Determination of Cyclopropane by Selective Absorption FRANCIS R. BROOKS, ROBERT E. MURDOCK,
AND
VICTOR Z.4"
Shell Decelopment Company, Emeryeille, Calif.
Cyclopropane can be determined in gases containing both olefins and paraffins by absorption of the olefins in acid mercuric sulfate reagent followed by absorption of the cyclopropane in 87% sulfuric acid. Ethylene can be distinguished from other olefins by a second analysis in which the order of absorbents is reversed, and cyclopropane plus olefins other than ethylene is absorbed in 8i70 sulfuric acid and then ethylene is absorbed in acid mercuric sulfate reagent.
C
PROCEDURE
YCLOPROPANE is occasionally encountered as a component of hydrocarbon gases and a simple and direct method, using conventional gas analysis apparatus, for its determination in the presence of other hydrocarbon gases would be desirable. Mixtures of propylene and cyclopropane have been analyzed by absorption of the propylene in 3% aqueous potassium permanganate solution, and the cyclopropane has been determined by difference after the necessary corrections have been made for solubility of cyclopropane in this reagent (1, 5, '7). Such mixtures have also been analyzed by a micromethod, using saturated aqueous silver nitrate solution for removal of propylene (6). A method for the determination of cyclopropane in the presence of both propylene and inerts, described by Corner and Pease (Z), is based on the' selective hydrogenation of propylene over a nickel-kieselguhr catalyst partially poisoned with mercury, followed by hydrogenation of cyclopropane over a nonpoisoned catalyst. Although this method is apparently capable of yielding data of a high order of accuracy, difficulty might be experienced in securing or preparing catalysts of the required activity level. During a study of the behavior of cyclopropane toward absorption reagents used in this laboratory for the determination of olefinic gases, it m s found that cyclopropane is not absorbed by acid mercuric sulfate reagent (4), but is rapidly absorbed by 87% sulfuric acid, which is used for the selective absorption of propylene in the presence of ethylene. These observations suggested a simple means for the determination of cyclopropane in gases containing both saturated and unsaturated hydrocarbons. This paper presents the results of an investigation of this method of analysis.
Two methods of analysis are presented. The first procedure concerns samples containing propylene, cyclopropane, and inert gases; the second, samples which contain ethylene as well. For most accurate analyses, each reagent should be saturated n i t h the residual gas that will be present in the mixture after absorption of the gas reactive with that particular reagent'. For this purpose, measure approximately 50 ml. of sample into the gas buret and pass through the reagents in the manner described for analysis of the sample, but make no effort to read volumes accurately or to record data. If a series of samples of similar composition is being analyzed, the presaturation need be carried out only for the first sample, as each analysis can serve as a presaturation for the one follom-ing. A. Samples Containing Propylene, Cyclopropane, and Inerts. Measure a 100-nil. portion of the gas sample into the gas buret. To determine the propylene content, pass the gas into the acid mercuric sulfate reagent five rimes and record the residual gas volume. Repeat in groups of five passes until a constant volume or a constant absorption per group of passes is obtained. Samples containing high concentrations of cyclopropane vi11 give a small constant coabsorption of cyclopropane in this reagent, for which appropriate correction must be made. To determine the cyclopropane content, pass the residual gas into the second sulfuric acid pipet in groups of five passes until R constant residual volume is obtained. B. Samples Containing Ethylene, Propylene, Cyclopropane, and Inerts. Measure a 100-ml. portion of the gas sample into the gas buret and treat as described in Procedure -1. I n this case the absorption in acid mercuric sulfate reagent is a measure of propylene plus ethylene. If ethylene and propylene values are desired, measure another 100-ml. poption of sample into the buret and pass into the first 87% sulfuric acid pipet to determine propylene plus cyclopropane and then pass the residual gas into the acid mercuric sulfate reagent to determine the erhylene content,. Samples containing high concentrations of ethylene give a small, constant coabsorption of ethylene in the 877, sulfuric acid, for which appropriate correction must be made. The propylene content is cnlculatpd as the difference between the propylene plus ethylene value found in the first analysis and the ethylene value found i n the second analysis.
