each passage of gas between buret and pipet, the first absorption required 14 x (30 10) = 560 seconds; the second required 8 X (75 10) = 680 seconds. Complete absorption required 23 per cent more time for the 75-second passes than for the 30-second passes. A pass during which the sample is kept in the absorption pipet for 30 seconds is highly satigfactory. For the absorbents specified REPLACEMENT SCHEDCLE. in Table VI and for 30-second passes, replacement of absorbent after every two passes is satisfactory. PIPETWASHING. Since correction for incomplete specificity of a final absorbent can be made accurately only if the absorbent strength is constant, two or three 1- to 2-cc. portions of such absorbent must be used prior to absorption t o
+
+
VOL. 10, NO. 7
wash any weaker acid from the pipet. To spread the acid over the contact tubes, air is drawn in through the acid-discharge tube. When, however, a preliminary absorbent is to be used, pipet-m-ashing is preferably omitted. This has the advantage, important for samples of high olefin content, especially of n-butenes, that the absorption by the first few portions, which undergo dilution, is kept from exceeding about 15 cc. per portion. After each analysis, the pipet is washed once with about 5 cc. of water, which is first run into the adjacent part of the manifold to remove any acid accidentally carried there. MOISTURE.Several considerations indicate that buret readings should be made with the sample saturated with moisture: (1) No preliminary desiccation of moisture-containing samples is then necessary; (2) it is next to impossible to keep an Orsat apparatus reliably dry, because of the aqueous solutions and the moisture from combustions; (3) sulfuric acid solutions have an appreciable aqueous tension, which varies with concentration and temperature and for which correction (6, 10) requires time-consuming calculations; and (4) the simplest method of taking account of the aqueous tension of absorbents or of moisture in the sample is the automatic saturation of the sample with water vapor in the buret. Because sulfuric acid has a strong desiccating action, careful attention must be paid to keeping sufficient water in the buret. If the rapid or short passes customary to Orsat analyses are used, deeiccatioii of the upper part of the buret and the resultant unsaturation can produce a not inconsiderable error. Even when sufficient water is present in the buret, such an error can occur unless the sample is kept in the absorption pipet long enough for a part of the water to work its way up to the top of the buret. For addition of water to the buret during the course of a n analysis, a small water-reservoir connected through a stopcock to the manifold between the buret and the manometer, as in Figure 2, is convenient OF DRAINAGE ON TIMEOF TABLE VIII. IXFLUENCE
.4BSORPTIoN OF ISOBUTYLEXE --30-Second PassesKO.of Buret passes reading Absorbed
cc.
-75-Second PassesNo. of Buret passes reading Absorbed
cc.
DILUTION.Since the residual volume after analysis should not be less than 15 to 20 cc., the sample must be diluted with air or nitrogen if it contains more than 85 per cent of olefins. About 15 to 20 cc. of diluent are taken, measured, and stored in the sulfuric acid pipet. Then 80 to 85 cc. of the sample are taken; this is preferably but not necessarily measured, and is diluted by adding the diluent to it. After mixing is effected by raising the leveling bulb for the buret a few times, the buret being closed a t the top if the manometer is open to the manifold, the total volume is measured. I n order to minimize the effect of deviations from the gas laws, the volume of the sample should be taken as the difference between the volumes of the mixture and the diluent instead of that directly measured before dilution.
-4bsorption Procedure After a 1-cc. portion of the appropriate preliminary absorbent is placed above the mercury in the pipet, two 30-second passes of the accurately measured gas sample are made. Then the acid is drawn up t o the pipet stopcock, which is turned to discharge the
AKALYTICAL EDITION
JULY 15, 1938
spent acid, and the gas volume is read. This procedure is repeated until the absorption per portion of full-strength preliminary absorbent becomes less than 5 cc. In this manner, the residual olefin is reduced to 5 to 15 cc. (sometimes more than 15 cc. if the olefin is ethylene). If the pipet previously contained a weaker acid, at least tn-o or three portions should be used before the preliminary absorption is discontinued. If the sample is knon-n t o contain less than about 15 cc. of the olefin, the preliminary absorbent should not be used at all. Then the pipet is washed at least twice with the corresponding final absorbent and absorption is continued with two 30-second passes into each portion of fresh absorbent until a constant small absorption is obtained for about three portions (only one or two portions if it is zero). A total of 5 to 8 portions is generally required. .Ifter the constant absorption has been determined, the foregoing procedure is repeated for the next set of absorbents. The buret readings need not be made Tvith accuracy until the rate of absorption is small. This condition is indicated independently by a transition, pronounced for the olefins heavier then ethylene, from a “rough” appearance of the acid on the contact tubes to a “smooth” appearance.