APPARATUS AND REAGENTS
The apparatus used for the tests described consisted of a 100-ml. gas buret graduated in 0.2-ml. divisions and three gas pipets xyhich communicated to the buret through an all-glass manifold. Mercury was used as a displacement liquid in the buret, and the gas was saturated with water vapor before each volume measurement EXPERI3IEIVT.4 L by maintaining a film of water upon the wall of the buret. The pipets each contained approximately 180 ml. of reagent and were A series of hydrocarbon niixtures was prepared from the pure packed with vertical tubes to provide contact with a large film components to contain various concentrations of ethylene, proof reagent. TKOof the pipets containing 877, sulfuric acid and pylene, cyclopropane, and propane. The pure components used one containing acid mercuric sulfate reagent were attached to the manifold so that the latter was located between the sulfuric acid pipets. The 877, sulfuric acid nas Table I. Anal>-sisof Hydrocarbon 3Iixtures oreoared bv dilution of C.P. Component, Mole Per Cent coicentrate'b. acid, and is used Propane Propylene Ethylene Blend Cyclopropane in this laboratory for the seFounda Calod. Calcd. Found Found Calcd. Found Calcd KO. Method lective absorption of propylene 34.3 34.0 23.3 28.4 28.7 B 13.9 14.0 23.3 in the presence of ethylene. 66.2 66.5 11.3 11.2 11.3 11.2 B 11.2 11.2 The acid mercuric sulfate re46.0 46.3 60.4 50.4 -0.1 0.0 B 3.7 3.4 33.4 33.4 0.0 0.0 33.3 33.3 agent was prepared accord33.3 33.3 B 49.7 49.7 47.0 0.0 0.0 B 47.2 3.1 3.4 ing to Francis and Lukasiexicz 5 0 . 1 49.8 0 . 0 3.1 3.3 A 46.8 46.9 ( 4 ) and modified by satura66.6 66.6 0.0 33.4 33.4 A 0.0 0.0 1 5 . 3 15.0 0 . 0 tion with magnesium sul7 6 . 0 7 5 . 0 9 7 1 0 . 0 A 3 . 2 3.3 0 . 0 A 9 3 . 6 9 3 . 4 3 . 3 3 . 2 fate iRlnSOa.7HoOi. . - , , as suna By difference. gested bYDavidson and And&son ( 3 ) . 62
V O L U M E 20, N O . 1, J A N U A R Y 1 9 4 8 were analyzed mass-spectrometrically and found to contain not more than 0.5% of impurities. The results from the analysis of these mixtures, using the two procedures described above, are presented in Table I, which shows that the method yields accurate values over a wide range of concentrations. LITERATURE CITED (1) Chambers, T. S., and Kistialion-sky, G. B., J . Am. Chem. Soc., 56, 309 (1934).
(2) Corner, E. S.,and Pease, R. N., 17,504 (1945).
63 and Anderson, J. S., Esso Laboratories, Stand(3) Davidson, V. M., ard Oil Co. of Xew Jersey, Louisiana Division, private communication (paper in preparation). Francis, -1.W., and Lukasiewicz, S. J., IKD.EKQ.CHEX.,ANAL. ED..17. 703 (1945). (5) Morikawa, K.,‘Trenner, N. R., and Taylor, H. S., J . Am. Chem. Soc., 59,1103 (1937). J r . , and Priest, IT. J . , J . Chem. Phys., 7, 736 (1939). (6) Ogg, R . -1., (7) Roginski, S. Z., and Rathman, F. H., J . Am. Chem. Soc., 55, 2800 (1933). (4)
ISD. ENG.CHEX.,A s a ~ ED., . RECEIVED J u l y 12, 1947.
Selenium as Catalyst in Kjeldahl Digestions S. M. PATEL AND ARUNACHALA SREESIVASAN Department of Chemical Technology, University of Bombay, Bombay 19, India
1n the contersion of organic to ammoniacal nitrogen b j the Kjeldahl method, the use of zelenium to cataljze the oxidation results in loss of nitrogen during prolonged heating follow ing clearance of the digest. Comparatitely better recoier) of nitrogen is obtained when a combination of selenium and mercuric oxide is,used as cataljst; this is due t o the
T
HE catalytic effect of small amounts of selenium, selenium
oxychloride, selinites, or selenates in the oxidative digestion of nitrogenous organic matter by the Kjeldahl method is well known (6,I S , 17, 20, 28, 3 2 ) . A combination of mercuric oxide and selenium acts much more effectively than either of them singly in accelerating the decomposition of proteins (1, 4, 22, 65, 33, 35). 111 a study of the mechanism of selenium catalysis, Srecnivasaii and Sadasivan (31, 32) have shown that selenium acts as an efficient carrier of oxygen n-hen heated with sulfuric acid and reducing organic matter by means of rapid, reversible reactions involving the formation of selenic and selenious acids in the presence and absence, respectively, of mercuric oxide; selenic acid is comparatively unstable, being easily decomposed into, and having therefore a greater oxidizing capacity than, the lower (selenious) acid. This explains the synergetic action of seleniuni and mercuric oxide in catalyzing the oxidation. Although the use of selenium in Kjeldahl digestions is extensive, scattered references in the literature question the advisability of its use. Sandstedt ( 2 7 ) reported, soon after the original observation of Lauro on the effect of selenium (,El),that prolonged digestion with selenium catalysts resulted in lon- values for total nitrogen. Similar losses of nitrogen were noted by Davis and X s e ( I C ) , Osborn and lirasnitz (%?),Snider and Coleman (29), Dalrymple and King ( 1 3 ) ,and others. Illarionov and Ssolovjeva ( 1 6 ) observed that the catalytic action of selenium Tsas proportional to the quantity added. while Bradstreet (9) stated that an increase in the amount of selenium used gave l o n results. Miller and Houghton (21) failed t o obtain quantitative values for the nitrogen contents of lysine and tyrosine using selenium as catalyst. Van Slyke et al. ( S i ) found that none of the catalysts recommended in the literature for the Kjeldahl procedure, including selenium, yielded more than 902. of the total nitrogen of tryptophane and lysine. I n experiments on the use of oxidizing agents like peroxides, permanganate, and dichromate in Kjeldahl digestions, Sreenivasan (30) also observed that the use of these catalyst,s, in more than small quantities, resulted in loss of nitrogen. Wicks and Firminger (%), Kaye and Weiner ( l a ) , and Jonnard ( 1 7 ) , using perchloric acid as an aid to hasten digestion (E?),reported low resu1t.s for nitrogen. Some of the discrepancies in the values obtained for total nitrogen by using modifications of the Kjeldahl method have been attributed t o the formation of free nitrogen ( 2 4 )and to undecomposed amines (15).
formation of mercury-ammonium complexes which resist oxidation to a greater degree than do free ammonium salts. The period of afterboil necessary to obtain theoretical yields of nitrogen must be accurately standardized for each type of nitrogenous organic matter and selenium cannot be recommended as a general reagent for Kjeldahl determinations.
On the other hand, provided an adequate period of afterboil folloLving clearance of the digest is given, correct results are obtained (2, 6, 10, 26) and the erratic results sometimes obtained with selenium compounds may frequently be due to insufficient digestion (3, 25, 28). Clark (11) was of the opinion that many compounds, although requiring a very long digestion period, will nevertheless yield their nitrogen quantitatively to one or the other of the modified Kjeldahl procedures. According to Miller and Houghton ( 2 1 ) and Jonnard ( 1 7 ) ,the catalyst and the period of digestion are not necessarily identical for all compounds of the same general class and the low values obtained with selenium are due to conditions of heating during digestion and distillation. I n view of the divergent observations of earlier workers, a reinvestigation of the catalytic action of selenium on Kjeldahl digestion suggested itself and this paper presents the results of a comparative study of the influence of the duration of digestion, with and without selenium or selenium and mercuric oxide, on the extent of recovery of nitrogen by the Kjeldahl method. EXPERIMENT4L
Procedure. All determinations were carried out with the conventional Kjeldahl apparatus in the usual manner, using as source of heat gas burners of more or less uniform heating poTver. Chemicals meie of analytical grade and were specially purified where required. Digestions were carried out with substances in amounts such that the nitrogen content of each sample was about 10 mg. together with 20 ml. of sulfuric acid, 6 grams of potassium kulfate, and 0.20 grain of crystalline copper sulfate, the last two referred to a i salt mixture below. The quantities of elemental selenium and mercuric oxide, where employed, amounted t o 50 mg. and 0.50 gram, respectively; in the latter case, the mercuryammonium complexes were destroyed during distillation by addition of 0.50 gram of sodiuni thiosulfate. All results represent the average of a t least t n o determinations except where duplicates were not in good agreement, when three or four determinations were carried out. The conditions of digestion and distillation xere carefully checked, using standard solutions of ammonium sulfate which, in several trials and, by employing the Kjeldahl procedure without selenium and for varying periods of digestion up t o 3 hours, gave results which 7%-erein error by not more than 0.4”0. Digestions with Casein. In Table I are presented the rksults obtained using casein.