TABLE
Time
k
~ OF VXDILUTED ~ ~ OLEFIX ~ ~ 30-SECOSD PASSES
H&Or,
Butene-2
62.0 70.0 82 0 90 0
0.20 2.15
%
(99.8%) ..
Butene-1 (99.6%) 0.15 L25
.. ..
Propylene (97.4%)
..
I N ~T W O ~
AkS.4LYsIS O F SYNTHETIC
OLEFIXS
HzSOa
R,eading
Absorbed
%
cc.
cc
6210 62 0
100 00 93.20 66.40 81 60 7 5 50 74 05 7 3 40 i 2 80 73 30 71 80 71 40 70 90 62.70
Time 3:13
MIXTUREO F GASEOUS H&04
Reading
%
cc.
Cc.
44.80 44.50 44 25 44.00
9.05 0.30 0.25 0.25
43.75 38.15 34.00 25.60 21.10 19.75 19.45 19.25 19.10 18 90 18 70
0.33 5.60 4.15 8 40 4.50 1 35 0.30 0.20 0.15 0.20 0 20
90.0
6’80
90.0
6 80 90.0 61 0 4 80 90 0 2 28 70 0 G 10 90 0 70 0 1 45 3 30 90 0 Aga 70 0 0 65 YO 0 Ag 70 O 0 60 3 40 100 A g b 70 0 0 50 100 h g 70 0 0 50 100 kg 70 0 0 40 100 -ip 70 0 0 50 100 .kg 2:69 82 0 8.20 100 Ag 82.0 5 i . 6 5 5 05 100 A g 82.0 63.85 3.80 4.U1 100.Ag a 90.0 Ag = 90.0% H&O< containing 1.007GA g S O I . 6 100 .4g = 100% H:SO< containiug 0.,:0% .kg!YOt.
Calculations: Isobutylene found = 100 00 - (70 90 8 X 1 ‘3014) Isobutylene raken Difference n-Butenes propylene found = (70.00 f 8 X 1.90/4) (43.75 5 X 0.25) Butene-2 propylene taken = 14.75 14.75 Difference Ethylene found = (43.75 5 X 0.23) - (18.70 8 X
+
+++
I n general, physical solubility of gaseous paraffins is more than negligible only in the final absorption of ethylene. A correction for it is found by continuation of the absorption until it becomes constant for each 1-cc. portion of final absorbent and multiplication of the constant absorption by the number of portions. Ix. A
x.
2 : 14
Corrections
TABLE
359
-
+
+
0.i514j
~
-
Eth>-lene taken Difference Paraffins and inerts iound = 18.70 n-Butane and inerts taken = 19.60 Difference S
++ 80.60 X 0.75/4
Absorbed
= 23 30% = =
25.3070 0 ooa ~
,I
2‘3.70% = 29 joy,
=
+o.2070
= 24.80YG = 24 90Lh = - 0 10% .= 20.207~ = 20 20cyo = 0 00%
Ethylene (9’3.8%)
0:0i5 0 05 0 90
I n the analysis of samples containing principally fourcarbon hydrocarbons, a n appreciable physical solubility effect can be present also during the use of 88 to 90 per cent acid. If both butanes are present in considerable amounts, or if the temperature happens to be much belom- 2.5” C., i t can amount to eren more than that indicated by Table I-e. g., 0.05 cc. per 1-cc. portion of acid. I n such analyses, especially when ethylene is known to be absent, corrections should be determined in the manner just described. The same procedure IS used to obtain corrections for incomplete specificity of absorbents for olefins heavier than ethylene. As indicated by Table IX, which gives the absorption obtained in t7T-o 30-second passes of about 100 cc. of substantially pure olefin into acids of the maximum recommended strengths, the correction is largest for butene-2 absorbed a t the end of the isobutylene determination. I n actual analyses, because of dilution of olefins with other gases and generally smaller effective acid-surface areas, the absorptions indicated in Table I S are never obtained. 9 s alkali solutions dissolve paraffins to a considerable extent and as the absorbents here recommended are nonfuming, the usual procedure of passing the gas residue from the ethylene absorption into a n alkali pipet, which is necessary in determinations by fuming acid, should not be used.
Examples A typical example illustrating the use and the accuracy of the improved method is presented in Table X. The pipet was washed out twice with 1 to 2 cc. of acid of each new concentration before the acid was used for absorption. Buret readings and replacement of absorbent were made after every two passes of 30 seconds each (exclusive of time of inflow and outflow). The accuracy is all that can be expected. I n a series of analyses of synthetic mixtures of four-carbon hydrocarbons,
the value for n-butenes appeared to be consistently slightly too high, probably because of the individually negligible but cumulatively appreciable solution of bufane in the 15 to 20 portions of acid that were used. Hence, when the residual gas is known to consist mainly of one or both butanes, i t appears justifiable to subtract a small supplementary and somewhat arbitrary correction of 0.10 cc. from the value for the n-butene fraction. I n the present example, this reduces the difference between the amounts found and taken froln 0.20 to 0.10 per cent. TABLE XI.
h A L I ’ S I S O F SYSTHETIC BUTENE-2
Air taken Butene-2 taken Total, calculated I‘otal, measured Sample volume = 9‘3 60 Time
&SO4
qo 9:00
.. 70.0 70.0 70.0 70 0 z0.0 rO.O
Reading Absorbed
cc. 99.60 97.65 96.05 94.60 93.20 Y1.80 Y0.40 79.20
-
= = = = 17.70 =
Time
cc.
17.70 cc. 81.80 cc. 99.50 cc. 99.60 cc. 81.90 cc.
HzSOr Reading Absorbed 70
..
82.0
2.05
82.0 82.0 82.0 82.0 90.0 90.0 90.0
1.50
1.45 1.40 1.40 1.40 11.20
Y:37
cc.
cc.
63.50 48.10 36.90 29.70 25.10 18.00 17 go 17.90
15.70 15.40 11.20 7.20 4.60 7.10
~~
n. io . ~.
9:20 82.0 9:43 0.00 Calculations: Isobutylene iound = 99.60 - (90.40 6 X 1.40) = 0.80 cc. = 1.0$& Butene-2 iound = (90.40 f 6 X 1.40) - 17.90 = 80.90 cc. = 98.87G Inert impurities found = 17.90 - 17.70 = 0 . 2 0 cc. = 0,2Y0
+
Table XI illustrates the analysis of one of the most difficult samples likely to be encountered, butene-2 (prepared from the corresponding technical alcohol) containing a small concentration of isobutylene. The pipet was first washed 3 or 4 times with the 70.0 per cent acid. No pipet-washing was made with the 82.0 per cent acid; hence, no portion absorbed much in excess of 15 cc. of butene-2. As ethylene was known to be absent, no pipet-washing mas made with the 90.0 per cent acid. I n spite of the high final constant absorption rate of 1.40 cc. per portion, no appreciable uncertainty was present in the
INDUSTRIAL AND ESGINEERING CHEMISTRY
360
isobutylene determination. I n the absence of isobutylene, the rate would have been constant from the very first. Whenever this synthetic butene-2 was used, its known content of isobutylene was taken into account or, if necessary, as for the determination of the data in Table I X , the isobutylene was first removed with 68 to 70 per cent acid. The other olefin samples contained no extraneous olefin; the impurities were the corresponding paraffins or air. The time required for an analysis depends somewhat on the details of the apparatus and on the skill of the operator. It is about half an hour for each olefin. For the foregoing two analyses, all absorbents were in bottles, as each had been adjusted to the maximum recommended strength; hence, the total time, because of additional manipulations, was slightly greater than if the reservoir4 of Figure 2 had been used.
Jlodifications It is perhaps obvious that some modifications in the method may be made without introducing errors for samples of more or less known composition. For example, in the analysis of samples having small concentrations of olefins, only one concentration of acid need be used for each olefin. Also, the number of passes per portion of acid, especially in determinations of propylene and ethylene, may be increased. Isobutylene in samples containing much butene-2 or much
VOL. 10. NO. 7
butadiene can be determined with a somewhat advantageously increased specificity with acids weaker than 68 to 70 per cent. The data presented should prove helpful in selecting conditions for such modifications. For the analysis of any sample in general, however, it is believed that the recommendations given represent an optimum over-all compromise among the several conflicting factors involved and that observance of them will yield rapid and reliable analytical results.
Literature Cited (1) Christoff, 2. p h y s i k . Chem., 55, 622 (1906); International Critical Tables, hfcGraw-Hill, Vol. 111, p. 280 (1926). (2) Davis, J . Am. Chem. SOC.,50, 2780-2 (1928). (3) Davis and Crandall, Ibid., 52, 3769-85 (1930). (4) Davis and Daugherty, IND.ENQ.CHEM.,Anal. Ed., 4, 193-7 (1932). (5) Davis and Schuler, J. Am. Chern. Soc., 52, 721-38 (1930). (6) Dobryanski, Neftyanoe Khoz., 9, 565-73 (1925). (7) Frey and Huppke, IND. ENQ.CHEM.,25, 55 (1933). (8) Hurd and Spence, J . A m . Chem. SOC.,51, 3356-7 (1929). (9) Manning, King, and Sinnatt, Dept. Scientific and Industrial Research, Tech. Paper 19 (1928); Ellis, “Chemistry of Petroleum Derivatives,” p. 1120, New York, Chemical Catalog Co., 1934. (10) Markovich and Dementera, Khimteoret, 2, 131-43 (19353. (11) Markovich and Moor, .Veffyanoe Khoz., 19, 604-13 (1930) RECEIVEDl p r i l 18, 1938. Presented before t h e Division of Petroleum Chemistry a t the 95th Meeting of the hmerican Chemical Society, Dallas, Teras, l p r i l 18 t o 2 2 , 1938.
Volatilizing Chromium as Chromyl Chloride A Rapid Method Applicable to Determination of Manganese in Stainless Steel FRED WILSON SMITH Carnegie-Illinois Steel Corporation, South Works Chemical Laboratory, Chicago, Ill.
A new and rapid method is described for the accurate determination of manganese in stainless and other high-chromium steels, in which the chromium is rapidly volatilized from a perchloric acid solution of the sample. The method is also applicable to other determinations in which large quantities of chromium are objectionable.
C
HROMIUM, when present to the extent of more than 2 or 3 per cent, interferes with the determination of manganese by the persulfate-arsenite method. Consequently, high-chromium steels require a separation of chromium prior to the determination of manganese. Heretofore, the author has used the zinc oxide separation, which is effected by adding zinc oxide paste to R dilute sulfuric acid solution of the steel which has been oxidized by nitric acid. The mixture containing ferric and chromium hydroxides and the excess zinc oxide is diluted to 200 ml., and filtered, and the manganese is determined on an aliquot of the filtrate. This method is time-consuming for two reasons: Two-gram samples of stainless steels may require from 0.5 t o 2 hours t o dissolve in 9 iV sulfuric acid, and the subsequent filtration of the bulky precipitate formed by the Bine oxide is also slow. T h e author h s t attempted to overcome this drawback by using perchloric acid to dissolve the steel, which makes solu-
tion possible in 5 to 10 minutes. He also confirmed the experience of other chemists in this laboratory that if oxidation of the iron following solution in sulfuric acid is postponed until after the separation of chromium, the filtration is much faster. An attempt to combine these two improvements by reducing the iron with sulfurous acid after solution in perchloric acid indicated that it is very difficult, from a practicable analytical viewpoint, to keep the iron in the reduced state in perchloric acid solution. The object of the first part of this investigation was to find a means of speeding up the preliminary operations of dissolving the steel and separating the chromium. A possible solution of this problem was suggested by a consideration of the possibility of removing chromiun by conversion t o a gaseous rather than a solid compound. The formation of the red gas chromyl chloride, Cr02C12,was studied for this purpose. The fact that chromyl chloride can be evolved from a mixture of a solid chloride and potassium dichromate in h o t concentrated sulfuric acid is known from the old qualitative test used to distinguish chlorides from bromides. I n this test the gases evolved, bromine from bromides and chromyl chloride from chlorides, are absorbed in a dilute ammonium hydroxide solution, The ammonium bromide and hypobromite formed are colorless, while the chromyl chloride forms yellow chromate ions. I t follows that the addition of a solid chloride to a hot sulfuric acid solution of hexavalent chromium will volatilize some of the chromium and it is possible that the reaction may be driven nearly to completion by excess of salt, provided that the chromium which is reduced to the trivalent
